Homeostasis - osmoregulation - water content control - urea and ion concentration - kidney function - kidney failure - dialysis

See also homeostasis blood sugar control - diabetes

See also homeostasis - thermoregulation and temperature control

Doc Brown's Biology Revision Notes

Suitable for GCSE/IGCSE/O level Biology/Science courses or equivalent

 This page will help you answer questions such as ...

How does the body control its water content?

 Why is it important for the body to control its water content?



Homeostasis control in the human body

Homeostasis is a word that is sometimes used to describe your bodily functions that try to maintain a stable constant internal environment including the factors listed above.

Know that internal conditions that are controlled in the body include water control, ion and urea concentrations.

Know and understand that water and ion content, body temperature and blood glucose levels must be kept within very narrow ranges.

Know and understand that waste products that have to be removed from the body include:

Carbon dioxide, produced by respiration and removed via the lungs when we breathe out,

urea, produced in the liver by the breakdown of amino acids and removed by the kidneys in the urine, which is temporarily stored in the bladder.

The body's control of water content and ion concentration is called osmoregulation.


The function of the kidneys

The kidneys are an important 'filtration' system for removing unwanted materials and waste products out of the blood, which would otherwise cause harmful effects on the body if not removed.

The kidneys filter out these substances as the blood through - hence the process name of filtration.

Glucose, ions and other useful substances and appropriate amount of water are re-absorbed back into the blood stream - this process is called selective reabsorption.

 

What the kidney removes to maintain the right balance of various substances

The substances removed by the body in urine include:

Controlling Water Content

"You can survive for several weeks without food, but only a few days without water"!

Water is extremely important for a healthy life for the following reasons ....

  1. 65% of our body is water, its in every cell and makes up most of the blood.

  2. It is a versatile solvent - nutrients like glucose, salts, amino acids are all soluble in blood, carbon dioxide and oxygen are exchanged via blood and other waste products like urea are dissolved and removed in urine.

  3. It is essential for enzyme action - water is the medium for their substrates and products.

  4. Water has a specific high heat capacity, that is important for temperature regulation - thermoregulation.

Typical daily human input and output of volumes of water - which must be constantly kept in balance

Volume taken in cm3: drink 1050;  food 600;  from respiration 250

Volume given out cm3: urine 1000;  exhaled air 350;  skin 400;  faeces 150

If the water or ion content of the body is wrong, too much water may move into or out of the cells and damage them.

If your blood becomes too concentrated water is drawn out of the cells and tissues by osmotic action and they become dehydrated. Under these circumstances enzymes cease to work efficiently and so do the cells.

If the blood becomes too dilute the reverse osmotic action happens. Water will collect in the tissues and the cells swell up - again a situation of imbalance.

The water content of the blood is continually influenced by the temperature around us (how much we sweat), our diet and the amount of water we lose (see below).

The water content of the body – our skin keeps us waterproof but water leaves the body via ...

(i) the lungs when we breathe out (you see the condensed water from our breath in the winter),

(ii) the skin when we sweat to cool us down,

(iii) excess water is lost via the kidneys in the urine,

(iv) and in our faeces.

and this 'output' is balanced by the 'input' of water we take in from food and drinks,

BUT, we can't control how much we lose in the ways described above, so we need a balancing system between the amount of water we consume and the amount of water removed by the kidneys in urine.

Details of the feedback system of water control

We need to, and are continually taking in water via drinks and food.

Any loss needs to be replaced, more so in the summer when we sweat more than in the winter.

On cold days you sweat less and pass more pale dilute urine, and on warm days you sweat more and pass more darker coloured more concentrated urine (assuming a similar fluid intake each day), either way the water balance is maintained.

After vigorous exercise (keeping fit or sport) you will have lost water and ions through sweat as well as burning up more glucose than usual.

Sports drinks contain water, ions and sugar to replace those lost but there are many products on the market competing for this lucrative revenue stream and each claiming to be just the right drink to take!

I gather from a TV program using a cheap, 'to dilute' fruit juice drink with a spoonful of sugar in, is just as effective - but my doctorate is in chemistry!

See also on this page ADH and the hormone controlled concentration of the blood and urine

 

Controlling ion concentrations

The ions essential for life are absorbed into the blood stream in the gut from digested food.

If the water or ion content of the body is wrong, too much water may move into or out of the cells and damage them.

The ion content of the body – ions are lost via the skin when we sweat (you can taste it on your skin) and excess ions are lost via the kidneys in the urine.

For example, the kidney controls the sodium ion (Na+) concentration from digested food containing salt.

Any excess of any ion needs to be removed e.g. by the kidneys and subsequent excretion of urine.

Water and ions, such as sodium (Na+) enter the body when we eat and drink and are then absorbed into the bloodstream.

Any imbalance in the ion concentrations of the cells will result in the wrong quantity of water being absorbed by osmosis and this reduces the correct functioning of the cell.

It is the kidney maintains the ion concentration balance in your blood.

If the ion or water content of the body is not right it can upset the balance between ions and water. It can result in too little or too much water being absorbed into the cells by osmosis. The wrong amount of water in cells damages them inhibiting their normal function.

Ions are lost in sweat, but this is not a regulated system, which is why the correct balance of ions is maintained by the kidneys.

The appropriate amount of ions is reabsorbed into the blood after filtration in the kidney and any excess ions removed from the body in the urine.

 

Controlling waste products - urea and kidney function

Humans need to remove waste products from their bodies to keep their internal environment relatively constant.

Know and understand that waste products that have to be removed from the body include urea, produced in the liver by the breakdown of amino acids and removed by the kidneys in the urine, which is temporarily stored in the bladder.

We take in protein in our food e.g. meat and cheese, which is broken down into amino acids (H2NCHRCOOH, R is variable). The amino acids are then converted (via DNA/RNA coding) into the proteins we actually need. Unfortunately, we cannot store amino acids in our body, so the excess must be dealt with in some way. The amino acids can be converted into fats and carbohydrates (C, H and O compounds) which can be stored in the body, but neither of these groups of molecules contain nitrogen. This conversion occurs in the liver and involves a process called deamination. Toxic ammonia (NH3) is produced as a waste product of the process and this must be converted by the liver into non-toxic urea, formula O=C(NH2)2. The urea is soluble and transported to the kidneys, where it can be filtered out and excreted from your body in urine.

The kidney has three primary functions in terms of cleaning the blood of waste products and maintaining the correct concentrations of various substances.

(i) The kidney removes urea, formed from the waste of excess amino acids.

The body can't store proteins, so excess amino acids are converted into storable fats and carbohydrates and the waste product of the process is urea.

Urea is a poisonous substance so on its release from the liver into the bloodstream it is filtered out by the kidneys before storage in the bladder prior to excretion in urine.

(ii) The kidney adjusts the water content of the blood e.g. to avoid you becoming dehydrated.

(iii) The kidney also adjusts the concentration of ions like sodium (Na+), potassium (K+) and chloride (Cl) in the blood.

A healthy kidney produces urine by:

first filtering the blood, excess ions such as sodium are removed, though some ions are lost in sweat,

reabsorbing all the sugar,

reabsorbing the dissolved ions needed by the body,

reabsorbing as much water as the body needs,

releasing urea, excess ions and water as urine.

 


The hormone controlled concentration of the blood and urine - ADH negative feedback mechanism

Urine volume and concentration is regulated through the same processes that regulate blood volume. Antidiuretic hormone (ADH), produced by the pituitary gland, increases the amount of water reabsorbed in the kidney tubules.

The concentration of urine in your body fluids is controlled by the anti-diuretic hormone (ADH) - which makes the kidney tubules more permeable - so more water can be reabsorbed into the bloodstream

ADH is released into the blood by the pituitary gland in the brain.

The brain's hypothalamus monitors the water content of the blood via osmotic pressure changes - osmoregulation.

If the water content is not right, the brain instructs the pituitary gland to release ADH into the bloodstream depending on how much is needed.

This process is controlled by a negative feedback system so that if the water content of your body gets too high or too low the ADH mechanism is initiated to bring the water levels to normal.

(a) If you drink a lot of water it may dilute the blood too much. If this happens, little, if any ADH is released into the bloodstream. This results in less water being reabsorbed by the kidney and more water passes through the tubules to produce urine.

In passing (if you will pardon the pun), drinking alcohol inhibits the production of ADH so you produce much more urine. Heavy drinking can produce dehydration.

(b) If the blood starts to become too concentrated it makes you feel thirsty and the brain instructs the pituitary gland to release antidiuretic hormone ADH into the bloodstream. ADH stimulates the kidneys to reabsorb water into the blood from the urine. Reducing water loss in the urine helps to conserve water and reduce the likelihood of dehydration.

A description of a body's homeostasis negative feedback system for controlling water-urine content via ADH

(a) The negative feedback in action for too high level of water content in the body e.g. after drinking a large volume of water (b) The negative feedback in action for too low level of water content in the body e.g. hot and thirsty due to high outside temperature or physical exercise
1. A receptor in the brain's hypothalamus detects a stimulus that the water level is too high - blood solute concentrations too low - too dilute - detection is to do with osmotic pressure (osmoregulators). 1. A receptor in the brain's hypothalamus detects a stimulus that the water level is too low - blood solute concentrations too high - too concentrated - detection is to do with osmotic pressure (osmoregulators).
2. The hypothalamus coordination centre in the brain receives and processes the stimulus information and then organises a response by the effector - pituitary gland. 2. The hypothalamus coordination centre in the brain receives and processes the stimulus information and then organises a response by the effector - pituitary gland.
3. The effector, the pituitary gland, responds by releasing none or very little ADH - so less water is reabsorbed by the blood from the kidney tubules. This results in larger volumes of dilute urine - pale yellow colour. 3. The effector, the pituitary gland, responds by releasing more ADH into the blood stream - so more water is reabsorbed into the blood from the kidney tubules . This results in a smaller volume of concentrated urine - dark yellow colour.
4. The pituitary gland will carry on producing less of the ADH - the 'water reducing' response, as long as the coordination centre is stimulated by the receptors are saying ,too much water'. Eventually the correct water balance is reached. 4. The pituitary gland will carry on producing the increased ADH 'water increasing' response as long as the coordination centre is stimulated by the receptors are saying 'too little water'. Eventually the correct water balance is reached.
5. If the pituitary effector response is more than required, and the water content becomes too low, the receptors will detect this, and the negative feedback will stimulate the effectors to increase the water level (1. - 3. on the right). 5. The pituitary effector response is more than required, and the water content becomes too high, the receptors will detect this and the negative feedback will stimulate the effectors to decrease the water level (1. - 3. on the left).
This is all automatically done by the organism's complex control systems and enables the organism e.g. your body, to maintain as near as possible the 'ideal' water content of your body.

 


Problems with malfunctioning kidneys and kidney failure

Urea, the waste product from excess amino acids, is released by the liver and filtered out by the kidneys and excreted via the bladder in urine.

People whose kidneys do not function properly may die because toxic substances accumulate in their blood.

If the kidneys are not working properly-efficiently then potentially toxic substances can build up in your blood stream.

If you lose your ability to control the levels of ions and water in your body it can have fatal consequences.

If kidney function deteriorates to the point of complete kidney failure your life is in danger.

Their lives can be saved by using dialysis machines or having a healthy kidney transplanted.

 


Dialysis - an artificial means of purifying the blood

People who suffer from kidney failure may be treated either by using a kidney dialysis machine or by having a healthy kidney transplanted.

If the kidneys don't function properly then waste products build up in the body's bloodstream and the body's ability to control the water content and ion concentrations is lost, a situation that can be fatal - kidney failure is a bad situation - a major lack of this aspect of homeostasis control.

Though uncommon, kidney failure can be caused by infection, severe poisoning, an injury with severe loss of blood or very high blood pressure

Treatment by dialysis restores the concentrations of dissolved substances in the blood to normal levels and has to be carried out at regular intervals.

In particular, keeping the ion concentrations at the right level and removing waste products is essential and a dialysis machine can perform some of the functions of a real kidney..

In a dialysis machine a person’s blood flows between partially permeable membranes (selectively permeable membranes).

Diagram of kidney dialysis machine procedure - the dialyser connected to a patient's arm - blood is pumped from an artery to the dialysis machine and the 'cleaned' blood pumped back into a vein.

In a dialysis machine there is a selectively permeable membrane (partially permeable) where the patient's blood flows on one side and the dialysis fluid on the other. The membrane allows substances like ions and urea waste to pass through the membrane from the blood to the dialysing fluid, but, does not allow larger molecules like proteins through. This is a similar action to the membranes in the kidney.

The dialysis fluid contains the same concentration of useful substances e.g. ions and glucose as healthy blood to ensure that glucose and useful mineral ions are not lost in the dialysis process.

Waste substances such as urea and excess ions pass out from the blood through the membrane into the dialysis fluid. This will naturally happen by diffusion because the dialysis fluid contains no urea, so, the diffusion gradient is from blood to dialysis solution.

The levels of ions and glucose in the dialysis fluid are set at approximately those required in the patient's blood so that diffusion may occur in either direction to maintain the correct concentrations in the blood.

For people with poorly functioning kidneys, dialysis might be required several times a week with each session lasting several hours (3-4 hours).

Regular dialysis will keep the concentrations of dissolved substances in the blood at the normal levels and remove potentially toxic waste substances.

BUT, dialysis is not without risks:

Dialysis may cause blood clots.

There is an increased risk of blood infection.

It is also an expensive unpleasant experience, even if its free on the NHS.

These risks and unpleasantness must be balanced against giving the patient with kidney failure some time, and therefore a chance, getting a life-saving transplant of a donor organ.

In kidney transplants a diseased kidney is replaced with a healthy one from a donor.

Source of kidney organs

Kidney transplants are one of the most common organ transplant operations done in our modern health service (eg the NHS of the UK).

The only cure for serious kidney disease is to have a kidney transplant, hopefully from a suitable matching donor.

The donor can be a relative donating one of their kidney's or a fatal accident victim e.g. somebody who has died suddenly.

The person who has died must be on the donor organ register or carrying a donor card (providing relatives agree to the organ donation).

Since we have two kidneys, one can be transplanted from a living person to the kidney failure recipient, but with some residual risk to the donor.

Problems

However, the donor kidney may be rejected by the immune system unless precautions are taken.

The patient needs to be treating with drugs to prevent organ rejection, but it may still happen.

There are also problems with availability of kidneys, e.g. a long waiting list and the cheaper dialysis is often used as an interim treatment.

Antigens are proteins on the surface of cells., and a recipient’s antibodies (from white blood cells of the immune system) may attack the antigens on the donor organ as they do not recognise them as part of the recipient’s body and so the transplanted kidney is rejected.

In order to prevent rejection of the transplanted kidney:

A donor kidney with a ‘tissue-type’ similar to that of the recipient is used - which is where a generous relative can mean the difference between life and death.

The recipient is treated with drugs that suppress their immune system response to reject the kidney transplant.

Tests for blood grouping and compatibility tables are part of the procedure to give the transplant the best chance of not being rejected.

Dialysis or transplant?

In the long run, transplants are cheaper than dialysis, but there is a shortage of kidney organs for transplant operations.

There is a long waiting list, but at least dialysis keeps patients with kidney failure alive!


Practical work to help develop your skills and understanding may have included the following:

dissecting and making observations of a kidney

designing a model kidney dialysis machine using Visking tubing as the filter

testing urine from diabetic and non-diabetic people using Clinistix.

Be able to evaluate the advantages and disadvantages of treating kidney failure by dialysis or kidney transplant,

 


 

Homeostasis notes index:

Homeostasis - introduction to how it functions (negative feedback systems explained)

Homeostasis - control of blood sugar level - insulin and diabetes

Homeostasis - osmoregulation, ADH, water control, urea and ion concentrations and kidney function, dialysis

Homeostasis - thermoregulation, control of temperature

 


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