How does the body keep its internal environment constant, and what is the role of the kidney?
The principle of homeostasis and negative feedback, the control of body temperature and blood glucose, and the structure and function of the kidney in osmoregulation and excretion.
An Edexcel A-Level Biology B (Salters-Nuffield) answer on homeostasis and the kidney, covering negative feedback, the control of temperature and blood glucose, and the structure and function of the kidney in excretion and osmoregulation.
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What this dot point is asking
Edexcel wants you to explain homeostasis and negative feedback, describe the control of body temperature and blood glucose, and describe the structure and function of the kidney in excretion and osmoregulation. Linking nephron structure to ultrafiltration and reabsorption, and explaining ADH control, are common exam tasks.
Homeostasis and negative feedback
The general pattern is: a receptor detects the change, a coordination centre (often the hypothalamus or pancreas) processes it, and an effector (muscle or gland) acts to reverse it. Negative feedback keeps each factor within narrow limits, which matters because enzymes and cell processes work only over a limited range of temperature, pH and solute concentration.
Examples include the control of body temperature (sweating and vasodilation of skin arterioles to lose heat, shivering and vasoconstriction to conserve heat), and blood glucose (insulin lowers it by promoting glucose uptake and glycogen storage; glucagon raises it by breaking down glycogen; both are made by the islets of Langerhans in the pancreas).
The kidney
The kidney both excretes waste (urea) and carries out osmoregulation (controlling the water potential of the blood).
Osmoregulation and ADH
The amount of water reabsorbed is controlled by antidiuretic hormone (ADH). Osmoreceptors in the hypothalamus detect blood water potential. When it falls (dehydration, too little water), more ADH is released from the posterior pituitary; ADH makes the collecting duct more permeable to water (by inserting aquaporin channels), so more water is reabsorbed into the blood and a small volume of concentrated urine is produced. When blood water potential is high (too much water), less ADH is released, the collecting duct is less permeable, less water is reabsorbed and a large volume of dilute urine is produced. This is itself a negative feedback loop.
Examples in context
Example 1. Type 1 diabetes and glucose in urine. In type 1 diabetes the beta cells are destroyed, so no insulin is made and blood glucose stays high. So much glucose is filtered at the glomerulus that the proximal convoluted tubule cannot reabsorb it all, so glucose appears in the urine. The glucose lowers the water potential of the filtrate, so less water is reabsorbed and the person produces large volumes of urine and feels thirsty. This connects kidney function directly to the failure of glucose homeostasis.
Example 2. Marathon runners and ADH. During a long race a runner loses water in sweat, lowering blood water potential. Osmoreceptors trigger ADH release, the collecting ducts reabsorb more water, and urine becomes scant and concentrated to conserve water. Drinking too much plain water can over-dilute the blood (hyponatraemia), showing why osmoregulation around a set point matters.
Try this
Q1. Explain how negative feedback controls blood glucose after a meal. [3 marks]
- Cue. Rising glucose is detected, insulin is released, cells take up glucose and the liver stores it, so glucose falls back towards normal.
Q2. Explain how ADH affects the kidney when the body is dehydrated. [2 marks]
- Cue. More ADH makes the collecting duct more permeable, so more water is reabsorbed and concentrated urine is produced.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20195 marksDescribe how the structure of the nephron allows ultrafiltration at the glomerulus and the selective reabsorption of glucose at the proximal convoluted tubule.Show worked answer →
Markers want structure linked to each function.
The afferent arteriole is wider than the efferent arteriole, so blood in the glomerular capillaries is at high hydrostatic pressure. This forces water, glucose, ions and urea through the fenestrated capillary endothelium, the basement membrane and the podocytes of the Bowman capsule into the tubule (ultrafiltration); large proteins and cells stay in the blood. At the proximal convoluted tubule, the cells have microvilli (large surface area) and many mitochondria. Sodium ions are actively transported out, and glucose is reabsorbed by co-transport with sodium, then moves into the blood by facilitated diffusion, so all glucose is reabsorbed.
Award marks for: afferent wider than efferent giving high pressure; small molecules forced out, proteins retained; microvilli and mitochondria at PCT; glucose co-transport with sodium; all glucose reabsorbed in a healthy person.
Edexcel 20224 marksExplain how the control of blood glucose concentration by insulin and glucagon is an example of negative feedback.Show worked answer →
Markers want the two hormones acting in opposite directions around a set point.
When blood glucose rises (after a meal), beta cells in the pancreatic islets detect it and secrete insulin. Insulin makes liver and muscle cells take up glucose and convert it to glycogen (glycogenesis), so blood glucose falls back to normal. When blood glucose falls (during fasting or exercise), alpha cells secrete glucagon, which makes the liver break down glycogen to glucose (glycogenolysis), raising blood glucose back to normal. Because each response reverses the change and returns glucose towards the set point, this is negative feedback.
Award marks for: rise detected, insulin lowers glucose by glycogenesis; fall detected, glucagon raises glucose by glycogenolysis; each response reverses the change (negative feedback).
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Sources & how we know this
- Pearson Edexcel A-Level Biology B (9BN0) specification — Pearson Edexcel (2015)