Homeostasis
Monacia Williams, Contributor

Hello students. Here we are sharing another lesson together again. How was your week? I hope it was good and productive.

Last week we finished looking at the kidney but towards the end I introduced a word that takes us right into this week's lesson. Can you guess what word it was? Of course you can. It was osmoregulation! Remember what it means? You don't? Look it up! Okay, I will help. It is the regulation or control of the water balance within an organism by osmosis.

What then is homeostasis? Homeostasis is the means by which constant conditions in the body are maintained despite changes in the surroundings. These changes can be external to the organism or they can be internal. Homeostasis is controlled by self-regulatory processes in the body. The body has mechanisms which can detect too little or too much of a particular substance. Too much of a substance and the body initiates a mechanism to get rid of it; too little and the body initiates a mechanism to increase it. This response by the body is known as a negative feedback mechanism. Examples of negative feedback mechanism include:

• Control of water balance
• Control of blood sugar
• Control of body temperature

The mechanism consists of three main parts:

• A sense organ
• A responding organ
• A control centre

In the control of water, the sense organ detects the concentration of the blood. Here, the osmoreceptors in the hypothalamus of the brain represent the sense organ and the controlling centre. The hypothalamus stimulates the pituitary to secrete or not secrete antidiuretic hormone - ADH. The walls of the distal convoluted tubules of the nephron respond to ADH, hence the kidneys would be the responding organs.

Look at the diagram below:

This diagram can be used as a template for all the body's homeostatic functions. First, let us try it using what we are familiar with; of course, the regulation of the water balance! We will begin from the normal level. If level of water in the blood drops below the normal due to:

• excessive sweating
• ingesting large amounts of salt or salty foods
• drinking too little water, then the corrective mechanism kicks in. We have already gone through this. The hypothalamus stimulates the pituitary and the pituitary increases the amount of ADH produced so a lot of water is reabsorbed by the distal convoluted tubule. This re-absorption of water helps to reduce the concentration of the blood, bringing it back to normal. The individual will produce a small amount of concentrated urine.

If the level of water in the blood rises above normal due to:

• too little sweating, for example, on a cold day
• drinking large amounts of water
• low salt intake in the diet, again, the corrective mechanism will set in. The hypothalamus detects this and stimulates the pituitary to decrease the production of ADH, hence decreasing the amount of water reabsorbed by the distal convoluted tubule, hence raising the concentration of the blood. This causes the individual to produce a large amount of dilute urine.

Let's see if we can apply another homeostatic function to our diagram - control of blood sugar. Blood sugar level is also under hormonal control.

This time, however, two hormones are involved: insulin and glucagon. Both hormones are produced by cells in the islets of Langerhans of the pancreas. The beta cells produce insulin and the alpha cells produce glucagon. The level of glucose that is normal for the body is 90mg/100cm3 of blood. This is known as the set point, levels above and below this limit result in corrective action. Diabetes occurs when blood sugar levels increase above 169mg/100cm3 of blood and the body can no longer respond with the required corrective actions.

If the blood sugar falls below the set point, the body responds by stimulating the alpha cells of the islets to produce glucagon. Glucagon stimulates the conversion of glycogen in the liver or the muscles to glucose, releasing it into the blood. This increases the level in the blood, bringing it back to normal. If the level rises above the set point, for example, after a meal, the beta cells are stimulated to produce insulin which stimulates the liver to convert the excess glucose to glycogen, reducing the amount in the blood, bringing back the level to normal.

Diabetes is a debilitating illness which can lead to kidney failure. This means that the kidneys are not able to carry out their function and this has to be done by a dialysis machine. You can read up on how the machine works in your text; you need to! Next week we will look at the other regulatory function - that of controlling body temperature. See you then!

Monacia Williams teaches at Glenmuir High School. Send questions and comments to kerry-ann.hepburn@gleanerjm.com