Site author Richard Steane
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Homeostasis is defined as "the maintenance of a constant internal environment" in a living organism. Every organism carries out some form of regulation, although it may somehow find a place to live where it can tolerate variation in the external environment.

In an advanced animal such as Man (and other mammals), this involves a series of automatic control mechanisms which keep conditions more or less stable for all the cells of the body, thus providing exactly what each one requires. These conditions are adjusted by negative feedback mechanisms (see later).
The same applies in embryonic development (whether inside an egg or the uterus), so that animals usually take steps to ensure that conditions are held constant as far as possible.

The requirements of the cells of the body include:
Supply of raw materials for respiration and growth:
e.g. oxygen,glucose and other "foods"
Removal of waste substances:
e.g. carbon dioxide, urea
Suitable physical conditions:
e.g. solute concentration (salt/water balance), pH, temperature
Hygienic conditions:
absence of infecting micro-organisms (bacteria, viruses, etc.)

All of these are provided by means of 2 liquids with close affinities with blood, which bathe all the cells of the body: tissue fluid and lymph.

By reference to previous work, you should be able to say how the supply of oxygen and glucose via the blood is kept fairly constant (see the human respiratory system, digestive system and absorption and the liver, as well as the more recent work on the functions of blood and the heart).

We shall now concentrate on the kidneys, which carry out 2 processes which can also be seen as aspects of homeostasis:
- excretion.
removal of waste substances [formed in the cells of the body].
- osmoregulation
control of salt/water balance [in the blood] - so as to provide a constant osmotic environment around the body cells.
Incidentally, which waste substance is the body's main waste, formed in every cell of the body?
> carbon dioxide CO2
Which organ excretes it?
> lung(s)

The liver and the production of urea

amino acid - molecular structure urea - molecular structure Urea is a nitrogen-containing substance which is produced in the liver, in order to deal with excess amino-acids in the body, which is a natural consequence of the fact that we almost always eat more protein than our body needs to assimilate.
The process is called deamination. The amino (-NH2) groups are removed and combined into the fairly harmless soluble compound urea which is sometimes called a nitrogenous excretory product.

The urea is then removed from the body by the kidneys, in a separate process. As urea is produced, it leaves the liver in the bloodstream and passes via the circulatory system to all parts of the body. If it builds up, it may have the effect of a poison on the body. However, the kidneys are continuously removing urea from the blood and producing urine as a result.

It is sometimes said that the kidneys "clean" the blood by filtering it, but this is an over-simplification, because filtration would only take out solids. Also, urea is not a "dirty" substance in itself. For that matter, nor is fresh urine - it usually only becomes rank when bacteria grow in it. Urea is only one of the waste substances that the kidneys get rid of, and there are other excretory organs besides the kidneys. Faeces is not really an excretory product because it was not taken into the cells of the body.

List (in order) as many structures (blood vessels, organs, etc) as possible through which the blood containing urea passes on its way between the liver and the kidneys.
liver > hepatic vein, vena cava, heart (rhs)/right atrium, right ventricle,pulmonary artery, lungs, pulmonary vein, heart (lhs)/left atrium, left ventricle, aorta, renal arteries kidneys

The kidneys and the urinary system

Many more kidneys and
other anatomical models
available online
via this link
The adjective associated with the kidney is "renal".
What would be the names of the following blood vessels?
- leading to the kidneys > renal artery
- leading from the kidneys > renal vein

Can you give the full names for the 2 vertical blood vessels X and Y below (of which only parts are shown)?
X (blue) > vena cava
Y (red) > aorta

Use the mouse, or tap the screen, to label the following structures on this generalised diagram of the urinary system.
renal artery
renal vein
The urethra carries urine to the outside of the body. It passes through the penis in males, and opens in front of the vagina in females.

Nephrons as kidney processing units

Each kidney contains about a million very fine tubes called nephrons. Each has its own blood supply, so the "dirty" blood is divided between them, and each nephron processes the blood and returns it in a "cleaned" state, so urine is produced as a result of their combined effect.
There are 2 stages to this processing:
Filtration (ultrafiltration), and Selective reabsorption, which happen in different parts of the nephron.
The diagram below shows more detail than some textbooks, but the names of the parts and the following principles of action must be understood.

Use the mouse, or tap the screen, to label the following on the diagram below:
Bowman's capsule (renal capsule),
first and second convoluted tubules,
collecting duct,
arteriole (branch of renal artery),


Filtration - in Bowman's capsule

Most of the simple substances dissolved in the plasma - those with "small" molecules - are forced (by blood pressure) out of the blood which passes into the capillaries of the glomerulus.
What substances are likely to be dissolved in this liquid, called glomerular filtrate?
> glucose, amino-acids, urea, salts (vitamins, hormones) - [dissolved in water]
Note the difference in diameter between the two arterioles a & b. What might be the effect of this?

> pressure buildup
What sort of "things" are likely to be found in blood vessel b leading immediately away from the glomerulus?
> red cells, white cells, plasma proteins

Selective reabsorption

- in the first convoluted tubule

Only useful substances are passed through the wall of the nephron, back into the blood in region c. Depending on the body's requirements, different amounts of some substances will also be reabsorbed.

Name the useful substances that are likely to be reabsorbed.
> glucose, amino-acids, (salts, water)

Later on in sections d and e, adjustments will be made to the water and salts content, but no more glucose will be absorbed. For this reason, sufferers from diabetes will excrete glucose in their urine after a meal containing carbohydrate.

The amount and content of urine varies considerably: normally about 1.5 - 2 litres per day - 2% urea, 1% sodium chloride.
What makes up the rest?
> water
What would cause extra urea to be made?
> consuming extra protein

This leaves unwanted substances to run on through the tubule, and eventually form urine.
What will these substances be?
> urea (& excess salts & water)

Kidneys and osmoregulation

As well as getting rid of urea, the kidneys deal with water and salts in the blood. Depending on the diet, there is likely to be more than the body needs, so the kidneys remove whatever is in excess. In order to keep the correct salt/water balance in the body, the composition of the urine is varied. This may to some extent be linked with fluid loss due to sweating in order to keep constant body temperature - (another aspect of homeostasis).

Blood passing through part of the brain (hypothalamus) is constantly monitored; if its water content falls, then a hormone ADH (a "chemical message") is released into the blood (from the pituitary gland, near to the hypothalamus). The ADH passes via the bloodstream to the kidneys, where it causes more water to be reabsorbed from the nephrons. As a result, more water enters the blood, so that its water content is kept constant, and the urine becomes more concentrated.
What might a person do to cause the water content of their blood to fall?
> eating salty food, not drinking enough, [sweating]

This system is an example of negative feedback. The names of the actual organs, etc., have been inserted in brackets.
Negatve feedback loop What happens if the brain detects a rise in the water content of the blood?
> less ADH from pituitary, less water reabsorbed

If "sugar" is excreted from the body in urine, it is a symptom of diabetes, in which the normal negative feedback mechanism has been upset by a lack of the hormone insulin. As a result, blood sugar level is unstable, and can rise after a meal so much that the kidney cannot reabsorb it all (in the first convoluted tubules), and some glucose escapes from the body.

In the normal condition, the hormone insulin usually causes excess sugars entering the liver from the digestive system to be stored as glycogen. Later, this can be reconverted into glucose under the influence of another hormone called glucagon, and allowed out into the blood stream, as required by the body. These actions also have the effect of keeping the concentration of glucose in the blood steady (another example of homeostasis).

Many other anatomical models and charts available by following this link.

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