Homeostasis
🎯What you need to be able to do
- Define homeostasis, and name body temperature, blood pH, blood glucose and blood osmotic concentration as homeostatic variables.
- Explain negative feedback, and why it rather than positive feedback is used in homeostasis.
- Explain the regulation of blood glucose by insulin and glucagon.
- Explain the physiological basis, risk factors, prevention and treatment of type 1 and type 2 diabetes.
- Explain thermoregulation, including thermoreceptors, the hypothalamus, pituitary, thyroxin, and effectors in muscle and adipose tissue.
- Explain vasodilation, vasoconstriction, shivering, sweating, uncoupled respiration in brown adipose tissue and hair erection.
- AHL Distinguish excretion from osmoregulation, and explain the roles of the glomerulus, Bowman’s capsule, proximal convoluted tubule and loop of Henle.
- AHL Explain osmoregulation by ADH and aquaporins in the collecting ducts, and changes in blood supply to organs with activity.
📚The biology
Homeostasis
Homeostasis is the maintenance of the internal environment of an organism: its variables are kept within preset limits, despite fluctuations in the external environment. In humans, homeostatic variables include:
about 37 °C
about 7.35–7.45
about 4–8 mmol dm−3
solute concentration of blood plasma
Keeping these steady allows enzymes to work at optimal rates, prevents damage to cells by osmosis, and gives cells a reliable supply of glucose.
Negative feedback
Homeostasis uses negative feedback: when a variable moves away from its set point, the change is detected and a response brings it back towards the set point. Crucially, negative feedback works in both directions — from values above the set point and from values below it — usually through two opposing responses (such as insulin and glucagon).
Positive feedback would amplify a change, pushing the variable further from the set point: a small rise in temperature would cause a further rise. That is useful for events that must be completed quickly, such as childbirth (D3.1), but it would destroy stability, so it cannot maintain a constant internal environment.
Regulation of blood glucose
Blood glucose is regulated by two hormones from the endocrine cells of the pancreas, found in groups called islets of Langerhans. These cells act as both receptors and control centre: they detect blood glucose concentration directly.
Beta cells secrete insulin into the blood.
Insulin is transported in the blood to target cells, mainly liver, muscle and adipose cells.
It stimulates them to take up glucose (by inserting glucose transporters into their membranes), and stimulates liver and muscle cells to convert glucose to glycogen, and adipose cells to convert it to fat.
Blood glucose falls back to the set point.
Alpha cells secrete glucagon into the blood.
Glucagon is transported to target cells, mainly in the liver.
It stimulates the liver to break down glycogen into glucose and release it into the blood, and to make glucose from other compounds.
Blood glucose rises back to the set point.
Type 1 and type 2 diabetes
Physiological change: the immune system destroys the beta cells (an autoimmune disease), so little or no insulin is produced.
Onset: usually in childhood or adolescence.
Risk factors: genetic predisposition, possibly triggered by a viral infection; not caused by lifestyle.
Prevention: no reliable method known.
Treatment: regular insulin injections (or a pump), with monitoring of blood glucose and a controlled diet.
Physiological change: target cells become insensitive (resistant) to insulin, because receptors respond less; insulin is produced, often in large amounts initially, but glucose is not taken up. Later, beta cell function may decline.
Onset: usually in adults, increasingly in young people.
Risk factors: obesity, lack of exercise, diets high in sugar and fat, age, family history and ethnic background.
Prevention: healthy diet, regular exercise, maintaining a healthy body mass.
Treatment: diet and exercise; drugs that increase insulin sensitivity or reduce glucose production; insulin in later stages.
In both types, blood glucose stays high after meals. Glucose appears in the urine because the kidneys cannot reabsorb all of it, and long-term high glucose damages blood vessels, nerves, the eyes and kidneys.
Thermoregulation: the negative feedback loop
Body temperature is controlled by negative feedback:
- Receptors: peripheral thermoreceptors in the skin detect changes in skin temperature; thermoreceptors in the hypothalamus detect the temperature of the blood.
- Control centre: the hypothalamus compares the information with the set point and coordinates responses, by nerve impulses to effectors and by hormones.
- Hormonal pathway: in cold conditions, the hypothalamus stimulates the pituitary gland to release TSH, which stimulates the thyroid gland to secrete thyroxin. Thyroxin increases the metabolic rate of body cells, generating more heat.
- Effectors: skeletal muscle (shivering), brown adipose tissue (heat generation), sweat glands, smooth muscle in arterioles supplying the skin, and hair erector muscles.
Thermoregulation mechanisms in humans
Birds and mammals regulate body temperature by both physiological and behavioural means (seeking shade, huddling, putting on clothes). The physiological details for humans:
Vasodilation: arterioles supplying the skin widen, so more warm blood flows near the surface and more heat is lost by radiation.
Sweating: sweat glands secrete sweat onto the skin; its evaporation removes heat.
Hair lies flat, trapping little air.
Vasoconstriction: arterioles supplying the skin narrow, so less blood flows near the surface and less heat is lost.
Shivering: rapid, involuntary contractions of skeletal muscle generate heat from respiration.
Uncoupled respiration in brown adipose tissue: in brown fat (important in babies), a protein lets protons leak back across the inner mitochondrial membrane without making ATP, so the energy from respiration is released directly as heat.
Hair erection: hair erector muscles raise body hairs (goosebumps), trapping a layer of insulating air; very effective in furry mammals, of little use in humans.
Thyroxin: raises metabolic rate.
Excretion and osmoregulation AHL
The removal from the body of waste products of metabolism, such as urea (from breakdown of excess amino acids) and carbon dioxide, and of toxins.
The regulation of osmotic concentration — the solute concentration of body fluids — by controlling the amounts of water and salts. Osmotic concentration is measured in osmoles per litre (osmol L−1).
The kidney carries out both. Each kidney contains about a million nephrons, the functional units.
Glomerulus, Bowman’s capsule and proximal convoluted tubule AHL
Ultrafiltration. The glomerulus is a knot of capillaries inside the cup-shaped Bowman’s capsule. Blood enters at high pressure, because the arteriole leaving the glomerulus is narrower than the one entering. The pressure forces fluid out through the fenestrated capillary walls, a basement membrane (the actual filter) and gaps between the podocyte cells lining the capsule. Water, glucose, amino acids, salts and urea pass into the filtrate; blood cells and large plasma proteins are too big and stay in the blood. Ultrafiltration therefore removes solutes from blood plasma non-selectively — both waste and useful substances.
Selective reabsorption. In the proximal convoluted tubule, useful substances are taken back into the blood:
- all glucose and amino acids, by cotransport with sodium ions (indirect active transport, B2.1);
- most sodium and other ions, by active transport;
- much of the water, which follows by osmosis.
The tubule cells have microvilli and many mitochondria for this (B2.3). Urea and toxins are largely left in the filtrate and are excreted in urine.
The loop of Henle AHL
The loop of Henle extends down into the medulla of the kidney. Cells of the ascending limb actively transport sodium ions out of the filtrate into the surrounding tissue fluid of the medulla. The ascending limb is impermeable to water, so water cannot follow. As a result, the tissue fluid of the medulla has a high osmotic concentration, which increases deeper into the medulla. This high concentration is what allows water to be reabsorbed from the collecting ducts, which pass through the medulla.
Osmoregulation by ADH AHL
Water reabsorption in the collecting ducts is adjusted to the body’s needs:
Osmoreceptors in the hypothalamus detect the high osmotic concentration.
The pituitary gland increases secretion of antidiuretic hormone (ADH).
ADH causes vesicles containing aquaporins to move to and fuse with the plasma membrane of collecting duct cells.
The ducts become more permeable to water, which is reabsorbed by osmosis into the concentrated medulla.
A small volume of concentrated urine is produced; blood osmotic concentration falls.
Osmoreceptors detect the low osmotic concentration.
The pituitary reduces ADH secretion.
Aquaporins are removed from the plasma membrane back into intracellular vesicles.
The ducts become less permeable to water, so less water is reabsorbed.
A large volume of dilute urine is produced; blood osmotic concentration rises.
Changing blood supply to organs AHL
The distribution of blood to organs changes with activity, by vasodilation and vasoconstriction of the arterioles supplying them:
Low total cardiac output. Skeletal muscles receive little. Brain supply is maintained. Gut and kidneys receive a relatively large share.
The gut (especially after a meal), kidneys and brain receive large shares; skeletal muscles receive a moderate share.
Cardiac output rises several-fold. Skeletal muscles receive by far the largest share (most of the total). Supply to the gut and kidneys is reduced by vasoconstriction. Brain supply stays about the same in absolute terms, so it becomes a smaller percentage.
The brain’s supply is kept nearly constant under all conditions, because it cannot tolerate even brief shortages of oxygen and glucose.
✏️Worked example
(a) Calculate the volume of filtrate produced per day in dm3.
(b) Calculate the percentage of the filtrate that is reabsorbed.
(c) The concentration of urea is 0.3 g dm−3 in plasma and 20 g dm−3 in urine. Calculate how many times more concentrated urea is in urine, and explain how this increase comes about.
(d) Explain why a person with untreated diabetes may have glucose in their urine.
(a)
(b)
(c) 20 ÷ 0.3 = about 67 times more concentrated. Urea is filtered from the plasma into the Bowman’s capsule at the same concentration as in plasma, but most of the water in the filtrate is then reabsorbed (in the proximal convoluted tubule and collecting ducts) while little urea is reabsorbed. The same mass of urea ends up in a much smaller volume of liquid, so its concentration rises.
(d) In untreated diabetes, blood glucose is abnormally high, so the filtrate contains a very high concentration of glucose. Glucose is reabsorbed in the proximal convoluted tubule by cotransporter proteins, and there is a limited number of them. When they are all working at their maximum rate, the excess glucose cannot all be reabsorbed and passes into the urine.
📝Practise
Work through these on paper, then reveal the answer. Questions 5 and 6 are AHL.
1. Explain why negative feedback, not positive feedback, is used in homeostasis.
2. Explain how blood glucose concentration is returned to normal after a meal rich in carbohydrate.
3. Compare type 1 and type 2 diabetes.
4. Explain how the body responds to a fall in core body temperature.
5. AHL Explain the role of the loop of Henle.
6. AHL Explain how the kidney responds when a person becomes dehydrated.
🔗Go deeper — other people’s work
These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.
- Diabetes UK and the International Diabetes Federation — accurate, up-to-date information on types, risk factors and treatment.
- You and Your Hormones (Society for Endocrinology) — profiles of insulin, glucagon, thyroxin and ADH.
- HHMI BioInteractive — animations and data activities on the nephron and on thermoregulation.