TAGS
4. Water gain:
There are three ways in which your body gets water:
If your body is dehydrated you will feel thirsty. Your hypothalamus has detected that your blood is too concentrated: as well as stimulating the pituitary gland to make ADH, it will stimulate you to drink. However, you will not be able to drink exactly the right amount of water to get you blood back to the exact concentration.
How much water you get out of your food depends upon what it is that you eat and how much you eat. You might end up very fat if you ate something every time that you felt thirsty. So the water content of you food does not help you to control your water content. Marine mammals cannot drink seawater to replace the water lost in their urine. They rely on the water in the food they eat; even so they do not eat more to get their water balance right.
When your cells respire glucose they produce water. How much water is produced depends upon how active you are. This does not help you to balance you water content. Camels can get water out of the fat in their humps by tissue respiration; however this is only temporary. A dehydrated camel will have a floppy hump. It will replace its water by drinking as soon as it can.
5. Water loss:
There are several ways in which your body can lose water:
Mammals are terrestrial animals and they must conserve water. Unfortunately we lose water in some rather uncontrolled ways. We cannot stop breathing even in the desert when it may kill us. Every time we breathe out we lose water. In a cold climate you can see this happening as the water vapour in our breath condenses. Water also evaporates through our skin, we are not 100% waterproof. When we get too hot we start to sweat, as the water in the sweat evaporates, we cool down. We do not do this to get rid of excess water.
The colon does not remove all the water from our faeces, so when we defaecate we lose some water. This is a problem when the colon is infected by some nasty bacterium. Dysentery is a killer disease, so much water is lost in the faeces that the body becomes dehydrated.
Some diseases make us vomit. Even if you drink water, because it all come up again none is absorbed and the body becomes dehydrated. Water is also lost if you spit.
Where do babies get all their water from? Well, they drink milk. If a woman breast feeds her baby, she must replace all the water she loses in her milk, but this does not help her to control her water content. A lactating woman cannot give her baby more or less milk to help her control how much water is in her body.
So it all comes down to how much water is lost in the urine. This can be controlled exactly. You never have to think about it. The hypothalamus detects the amount of water in the blood, it controls how much ADH is secreted by the pituitary gland. ADH controls how much water is excreted by the kidney.
At the beginning we decided to forget about what happens to the sugar and mineral salts. Other hormones control how much sugar and salts remain in the blood. If there is too much glucose it can be converted into glycogen by the liver; if there is too little glucose the liver can convert glycogen back into glucose. If there are excess salts they can be excreted by the kidney. The control mechanisms work in just the same way but usinfg different hormones.
6. Glossary:
Hypothalamus: a region of the brain which monitors the conditions of the blood; i.e. how much glucose, mineral salts and water are present. It controls the pituitary gland. go back up
Pituitary Gland: an endocrine gland at the base of the brain, just underneath the hypothalamus. This gland is sometimes called the master endocrine gland because it controls all the other endocrine organs in the body. go back up
Anti-Diuretic Hormone: a hormone (chemical messenger) produced by the pituitary gland. ADH stimulates the kidney to reabsorb water. The more ADH there is in the blood the harder the kidney works to reabsorb water. go back up
Negative feedback: a control process. When a hormone has had an effect on its target organ the process of negative feedback can switch the endocrine organ off. Here is a simple analogy to help you understand negative feedback.
Positive feedback is the reverse of negative feedback. When we are having an arguement, we do not really listen to each other. If I raise my voice to make you listen to me, you react by raising your voice. eventually we are both shouting at each other. We still don’t listen. We start to get angry. I throw a pot at you and you throw one back. If we are unlucky positive feedback results in a death. One of us has a heart attack, or we start hitting each other and one gets killed. Positive feedback always makes matters worse. Suppose one of us starts to cry, this is a signal to stop. The other one realises that matters have gone too far and stops shouting. Then we start to talk instead of shouting. Now it is possible to listen properly. Instead of killing each other, we listen and things get back to normal. The crying had a negative feedback effect.
.
