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Homeostasis: Class 12 Biology
What homeostasis is, negative feedback, osmoregulation, excretion and the structure and working of the human kidney, and the control of body temperature.
Key points
Homeostasis is the maintenance of a constant internal environment, such as stable temperature, water content, blood glucose and pH, in spite of changes outside the body. It is needed because enzymes and cells work well only within narrow limits.
Most homeostatic systems work by negative feedback. A receptor detects a change from the set point, a control centre compares it with the set point and sends a signal, and an effector acts to bring the value back. The correction reduces the original change, which is why the feedback is called negative.
Osmoregulation is the control of water and salt content. In freshwater organisms water enters by osmosis and must be removed. In marine bony fish water is lost by osmosis and must be replaced by drinking. Land animals lose water by evaporation and in urine, so they must conserve it.
Excretion is the removal of the metabolic waste products of the body. Deamination of amino acids in the liver forms ammonia, which is converted to urea, a much less toxic substance, in the liver by the ornithine cycle. Carbon dioxide is excreted by the lungs, and bile pigments from the breakdown of haemoglobin are excreted in bile.
The kidneys are the main excretory and osmoregulatory organs. Each kidney has an outer cortex, an inner medulla and a pelvis that leads to the ureter. The functional unit is the nephron, of which each kidney has about a million.
Parts of a nephron and what they do:
- Glomerulus and Bowman's capsule: ultrafiltration of blood under high pressure. Water, glucose, salts and urea pass into the capsule. Blood cells and large proteins stay in the blood.
- Proximal convoluted tubule: selective reabsorption of all the glucose and amino acids, and most of the salts and water.
- Loop of Henle: creates a high salt concentration in the medulla, so that water can be reabsorbed from the collecting duct.
- Distal convoluted tubule: adjusts the amount of salts and the pH.
- Collecting duct: the final adjustment of water, controlled by antidiuretic hormone (ADH).
Control of water balance. When the blood is too concentrated, the hypothalamus detects the low water potential, and the pituitary releases more ADH. ADH makes the collecting duct more permeable to water, more water is reabsorbed, and a small volume of concentrated urine is produced. When the blood is too dilute, less ADH is released and a large volume of dilute urine is produced.
Kidney failure can be treated by dialysis or by a kidney transplant. In haemodialysis, blood passes through a machine where urea and extra salts diffuse into a dialysis fluid across a partially permeable membrane. A transplant gives a permanent solution but needs a matching donor and drugs to prevent rejection.
Thermoregulation is the control of body temperature. Humans are endotherms and keep the core temperature at about 37 °C. The hypothalamus is the control centre. When the body is too hot, sweating, vasodilation of skin arterioles and flattening of body hair increase heat loss. When it is too cold, shivering, vasoconstriction and erection of hairs reduce heat loss and increase heat production.
Blood glucose is another example. When glucose rises after a meal, the pancreas secretes insulin, which makes the liver and muscles take up glucose and store it as glycogen. When it falls, glucagon causes glycogen to be broken down to glucose. Failure of this system causes diabetes mellitus.
Study tip: For any homeostasis answer, name the stimulus, the receptor, the control centre, the effector and the response, then say how the response reverses the original change.
Definitions and flashcards
Say the meaning first, then open the card to check.
Homeostasis
The maintenance of a constant internal environment.
Negative feedback
A change is detected and the body responds to reverse it, returning the value to the set point.
Osmoregulation
The control of the water and salt content of the body.
Ultrafiltration
Filtration of blood under high pressure through the glomerulus into Bowman's capsule.
Selective reabsorption
The return of useful substances from the filtrate to the blood in the nephron tubule.
Antidiuretic hormone (ADH)
A hormone from the pituitary that increases the permeability of the collecting duct to water.
Nephron
The structural and functional unit of the kidney.
Thermoregulation
The control of body temperature.
Short questions with model answers
These are the kind of questions asked for 2, 3 and 5 marks. Try each one before opening the answer.
Define homeostasis and give two examples. (2 marks)
Homeostasis is the maintenance of a constant internal environment. Examples are the control of body temperature at about 37 °C and of blood glucose concentration.
Explain negative feedback with an example. (3 marks)
A change away from the set point is detected by receptors, and effectors reverse it. When blood glucose rises, insulin is released, which lowers the glucose and returns it to normal, so the stimulus for insulin is removed.
Why does glucose not appear in the urine of a healthy person? (2 marks)
All the glucose that is filtered into Bowman's capsule is reabsorbed into the blood by active transport in the proximal convoluted tubule.
How does ADH help to conserve water? (3 marks)
ADH makes the walls of the collecting ducts more permeable to water. More water is reabsorbed by osmosis into the concentrated medulla, so less water is lost and the urine is small in volume and concentrated.
List the ways in which the body loses heat when it is too hot. (3 marks)
Sweating, so that evaporation cools the skin; vasodilation, so that more blood flows near the skin surface and loses heat; and flattening of the hairs, which reduces insulation.
Common mistakes
- Saying that the kidney filters out only waste. Ultrafiltration is not selective, and the selection occurs in reabsorption.
- Writing that positive feedback maintains homeostasis. Homeostasis depends on negative feedback.
- Mixing up insulin and glucagon.
- Saying that ADH is made by the kidney. It is released from the pituitary and acts on the kidney.
- Naming sweat glands and the skin as the control centre of temperature, when it is the hypothalamus.