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School biology notes: How the human circulatory system works

Describing & explaining how the human circulatory system works

heart, lungs and body connection; blood vessel system - structure of arteries, capillaries & veins; blood system - function of red blood cells, white blood cells, platelets & plasma; causes & treatment of cardiovascular disease

 Doc Brown's school biology revision notes: GCSE biology, IGCSE  biology, O level biology,  ~US grades 8, 9 and 10 school science courses or equivalent for ~14-16 year old students of biology

 This page will help you answer questions such as ...  What is the mammalian double circulatory system work?  What is the function of veins, arteries and capillaries in the blood circulation system?  How is the structure of veins, arteries and capillaries adapted for their functions?  What are the causes and problems of cardiovascular disease?

Sub-index for this page

1. A double circulatory system - heart and lung connections

2. The human heart and its pumping action - diagrams and explanation

3. The structure and function blood vessels, arteries, capillaries, veins, flow rates

4. What's in the blood? - red and white blood cells, platelets, plasma

5. Cardiovascular disease (CVD) & treatments, heart disease, stents, statins

6. Scientific developments in treating cardiovascular disease and emergencies e.g. surgery, artificial blood, and artificial hearts

See also Respiration - aerobic/anaerobic including in animals

and Examples of surfaces for the exchange of substances in animal organisms



1. A double circulatory system - advantages

Oxygen is needed to produce energy from respiration chemistry to power the cells of any organism. Carbon dioxide is the waste product from respiration. So, a gas exchange system is required - this takes place in the alveoli of the lungs.

This is described on Gas exchange in the lungs by diffusion, so here we concentrate on the double circulatory system and how the heart works.

The circulatory system transports substances around the body

We humans, like other mammals, have a double circulatory system consisting of two closed circuits joined together with the heart acting as the central pump.

From the right ventricle of the heart, the first circuit pumps deoxygenated blood to the lungs to absorb fresh oxygen before being returned to the heart (right diagram).

Deoxygenated means the haemoglobin molecules in red blood cells are not carrying oxygen molecules, therefore the blood itself, is not carrying oxygen.

Oxygenated means the haemoglobin molecules in red blood cells are carrying oxygen molecules, therefore the blood itself, is carrying oxygen.

In the second  circuit, the left ventricle of the heart pumps oxygenated blood from the lungs to all the organs and associated structures (the rest of the body except for the lungs) and returns it to the heart (left diagram).

After circulating round the body discharging its oxygen to all the cells of the body, the heart pumps the blood back to the lungs to be re-oxygenated.

Blood is pumped out of the heart under high pressure into the arteries.

The blood flows through the arteries to the capillaries and then the veins, in doing so , the pressure decreases.

 

The whole double circulation system is pretty complicated (right diagram).

The diagram is a simplified diagram of the double circulatory system of the human body as an example of the mammalian circulation structure.

Apart from the lungs, the blood vessel system must pass near all the rest of the exchange surfaces too e.g.

the villi in the gut (small intestine) to absorb food molecules and water,

the kidneys, where the blood is filtered to remove urea,

and the liver filters the blood from the digestion system e.g. one of its functions is the detoxification of various chemicals.

The advantages of the mammalian double circulatory system

Adaptations

Because the blood is returned to the heart after its absorbed oxygen, it can be pumped out around the body at a higher pressure AND at a faster rate.

This increases the rate of blood flow increasing the supply of oxygen to all the tissues and organs of the body.

This is vital for mammals to maintain their optimum body temperature - warm blooded animals.

Also, the oxygenated blood is flows separately from the deoxygenated blood.

Note on fish:

Fish have a single circulatory system in which deoxygenated blood from the fish's body is pumped to the heart, which then pumps it through the gills to absorb oxygen from the water and round through the rest of the body in one continuous loop - just one circuit in operation (unlike the double circulatory system of mammals).

For more on fish gills see Examples of surfaces for the exchange of substances in animal organisms

 


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2. The human heart and its pumping action - diagrams and explanation

The heart is an organ and pumps blood around the body

Need to refer to the diagram of the structure of the heart above.

Much of the wall of the heart is made from muscle tissue which continually contracts and relaxes.

This cardiac muscle contains lots of mitochondria to provide the cells with lots of ATP for respiration - lots of energy is needed to work, so to contract the heart muscles.

Heart muscles need their own blood supply to continually access a source of nutrients and oxygen so they can work non-stop - a continuously beating heart to keep all of us mammals alive!

The cardiac muscles are supplied with blood from two coronary arteries which branch from the base of the aorta. They ensure a constant supply of glucose and oxygen for respiration, that can diffuse through the thick walls of the heart into the heart muscle cells .

The heart has valve systems that ensure the blood only flows in the right direction - one way only, no reverse flow allowed.

The heart has two pumps to work the double circulation system that beat together, typically 60 - 100 times per minute.

Each pump has an upper chamber (atrium) that receives blood and a lower chamber (ventricle) that pumps blood out.

Both atria and ventricles fill and pump blood at the same time.

The natural resting heart rate is controlled by a group of cells in the right atrium which act as a pacemaker.

Know that there are four main chambers of the heart (left and right atria and ventricles) to pump the blood around and the associated control valves which only allow flow in one direction.

See the diagram of the heart above/below where the important parts of the organ are labelled and the white arrows show the direction of flow of the two circuits for:

(i) the oxygenated blood from the lungs to the rest of the body (the organs etc.)

and (ii) the deoxygenated blood to the lungs.

Note: For visual simplification, note that oxygenated blood is shown as bright red and deoxygenated blood as blue (even though it is a dark red colour in reality!).

Repeated diagram for visual convenience

The mechanism by which the heart works

The blood flows from the heart to the organs, including the lungs, through arteries and returns through veins.

Blood is supplied to the heart by two coronary arteries which branch from the base of the aorta - the largest artery in the body.

The heart itself needs a good supply of oxygen and glucose for lots of energy to keep the heart muscles continuously working.

The four major blood vessels associated with the heart are the pulmonary artery, pulmonary vein, aorta, vena cava.

The aorta (main artery) - transports oxygenated blood from the left ventricle to the body.

The vena cava (main vein) - transports the deoxygenated blood flow from the body into the right atrium.

The pulmonary artery - transports the deoxygenated blood from the right ventricle to the lungs

The pulmonary vein - brings the oxygenated blood to the heart from the lungs into the left atrium.

gcse biology heart diagram double circulation atria ventricles veins arteries

Flow and valve action:

The heart has valves to ensure the blood only flows in the one correct direction.

When the ventricles contract, the valves to the atria close and the valves to the blood vessels open.

This prevents any reverse blood flow

 Ventricles have thicker walls than atria because they operate at a higher pressure to pump the blood further.

The left ventricle has a thicker wall than the right ventricle because the muscle tissue must be stronger because it operates at a higher blood pressure to pump blood around all of the body.

The right ventricle muscle tissue is thinner because it only has to pump the blood to the lungs.

The blood enters the two atria (right atrium and left atrium, atria) of the heart from the vena carva and the pulmonary vein.

The left atrium and ventricle pump oxygenated blood from the lungs around the body (see diagram above).

The left atrium receives the oxygenated blood from the lungs via the pulmonary vein and is pumped through the aorta to the rest of the body. It passes into the left ventricle to be pumped around the body.

The right atrium and ventricle pump deoxygenated blood

The right atrium of the heart receives the deoxygenated blood from all of the body, entering the heart by the vena cava.

The deoxygenated blood flows through the right ventricle which pumps it to the lungs via the pulmonary artery to be oxygenated.

The basic sequence of heart function is:

The heart relaxes and the blood enters both atria.

The atria contract at the same time forcing blood into the ventricles.

The ventricles contract from the bottom upwards which forces blood out of the heart through the pulmonary artery and aorta.

The atria fill up again and the whole cycle is repeated

The valves in the heart ensure that blood flows in the correct direction.

 

How is it, that normally the heart can beat at a regular rate?

In the right atrium are a group of cells that act as a pacemaker.

They control the heart beat by producing tiny electrical impulses (signals) that spread into the surrounding muscle cells causing them to contract.

If this group of cells are malfunctioning leading to an irregular, and dangerous heartbeat, an artificial pacemaker can be surgically implanted under the skin and wired up to the heart.

This device produce tiny electrical signals at the right frequency to generate a correct and regular heartbeat.

 

The pumping rate of the heart

The cardiac output is the total volume of blood pumped by a ventricle every minute.

It is calculated using the following equation:

cardiac output (cm3/min)  =  heart rate (beats/min)  x stroke volume (cm3)

Abbreviated rearrangements:

heart rate = cardiac output / stroke volume  and  stroke volume = cardiac output / heart rate

Your heart rate is the number of beats per minute e.g. when you measure your pulse rate.

The stroke volume is the volume of blood pumped by one ventricle every time it contracts.

Example questions

Q1 A person has a heart rate of 60 beats/min and average stroke volume of 70 cm3.

Calculate the cardiac output.

cardiac output = heart rate x stroke volume = 60 x 70 = 4200 cm3/min

 

Q2 An athlete sprinter heart rate is 120 beats per minute and a cardiac output of 6000 cm3/min.

Calculate the athlete's stroke volume.

stroke volume = cardiac output / heart rate = 6000 / 120 = 50 cm3

 

Q3 The cardiac output from a person is 3600 cm3/min.

If the person's stroke volume is 60 cm3, what is the pulse rate?

heart rate beat = cardiac output / stroke volume = 3600 / 60 = 60 beats/min


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3. The structure and function blood vessels

The circulatory system operates by a network of blood vessels that carry all the substances that the body wants, and doesn't want, around the body to requisite locations.

The blood flows from the heart to arteries, arterioles (smaller arteries), capillaries, venules (smaller veins), veins and then returned to the heart - reminder diagram of the double circulation system below.

gcse biology heart diagram double circulation atria ventricles veins arteries

There are three types of blood vessel and each is designed ('adapted') for its particular function.

Arteries

Arteries transport (i) oxygenated blood (except for the pulmonary artery) from the heart to the tissues and organs of the body and (ii) deoxygenated blood away from the heart to the lungs.

The arteries carry blood under high pressure with a pulse, so the artery walls need to be thick and strong with muscle tissue-fibre but these blood vessels must be elastic too.

The thick walls are an adaptation for high pressure flow.

The artery walls are relatively thick compared to the size of the 'hole' the blood flows through.

The thick artery walls are not permeable.

The 'hole' is called the lumen and is small.

The artery walls are made of a combination of thick layers of strong muscle cells AND elastic fibres which allow flexibility i.e. allowing the artery walls to be stretches and spring back when the pressure is relaxed - a biological example of elasticity.

The arteries branch into the narrower arterioles.

 

Capillaries

Capillaries exchange materials with tissues.

Arteries branch into arterioles which then branch into numerous much thinner capillaries.

The pressure falls in arteries and the pulse disappears.

There are no valves in arteries.

 In the organs, in fact in all tissues, blood flows through very narrow, thin-walled blood vessels called capillaries which branch out from the arteries-arterioles.

The network of capillaries in tissues art called capillary beds.

The substances needed by the cells in body tissues pass out of the blood capillaries, and substances produced by the cells pass into the blood, through the thin permeable walls of the capillaries - which are only one cell thick - thin wall adapted for efficient exchange of materials.

This ensures a short distance and movement time for particles - fast diffusion in and out - fast exchange between capillaries and cells - sugars, minerals, amino acids, oxygen into surrounding cells and carbon dioxide, urea and other waste products out of cells.

The blood in capillaries will slowly lose its dissolved oxygen.

The capillaries are the smallest blood vessels, and although these fine blood tubes carrying blood are dispersed in all the tissues of every organ, you can't see individual capillaries, but they reach every cell.

The capillaries have adapted to have large surface area to make exchange of substances as fast and efficient as possible.

Capillaries are the main interface for material exchange eg of sugars, amino acids and other nutrients, oxygen and waste carbon dioxide.

Some capillaries are so fine that blood cells cannot pass through them.

The blood flow through capillaries is the slowest of any of the types of blood vessels. This is important because it allows more time for the exchange of substances through the capillary membranes - more efficient diffusion.

Capillaries eventually join up to form wider blood vessels called venules which connect to veins (next section).

 

Veins

From the capillaries, the venules eventually join up to form veins to carry deoxygenated  blood back to the heart.

Blood flows at a lower pressure in the veins compared to the blood pressure in the arteries and contain valves to ensure the blood flows in the right direction (diagram below).

The adaptation of a larger diameter lumen offers the least flow resistance for the returning blood in the veins (to the heart).

Veins return and transport (i) deoxygenated blood from the tissues and organs to the heart AND (ii) convey oxygenated blood to the heart from the lungs and exit the heart via the aorta arteries to the rest of the body.

Veins have non-permeable thinner walls (operating at lower pressure) and periodically have valves to prevent back-flow of blood.

The diagram above shows a cross-section of a vein and the valve system to ensure one-way flow.

If the blood attempts to flow 'backwards', the tissue flap valves close preventing 'back-flow' in the wrong direction - another wonderful adaptation of biological engineering!

Since the pressure in the veins is lower, their walls don't have to be as thick and the lumen is bigger in cross-sectional area.

The bigger lumen allows good blood flow despite the lower pressure, but bits of tissue act as one-way valves - best appreciated in the diagram above.

If the blood attempts to go in the 'wrong direction' the 'flaps' of the valve close together and stop any reverse flow happening.

 

The relative diameter, cross-section areas and flow rates of blood vessels

The larger the cross-sectional area of a blood vessel the average velocity of the blood decreases.

Therefore, blood flows more slowly through capillaries than arteries or veins.

However, although capillaries are relatively small in average diameter, their total cross-sectional area is very large, so all the large blood flow from the arteries is dispersed through them.

The slow flow through capillaries is important because it allows more time for the exchange of substances through the capillary membranes.

It also means that the mean blood pressure is highest in the arteries because they are directly connected to the heart (closest).

In fact the total cross-sectional area of the capillaries is greater than that of the arteries which actually causes a fall in blood pressure.

 

Summary note on 'connections'

Strictly speaking, arteries and veins do not connect directly with capillaries. Arteries branch into arterioles which have a much smaller diameter. Arterioles then branch out into the even smaller capillaries. The capillaries then connect to venules which join together to form veins.

Initially the blood vessels get narrower conveying fresh blood, nutrients and oxygen and then get larger conveying deoxygenated blood and waste products away from the tissues.


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4. What's in the blood?

Blood is a tissue - it is a mixture of cells (e.g. red and white), solutes (dissolved substances) and a liquid (mainly water).

The liquid is called plasma and is straw coloured when separated from the red blood cells.

Know the structure and function of the following parts of the blood.

Red blood cells

Red blood cells carry oxygen from the lungs to every living cell of the body via the blood stream.

It's the red blood cells that give blood its red colour and unlike white blood cells, they have no nucleus.

Red blood cells are a squashed from both sides 'doughnut' shape (biconcave disc) to give them a large surface area for the iron containing haemoglobin molecules to capture the oxygen.

The large surface area gives efficient absorption of oxygen to combine with the haemoglobin molecules by the reversible reaction:

haemoglobin  +  oxygen  (c) doc b  oxyhaemoglobin (bright red pigment molecule)

The oxygen molecule is weakly bonded to the haemoglobin molecule by a reversible reaction, so it is easily released to be consumed in respiration.

A high oxygen concentration favours the formation of oxyhaemoglobin (reaction moves to right), and low oxygen concentration favours its dissociation to free oxygen (reaction moves to left) - this follows from Le Chatelier's Principle you learning in chemistry about reversible reactions and chemical equilibrium. See GCSE chemistry notes:

Reversible Reactions and Reversible reactions and chemical equilibrium

 

The red blood cells do not have a nucleus, allowing more space for haemoglobin molecules.

Note the four red blood cell adaptations:

(i) the haemoglobin molecule is adapted to carry the oxygen molecule O2 for cellular respiration in all the body's tissues,

(ii) the biconcave disc shape gives a large surface area / volume ratio for absorbing oxygen molecules - increases efficiency of diffusion of oxygen in and out of cells and reduces distance to centre of cell,

(iii) no nucleus - extra space for oxygen carrying haemoglobin molecules - nucleus not required,

(iv) red blood cells are very small and flexible and can easily pass through tiny capillaries.

In the lungs the oxygen molecule attaches itself to the iron atom at the centre of the large complex haemoglobin molecule to form oxyhaemoglobin.

The process is reversed in the body tissues to release oxygen for cell respiration (equation above).

If we don't have enough iron in our diet we can suffer from anaemia, a potentially serious condition where the blood can't carry enough oxygen needed for all the respiration demands of the cells.

White blood cells

White blood cells have a nucleus and are part of the body's immune system to fight diseases.

Some white cells can change shape to engulf and destroy potentially harmful microorganisms.

These cells are called phagocytes and their action is called phagocytosis.

Other types of white blood cells called lymphocytes and have variety of functions.

Type B lymphocytes produce antibodies (type of protein) to inhibit and fight the action of invasive microorganisms e.g. harmful bacteria.

Some white blood cells produce antitoxins to counteract the effect of toxins produced by microorganisms.

When your body is subjected to an infection, your white cells multiply to fight it - giving you a high white blood cell count.

Measurement of your white blood cell count is an important diagnostic indicator of the state of your body's defences.

Having a low white blood cell count increases your risk of infection, but having a very high white blood cell count may mean you have an infection or even a more serious condition like a blood cancer e.g. leukaemia.

Platelets

Platelets do not have a nucleus.

Blood platelets are ever present small fragments of cells that help to clot blood and cover over an open wound in tissues e.g. cut in the skin.

This reduces blood loss from bleeding AND prevents potentially harmful bacteria (microorganisms) getting into your body via the blood system.

If this didn't happen, blood would just keep pouring out of your body!

If your platelet concentration is low you may suffer from excessive bleeding (blood loss) and bruising.

Plasma

Blood plasma is the straw coloured (pale yellow) liquid fluid that carries everything in the blood, so its the major transportation fluid in the body.

Plasma looks straw coloured when separated from the red blood cells.

The constituents of blood can be separated in a centrifuge - high speed rotation of a container (of liquid mixture), that separates substances out by sedimentation, with the most dense material  moving the most outwards in the radial direction.

About 55% of blood is plasma, which itself is 90% water.

Blood plasma carries everything needed for every cell in the body:

(i) red cells (oxygen), white cells (and the antibodies and antitoxins they produce) and platelets,

(ii) nutrients e.g. amino acid and sugar products from the digestive system - the soluble products of digestion from the gut,

(iii) waste products (carbon dioxide from the organs to the lungs, urea from the liver to the kidneys where it is removed in urine),

(iv) proteins for enzymes and tissue growth

(v) control system hormones ('chemical messengers') from the glands to their target and activate the functions of organs.


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5. Cardiovascular disease (CVD), heart attacks, strokes and treatments

Cardiovascular disease is a medical term to describe diseases of the heart or blood vessels.

CVD is usually associated with the narrowing and blocking of the blood vessels (arteries) that convey oxygenated blood away from the heart.

Arteries become narrowed due to the build up of fatty deposits on the lining of damaged artery walls - often the damage is caused by high blood pressure. The fatty deposits can also cause blood clots.

One such example is coronary heart disease.

Heart disease is one of the major causes of death in the UK and other developed countries.

Cholesterol is made in the liver and transported in the blood. Cholesterol is important and we need a small amount of blood cholesterol because the body uses it to: build the structure of cell membranes, make hormones like oestrogen, testosterone and adrenal hormones. Cholesterol also helps your metabolism work efficiently is essential for your body to produce vitamin D.

However, high levels of cholesterol are associated with heart disease.

Smoking is also very bad for you, and not just for causing lung cancer.

When breathed in, the chemicals in tobacco smoke harm your blood cells.

They also can damage the function of your heart and the structure and function of your blood vessels.

This damage increases your risk of atherosclerosis - a disease in which a waxy substance called plaque builds up in the arteries.

 

Coronary heart disease

Coronary heart disease is when the coronary arteries that supply blood to heart muscle become blocked with layers of cholesterol, fats and other materials deposited on the inside of artery walls.

This effectively narrows the arteries restricting the blood flow.

This decreased blood flow means less oxygen is carried to the heart muscles.

A lack of oxygen to work the heart muscles can lead to a heart attack.

There is also an increased risk of blood clots forming - these can block the flow of blood completely, greatly increasing the chance of a heart attack.

 

Strokes

A stroke occurs when a narrowed artery that supplies blood to the brain, becomes blocked by a blood clot.

This deprives the brain of oxygen leading to a life-threatening situation..

Without rapid treatment, the results can be disastrous - death, injury to the brain (e.g. memory) and central nervous system (paralysis and physical coordination).

Recovery can be prolonged and often never complete.

 

Treating cardiovascular diseases

If the heart or blood vessels are severely damaged major surgery might be required.

Surgical techniques and other procedures are described in a later section.

Use of Stents and bypass surgery

Coronary heart disease can be treated with stents - see diagram below.

Stents are very thin-walled wire mesh tubes inserted into arteries to widen them and keep them open to ensure a good supply of blood to the heart muscles.

A good blood flow keeps the heart beating regularly and so keeps you alive, particularly for people suffering from cardiovascular disease!

1. A normal artery, no hardened fat deposits (plaque) on the inner surface of the artery wall, so there is no restriction of blood flow away from the heart.

2. Fatty deposits (plaque) can build up on the inside wall of the artery restricting blood flow.

The cross-section of the lumen is reduced e.g. the equivalent of narrowing a 'pipe' and reducing blood flow - reducing the rate of oxygen transfer to heart muscles.

Cholesterol is a fatty substance the body needs to build cell membranes.

However, too much of fatty substances, including 'bad' LDL cholesterol, in your blood stream can lead to an excessive and dangerous build up of fatty deposits on the walls of the arteries.

The fatty deposits harden over time to form lots of atheromas which restricts blood flow.

These bits of atheromas damage the blood vessel and if one breaks off it can cause a blood clot.

This damage to the arteries causes high blood pressure with increased risk of heart attacks, angina and strokes.

Thick fat deposits can block a blood vessel or cause blood clots to form which can block the flow of blood completely.

If this involves an artery supply blood to the heart muscle, the heart muscle is deprived of oxygen and glucose and can cause a heart attack.

There is less energy available for the heart muscles to contract.

Such blockages can also deprive the brain of oxygen causing a stroke.

3. With the help of a catheter, whose position is monitored by X-rays, a very thin-walled tubular bare metal stent is inserted that pushes against the inside artery wall. The stent squashes the fat deposits and so widening the artery to allow normal blood flow - increases the cross-section area of the artery, so increasing flow rate - hopefully bringing the oxygen supply to the heart back to normal!

By allowing normal blood flow rate, stents are a very effective way of lowering the risk of a heart attack or a stroke for people suffering from coronary heart disease.

After the operation, recovery time is short and the stents have a long working life of many years.

Repeat surgery might be necessary if the artery narrows again.

However, such procedures are not without risk -

(i) there may be complications during the operation (including, ironically, a heart attack),

(ii) there is always a risk of infection with any surgical procedure, and,

(iii) a risk of a blood clot developing in the patient near the stent - known as a thrombosis. This restricts blood flow and possibly stopping the flow - not good!

NOTE An alternative stent

 A drug-eluting stent (DES) is a peripheral or coronary stent (a scaffold) placed into narrowed, diseased peripheral or coronary arteries that slowly releases a drug to block cell proliferation.

This prevents the artery from getting narrowed by the growth of tissue cells.

4. Bypass surgery

There is little risk in having stents implanted, but the fatty deposits can build up again.

However, if the coronary artery is too badly damaged, bypass surgery must be used.

The blood vessels may be so badly blocked that stents won't work.

In this situation, the blood flow to the heart can be improved with coronary bypass surgery.

A healthy blood vessel (e.g. a vein in the leg) is transplanted and surgically inserted and connected to bypass the blocked damage artery.

Bypass surgery has the advantage of no rejection, but still carries risks of any major surgery.

It is a more invasive procedure than having stents fitted, requires a longer hospital stay and requires a longer recovery time.

There is a lower chance that repeat bypass surgery would be necessary, whereas with stents, this may be needed if the artery narrows again.

 

Cholesterol and the use of statins

Cholesterol is an important lipid that is essential for the functioning of your body.

BUT, there are two types of cholesterol:

(i) Too much of LDL cholesterol ('bad cholesterol') can cause health issues.

Too much LDL cholesterol in your bloodstream contributes to fatty deposits on the inner walls of the arteries, restricting blood flow, causing coronary hear disease (see diagram in stents section).

(ii) 'Good cholesterol', known as HDL cholesterol, helps remove other forms of cholesterol from your bloodstream.

The ratio of LDL to HDL seems to be an important factor to have healthy levels of the different types of cholesterol.

Statins are drugs used to treat to high a level of bad cholesterol

Statins slow down the rate at which fatty deposits are formed on artery walls.

For some patients, they are now considered as life-long medication.

Advantages of statins

(i) Statins have been clinically proven to reduce the level of 'bad cholesterol' (LDL cholesterol) in the bloodstream. This in turns slows down the rate at which fat is deposited on the walls of the arteries. This reduces the risk of strokes, heart attacks and suffering from angina and coronary heart disease and.

Angina is chest pain caused by reduced blood flow to the heart muscles.

(ii) Statins can increase the amount of 'good cholesterol' (HDL cholesterol) in the bloodstream as well as reducing the level of 'bad cholesterol' (LDL cholesterol) - all helping the benefits described above.

(iii) There has been some evidence that statins may prevent other diseases.

I can't find out anything about (iii), but research references to Alzheimer's disease and motor neurone disease

Disadvantages of statins

(i) There are adverse side effects from taking statins e.g. headaches,  diarrhoea and aching muscle fatigue and damage. In extreme cases you can suffer kidney failure, liver damage and memory loss.

(ii) The artery can age and narrow over time because stints can cause irritation and make scar tissue grow, therefore a CVD patient may have to take extra medication to stop blood clots forming on the stent.

(iii) Statins are a drug that must be taken regularly over a long period of time and the patient should ensure they don't forget to take their medication.

(iv) It takes some time for statins to take effect in reducing your cholesterol level, you need to be a patient-patient!

 

Other drugs used to treat cardiovascular disease conditions

Anticoagulants - advantages and disadvantages

Anticoagulant drugs like Warfarin are prescribed to reduce the risk of blood clots forming with patients suffering from cardiovascular disease.

Anticoagulants allow the blood to flow more freely to vital organs and reduce the risks of heart attacks and strokes.

BUT, a disadvantage, there is a risk of excessive bleeding if you cut yourself in an accident.

If the wound is severe, there is the danger of losing a significant amount of blood leading to a life threatening situation. There is also a danger, that the injury, combined with taking an anticoagulant, that internal bleeding occurs.

Antihypertensives - advantages and disadvantages

Antihypertensives are medications prescribed to reduce blood pressure. They help prevent damage to blood vessel due to excess higher blood pressure and reduce the risk of excessive fatty deposits forming on the arteries.

There are different types of antihypertensives, that help reduce blood pressure in different ways e.g.

ACE inhibitors relax the blood vessels to ease blood flow.

Beta blockers cause the heart to beat more slowly and less forcefully.

Diuretics help remove excess water.

However, they can have side effects - dizziness, fainting, headaches, feeling sick - looking at the information on my own blood pressure tablets, there is quite a long list of possible side-effects !!

 

Other possible CVD treatments

Tissue replacement

A section of blocked blood vessel can be replaced by taking a section of healthy blood vessel and bypassing the affected section.

This is called coronary bypass surgery.

 

Alternatives to statins and other CVD medications - lifestyle 'action'

You can make lifestyle changes to reduce your risk of developing cardiovascular disease (CVD) AND such changes would be advised as part of your treatment if you already suffer from CVD - to reduce the risk of further heart attacks or strokes.

Lifestyle changes that can reduce your cholesterol level and  cardiovascular disease risk include:

Eating a healthy balanced diet - low in saturated fat, plenty of wholemeal grains (cereal or bread). fruit and vegetables etc.

Saturated fat can increase blood cholesterol levels,

Exercising regularly - doesn't have to be excessive - at 74 in 2020, I do, weather permitting), a brisk walk every day for a minimum of an hour! (now the regular Covid-19 pandemic routine!)

Maintaining a healthy weight - lose excess weight with diet and exercise.

Limiting the amount of alcohol you drink, stop smoking - not good for lungs, therefore not good for your vital oxygen circulation!

Just like type 2 diabetes, you can do a lot for yourself!

AND there are NO NEGATIVE SIDE-EFFECTS !!!!!


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6. More on scientific developments to help treat cardiovascular disease and emergency situations!

Appreciate that modern developments in biomedical and technological research enable us to help when the circulatory system is not working well.

Surgical techniques

Transplanted donated heart or an artificial heart

In the case of a total patient heart failure and severe cardiovascular disease, surgeons may perform a heart transplant or even a combined heart and lung transplant if the lungs are diseased to.

The donor organs must come from other people who have recently died.

However, donor organs might not be immediately available, or they are not for some other medical reason not the best option, doctors may fit an artificial heart.

Even after the heart transplant operation, the new heart does not always start pumping the blood immediately - stimulation might be required.

Also, the patient has to take suppressive drugs (immunosuppressant drugs) to stop the body rejecting the 'foreign' tissue of the new heart (rejection) - these drugs can have side effects and the patient more vulnerable to infection.

Any major surgery like a hear transplant operation carries its own risk of bleeding, blood clots and infection from microorganisms including some potentially fatal bacterial infections like MRSA.

A transplant can greatly improve the quality of life (a factor that applies to all these surgical techniques), BUT the disadvantages are:

it requires major risky intrusive surgery - unforeseen complications or infection,

patients must take immunosuppressive drugs for the rest of their life,

the application of anti-rejection drugs - may cause a greater infection risk because the body's natural immune defence system is suppressed,

and, sadly, a shortage of donors - does require the death of somebody and permission to use their heart.

BUT, we are balancing risks versus survival.

 

Artificial hearts are machines inserted by surgery into a patients chest to pump the blood around the circulatory system, ideally temporarily, until a donor heart can be found.

These mechanical devices are able to pump blood around a person who's heart has failed.

They are usually a temporary fix to keep the patient alive until a donor heart can be found.

They can also be used to help a patient recover while the heart is rested and healing.

Artificial hearts can be permanent solution, with the advantage of reducing the need for a donor heart.

Artificial hearts have an advantage of being much less likely to be rejected by the body, as donor hearts can be, because the immune system doesn't recognise the plastic or metal parts as an invasive ('foreign') microorganism to be attacked - like it might with living tissue.

However, any major surgery in fitting an artificial heart or transplant carries risks eg from bleeding, infection.

Also, artificial heart machines are subject to wear and tear themselves and are not as efficient as a real heart and there are still risks from heart attacks and strokes.

Parts of the heart machine can wear out or the electric motor fail.

The blood flow is not as smooth causing blood clots leading to strokes.

Heart patients have to take drugs to thin their blood to make sure this doesn't happen.

Unfortunately, such drugs cause problems if the patient has an accident and excessive bleeding occurs.

I'm afraid it just another case of medical treatment where you are balancing risks ('bad' outcomes') versus 'good' outcomes!

 

Surgical techniques - heart valve replacement

Heart valves can be damaged or weakened by heart attacks, infection or just old age - you can't stop aging processes!

The damage may cause the valve tissue to stiffen and prevent the valve from opening and closing properly.

If the valve leaks, blood can flow in the wrong direction instead of always going forward in the right direction.

This results in poor circulation of blood, which causes its own problems.

Tiredness and lack of energy and breathlessness are symptoms of leaky valves.

Poor circulation can cause pain in the legs, feet, arms, and hands.

Cold hands and feet may ache or throb.

When the blood does not circulate correctly, oxygen and nutrients cannot reach tissues effectively, which can result in stiffness and cramping.

Badly damaged valves can be replaced with artificial valves or other animal biological valves.

Replacement valves can be taken from other humans or animals such as cows or pigs.

Defective heart valves can be replaced by m an-made artificial mechanical valves that work in the same mechanical way as a real heart and the surgery is much simpler and less risky, but still risks of blood clot problems.

Replacing a valve is a less complicated surgery, so less drastic, and therefore less risky, than doing a full heart transplant operation.

Advantages of artificial valves:

No rejection

Disadvantages of artificial valves:

They can damage red blood cells. Patients need anti-clotting agents because of blood clot risk.

Advantages of biological valves:

Red blood cells not damaged.

Disadvantages of biological valves:

Biological valves can harden and need replacing.

 

Artificial pacemakers

For some patients the problem is an inability of their body to control the heart rate.

The steady beat of the contraction and relaxation of the heart muscles is obviously important.

Artificial pacemakers can be fitted under the skin and a wire connects it to a vein to the right atrium.

The pacemaker sends electrical impulses to stimulate and control the heartbeat.

Advantage of artificial pacemakers:

No major surgery is required.

Disadvantage of artificial pacemakers:

Our immune system may reject the pacemaker ('foreign' materials) and may need replacing.

 

Use of artificial blood

Blood loss from a serious accident can lead to death indirectly - loss of available blood means less oxygen and nutrients are getting the cells of all the tissues and organs - not good!

If you can keep the volume of blood 'topped up' life can be preserved for sometime.

Therefore, artificial blood can be used temporarily as a blood substitute when a patient has lost a lot of blood, but, as long as the heart can still pump the diluted fluid (diluted plasma) containing the remaining red blood cells around the circulatory system.

The simplest artificial blood (blood substitute) is a saline solution - an aqueous sodium chloride salt solution NaCl(aq).

Its safe to use, but must NOT contain air bubbles, and it can keep people alive even if they have lost 2/3 rds of their red blood cells!

This can now give enough time e.g. to get the patient to hospital and prepared for surgery if necessary and also time for the body to produce more red blood cells.

This can be followed up by a blood transfusion if the patient cannot make enough red blood cells in time!

Research is being done to develop artificial blood containing molecules that can carry oxygen just like haemoglobin in red blood cells, but limited progress so far.

This is an ideal solution, to produce a product that can temporarily act like the haemoglobin molecules to transport oxygen to the bodies cells - a sort of haemoglobin replacement.

This would avoid the need for a blood transfusion, and presumably the body would gradually produce enough red blood cells to allow the circulatory systems to work normally.

 


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Possible practical work help develop your skills and understanding the human circulatory system:

dissection of the heart

use software simulations of the work of the heart and blood vessels

observation of arteries and veins from slides

observation of blood smears

observation of valves in veins preventing backflow of blood using the ‘athletic’ arm / prominent vein

using sensors to measure blood pressure before, during and after exercise


Typical learning objectives for blood and blood system

  • Knowledge of the names of the blood vessels associated with the heart   ...

    • arteries are blood vessels that carry deoxygenated blood away from the heart to the lungs,

    • veins are blood vessels that carry oxygenated blood to the heart from the lungs and exit the heart via the aorta to the rest of the body

    • capillaries are the smallest blood vessels

    • aorta - the oxygenated blood flow exit to the organs

    • vena cava - the deoxygenated blood flow entry from the organs

    • pulmonary artery - carries the deoxygenated blood to the lungs

    • pulmonary vein - brings the oxygenated blood to the heart from the lungs

  • Know and understand that arteries have thick walls containing muscle and elastic fibres.

    • The arteries carry blood under high pressure, so their walls need to be thick and strong with muscle tissue-fibre but these blood vessels must be elastic too,

    • Know that veins have thinner walls (operating at lower pressure) and periodically have valves to prevent back-flow of blood.

  • Know and understand that if arteries begin to narrow and restrict blood flow stents are used to keep them open.

    • You should understand the importance of stents, particularly with reference to the coronary arteries.

    • Stents are narrow tubes capable of supplying a blood flow equivalent to an artery.

    • Fatty deposits building up on the inner surface of the arteries that supply blood to the heart muscles can cause coronary heart disease.

    • These fatty deposits restrict the blood flow increasing blood pressure and causing heart attacks.

    • To combat this potential fatal condition you can insert stents into the arteries to increase blood flow to the heart muscles.

    • However, there are some drawbacks eg the stents can irritate the artery lining and cause the growth of scar tissue and drugs must be taken to avoid blood clots forming in the stent itself.

  • Know that in the organs, blood flows through very narrow, thin-walled blood vessels called capillaries which branch out from the arteries.

    • Know that substances needed by the cells in body tissues pass out of the blood, and substances produced by the cells pass into the blood, through the walls of the capillaries.

    • The capillaries are the smallest blood vessels, and these fine blood tubes carrying blood are dispersed in all the tissues of every organ.

    • Capillaries are the main interface for material exchange eg of sugars, amino acids and other nutrients, oxygen and waste carbon dioxide.

    • Some capillaries are so fine that blood cells cannot pass through them.

    • The walls of capillaries are thin with permeable walls only one cell thick, allowing rapid diffusion of substances into the cells (sugars, minerals, amino acids, oxygen) or out of surrounding cells (carbon dioxide, urea and other waste products).

    • Capillaries eventually join up to form wider blood vessels called veins. The blood flows at a lower pressure in the veins compared to the blood pressure in the arteries and contain valves to ensure the blood flows in the right direction.

  • Know and understand that blood is a tissue and consists of a fluid called plasma in which red blood cells, white blood cells, and platelets are suspended.

    • Blood is considered an irregular tissue because it doesn't physically support or connect things but it does involve groups of cells performing particular functions.

    • Plasma is pale straw-coloured liquid which transports everything around the body

    • What is the function of blood plasma? .... What does it do? ...

  •  Know and understand that blood plasma transports:

    • carbon dioxide from the organs to the lungs,

    • soluble products of digestion from the small intestine to other organs eg glucose, amino acids, mineral salt ions etc.

    • urea from the liver to the kidney prior to excretion in urine,

    • hormones which control the function of various organs and their associated chemical processes in the body,

    • and obviously, as already mentioned, red blood cells, white blood cells and platelets are constantly being carried around the body as well associated oxygen as oxyhaemoglobin and antibodies and antitoxins produced by the white blood cells.

  • Know and understand that red blood cells transport oxygen from the lungs to the organs.

    • What do red blood cells do? What is their function? Know that red blood cells have no nucleus but a large surface area to chemically absorb oxygen.

    • Know that red blood cells are packed with a red pigment called haemoglobin which readily combines with oxygen

    • Know that in the lungs haemoglobin combines with oxygen to form oxyhaemoglobin.

    • Know that in other organs and all body tissue oxyhaemoglobin splits up into haemoglobin and oxygen for cell respiration.

  • Know that white blood cells have a nucleus. What do white blood cells do? What is their function?

    • Know that white blood cells form part of the body’s defence system against microorganisms eg harmful bacteria.

    • White blood cells can attack and destroy harmful 'foreign' microorganisms.

    • White blood cells can produce antibodies to fight microorganisms.

    • White blood cells produce antitoxins to combat the effect of waste toxins produced by microorganisms.

  • Know that platelets are small fragments of cells.

    • Know that platelets have no nucleus.

    • Know and understand that platelets help blood to clot at the site of a wound and a lack of platelets is potentially dangerous from excessive bleeding and bruising.

    • The clotting action of platelets allows a skin to form over a wound which hardens into a scab and this prevents infection of the wound by harmful bacteria or any other microorganism.


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