Regulation of water
When the body has too little water:
- The blood will have a lower water potential than the RBCs, causing them to crenate. This reduces the amount of substances they can transport.
- Information is sent to the hypothalamus, the “thirst centre” of the brain, from sensors in the blood vessels. It signals the organism to look for water to drink.
- The hypothalamus increases the synthesis of an antidiuretic hormone (ADH) called vasopressin, which is secreted by the pituitary gland and travels to the kidneys. There, it increases the amount of water reabsorbed from the urine, thus reducing urine flow and conserving water in the body until more fluids are consumed.
= Visible effect: Less urine produced. Urine is more concentrated/yellow.
When the body has too much water:
- The blood will have a higher water potential than the RBCs, causing them to cytolyses due to osmosis. Thus there will be fewer RBCs to transport substances around the body.
- Information is sent to the hypothalamus, the “thirst centre” of the brain, from sensors in the blood vessels. It signals the organism to stop drinking.
- The hypothalamus decreases the synthesis of vasopressin, thus decreasing the amount of water reabsorbed from the urine and increasing urine flow.
= Visible effect: More urine produced. Urine is less concentrated (lighter yellow/ clear).
Regulation of temperature
Our body temperature is affected by that of the surroundings. When the surrounding temperature increases, it increases as well.
When body temperature is too high:
- Behavioural response: Finding shade to reduce temperature difference between core temperature and external environment, wearing thinner clothes to allow more heat from skin to be lost to the external environment
- Receptors in the hypothalamus monitor the blood’s temperature as it passes through the brain (core temp). Receptors in the skin monitor the external temperature.
- They send information to the brain, specifically the hypothalamus, which activates cooling mechanisms:
The body secretes sweat onto the skin from sweat glands. When the sweat evaporates, it draws latent heat away from the skin and thus reduces body temperature.
The arterioles expand to allow more blood to flow through the surface capillaries. More latent heat escapes from the blood to the skin and is lost through sweating, convection radiation.
= Visible effect: More sweat is produced. Face appears red/flushed.
When the body temperature is too low:
- Behavioural response: Curling up to reduce surface area exposed to the external environment, putting on more clothes to reduce heat lost to the surroundings
- … The hypothalamus activates warming mechanisms:
Muscles around the vital organs involuntarily contract and relax repeatedly to convert more ATP to heat energy. This reduces the difference in temperature between the outside environment and the body.
The arterioles contract, causing less blood to flow through the surface capillaries. Less heat is carried to the skin.
(only effective for animals) Fluffing up hair
Erector pili muscles in skin contract, raising skin hairs to trap an insulating layer of still, warm air next to the skin. In humans, this is not very effective and only causes goosebumps.
= Visible effect: Extremities can turn blue or become damaged (frostbite).
Regulation of glucose concentration in blood
When there is too little glucose:
(e.g. after heavy exercise, lack of food for extended periods)
- Behavioural response: Eating food
- Receptor cells in the pancreas monitor glucose concentration in the blood.
- The pancreas releases glucagon, a hormone which stimulates the breakdown of glycogen (stored glucose) to glucose in the liver (glycogenolysis). The glucose is released into the bloodstream, increasing blood sugar levels.
When there is too much glucose:
(e.g. after a meal)
– Behavioural response: Stop eating
– The blood would have a lower water potential than the RBCs, causing the RBCs to crenate. This reduces the amount of substances they can transport.
– The pancreas releases insulin, causing the liver to convert more glucose into glycogen (glycogenesis) and stimulates the uptake of glucose by cells for respiration. This reduces blood sugar levels.
*Both glucagon and insulin cannot be secreted at the same time.