S-Cool Revision Summary

S-Cool Revision Summary

Homeostasis is the way the body maintains a stable internal environment. It is important for the body to have a stable environment for cells to function correctly.

There are several things that need to be regulated:

Body temperature

Amount of water within the body

The amount of glucose in the body

The amount of nitrogenous waste in the body

The vast majority of organisms function between 10-35°C. There are basically two ways to regulate body temperature and we use these to categorise organisms:

  1. Homoiotherms: These are organisms that that regulate their own body temperature internally. Their internal body temperature is independent of the external temperature. (Don't use the term 'warm-blooded').
  2. Poikilotherms: These are organisms that cannot regulate their own body temperature internally. Their internal temperature fluctuates with the external temperature. (Don't use the term 'cold-blooded').

In your brain is there is an area called the hypothalamus. The hypothalamus has a thermo-regulatory centre and this detects the temperature of your blood. You also have thermo-receptors in your skin and these detect the temperature outside.

Controlling the level of water is linked to getting rid of nitrogenous waste so we'll deal with them both together. As mentioned before, nitrogenous waste would be toxic if it accumulated so it must be removed from the body.

This is done in number of steps:

  1. Excess proteins (i.e, nitrogenous waste) are broken down into amino acids.
  2. These then have the nitrogenous part removed as ammonia (see equation 1 below).
  3. Within the liver, the ammonia is converted into urea (see equation 2 below). This process is called deamination.
  4. The urea is then transported in the blood to the kidney (where it is extracted and excreted via the bladder).

Urea, along with salt, water and glucose, etc., is extracted from the blood in the kidney by a process called ultrafiltration. Blood passing the top of the nephron is under high pressure, so fluid is forced through the sieve-like capillaries and into the capsule. This fluid is called the filtrate. It does not contain any blood cells or larger proteins, as they are too big to pass out of the capillaries and into the capsule.

Much of what has been filtered out needs to be returned to the blood - they are too precious to lose - so the next process is called selective reabsorption.

The concentration of the blood (water potential) is monitored by osmoreceptors in the hypothalamus. The higher the concentration of the blood the less water there is in the blood.

If the concentration is too high impulses are sent to the pituitary gland which then releases more ADH. The water levels will be brought back to normal and the impulses stop.

Glucose is needed for respiration so if the level falls below this, the normal body activities may not be able to continue. If the level rises too much the normal behaviour of cells is affected and serious problems can arise. The ideal level of blood glucose is about 1mg/cm3.

Insulin reduces the level of glucose in the blood plasma

Glucagon increases the level of Glucose in the blood plasma

Glucagon promotes the conversion of fatty acids. into glucose. This process is called Gluconeogenisis (remember, neogenesis means new formation).

As well as being involved in the control of blood glucose levels, the liver has other extremely important functions. To be able to fully understand these, the structure of the liver must first be understood.

The liver is made up of numerous lobules which are packed with virtually identical cells called hepatocytes. The liver is supplied with blood flowing in from the hepatic artery (bringing oxygen) and the hepatic portal vein (bringing blood from the gut).

The blood in this vein carries lots of products from digestion for example; glucose, amino acids, lipids, cholesterol, plasma proteins, urea, carbon dioxide.

Liver Functions:

  1. Carbohydrate metabolism: gluconeogenesis, glycolysis and glycogenesis all occur in the hepatocytes.
  2. Deamination: when excess proteins have the NH2 group removed to make ammonia. This is then converted into urea and released into the blood to be taken to the kidney for excretion.

Other functions include:

Fat metabolism

Making cholesterol

Bile production


Storage of Vitamins

Breakdown of haemoglobin (Hb)

Synthesis of plasma proteins