How does the body keep its internal environment stable, and what does the kidney do?
The principle of homeostasis and negative feedback, the structure of the kidney and nephron, ultrafiltration and selective reabsorption, and osmoregulation by ADH.
A CCEA A-Level Biology answer on the principle of homeostasis and negative feedback, the structure of the kidney and nephron, ultrafiltration and selective reabsorption, and osmoregulation by ADH.
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What this dot point is asking
CCEA wants you to explain homeostasis and negative feedback, describe the structure of the kidney and nephron, explain ultrafiltration and selective reabsorption, and explain how ADH controls the water potential of the blood.
Homeostasis and negative feedback
Negative feedback keeps factors such as body temperature, blood glucose and blood water potential close to their set points. Stable internal conditions matter because enzymes work best within a narrow range of temperature and pH, and cells need a steady water potential to avoid osmotic damage.
Kidney and nephron structure
High pressure for ultrafiltration is created because the afferent arteriole entering the glomerulus is wider than the efferent arteriole leaving it. Small molecules (water, glucose, ions, urea) are forced through the basement membrane into the capsule, while large plasma proteins and blood cells are too big to pass and stay in the blood. The proximal convoluted tubule cells have microvilli and many mitochondria because reabsorption of glucose and ions uses active transport.
Osmoregulation by ADH
When blood water potential falls (for example after sweating), osmoreceptors in the hypothalamus detect this, and the posterior pituitary releases more ADH (antidiuretic hormone). ADH makes the collecting duct walls more permeable to water (by inserting aquaporins), so more water is reabsorbed into the blood and a small volume of concentrated urine is made. When water potential rises (after drinking a lot), less ADH is released, the collecting duct is less permeable, and a large volume of dilute urine is produced. This is negative feedback.
Examples in context
Example 1. Diabetes insipidus. A person who cannot make or respond to ADH produces huge volumes of very dilute urine and becomes thirsty and dehydrated, because water is not reabsorbed from the collecting ducts. This condition demonstrates the role of ADH directly: remove the hormone or its effect, and osmoregulation fails. It is treated with synthetic ADH (desmopressin).
Example 2. Glucose in the urine of an untreated diabetic. Normally all glucose is reabsorbed in the proximal convoluted tubule by active transport. In untreated diabetes mellitus, blood glucose is so high that the carrier proteins become saturated and cannot reabsorb it all, so glucose appears in the urine. This was historically how diabetes was detected, and shows that selective reabsorption has a maximum rate set by the number of carrier proteins.
Try this
Q1. Explain what is meant by negative feedback in homeostasis. [2 marks]
- Cue. A change is detected and effectors act to reverse it, returning the factor to its set point.
Q2. Describe how ADH changes the urine produced when you are dehydrated. [2 marks]
- Cue. More ADH makes the collecting duct more permeable, more water is reabsorbed, and a small volume of concentrated urine is made.
Q3. Explain why proteins are found in the blood but not normally in the urine of a healthy person. [2 marks]
- Cue. Plasma proteins are too large to be filtered through the basement membrane at the glomerulus, so they remain in the blood and do not enter the filtrate.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA 20196 marksDescribe how the kidney produces urine, including the processes of ultrafiltration and selective reabsorption.Show worked answer →
A 6-mark answer should describe filtration at the glomerulus then reabsorption along the nephron.
Ultrafiltration: blood enters the glomerulus through a wide afferent arteriole and leaves through a narrower efferent arteriole, creating high pressure. This forces water, glucose, ions, urea and other small molecules out of the blood, through the basement membrane, into the Bowman's capsule, forming the filtrate. Large molecules such as proteins and blood cells are too big to pass and stay in the blood.
Selective reabsorption: in the proximal convoluted tubule, all the glucose and amino acids and much of the water and ions are reabsorbed back into the blood, by active transport and then osmosis. The cells here have microvilli and many mitochondria for active transport.
The loop of Henle sets up a low water potential in the medulla, and water is reabsorbed from the collecting duct as it passes through, concentrating the urine.
Markers reward the pressure difference, what is and is not filtered, reabsorption of glucose and water, and the role of the loop of Henle and collecting duct.
CCEA 20215 marksExplain how the body responds to a fall in the water potential of the blood after heavy sweating, naming the receptors, hormone and target involved.Show worked answer →
A 5-mark answer needs the negative feedback loop with named parts in order.
Detection: osmoreceptors in the hypothalamus detect the fall in water potential of the blood (the blood is too concentrated).
Hormone release: the hypothalamus signals the posterior pituitary gland to release more antidiuretic hormone (ADH) into the blood.
Target and effect: ADH travels to the collecting ducts of the kidney and makes their walls more permeable to water (more aquaporins inserted), so more water is reabsorbed into the blood.
Result: a small volume of concentrated urine is produced, the water potential of the blood rises back towards normal, and ADH release then decreases (negative feedback).
Markers reward osmoreceptors in the hypothalamus, ADH from the posterior pituitary, increased collecting duct permeability, more water reabsorbed, and the return to the set point.
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Sources & how we know this
- CCEA GCE Biology specification — CCEA (2016)