How does negative feedback keep the internal environment stable, and how do the kidneys filter blood and control its water potential?
5.1.2 Homeostasis and excretion: the principles of homeostasis and negative feedback; the role of the liver in deamination and detoxification; the structure of the nephron and the processes of ultrafiltration and selective reabsorption; and osmoregulation by ADH acting on the collecting duct.
A focused answer to the OCR H420 5.1.2 dot point on homeostasis and the kidney. Covers negative feedback, the liver's role in deamination and detoxification, the nephron, ultrafiltration and selective reabsorption, and osmoregulation by ADH acting on the collecting duct.
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What this dot point is asking
OCR wants you to explain the principles of homeostasis and negative feedback, the liver's role in deamination and detoxification, the structure of the nephron, the processes of ultrafiltration and selective reabsorption, and osmoregulation by ADH acting on the collecting duct.
The answer
Homeostasis and negative feedback
Homeostasis is the maintenance of a stable internal environment (for example temperature, blood glucose, water potential and pH) within narrow limits despite changes outside. It works by negative feedback: a change is detected by receptors, a coordinator (the nervous or endocrine system) signals effectors, and the response reverses the change, returning the factor towards its set point.
A stable internal environment matters because enzymes work best within narrow temperature and pH ranges, and cells need a steady water potential and glucose supply.
The role of the liver
The liver processes the products of digestion and removes wastes:
- Deamination: excess amino acids cannot be stored, so the liver removes the amino group, forming ammonia, which is combined with carbon dioxide in the ornithine cycle to form less toxic urea for excretion by the kidneys.
- Detoxification: the liver breaks down toxins such as alcohol (oxidised to ethanal then ethanoate) and hydrogen peroxide (using catalase), and processes drugs and hormones.
The nephron and ultrafiltration
The kidney's functional unit is the nephron. Blood is filtered in the glomerulus, a knot of capillaries in the Bowman's capsule:
- Blood enters through a wide afferent arteriole and leaves through a narrower efferent arteriole, creating a high hydrostatic pressure.
- This pressure forces small molecules (water, glucose, ions, urea) out through the basement membrane (the actual filter) and the gaps between podocytes into the capsule, forming the filtrate.
- Blood cells and large plasma proteins are too big to pass and remain in the blood.
Selective reabsorption
Most of the useful filtrate is reabsorbed, mainly in the proximal convoluted tubule (PCT):
- all glucose and amino acids and most ions are reabsorbed into the blood, much by active transport (often co-transport with sodium ions);
- the PCT cells have many microvilli (large surface area) and mitochondria (ATP for active transport);
- water follows by osmosis, then more water is reabsorbed in the loop of Henle and collecting duct.
The loop of Henle sets up a salt (sodium and chloride) concentration gradient in the medulla by a counter-current multiplier, allowing water to be reabsorbed from the collecting duct.
Osmoregulation by ADH
The water potential of the blood is controlled by negative feedback through ADH (antidiuretic hormone):
- Osmoreceptors in the hypothalamus detect a fall in blood water potential (for example after sweating).
- The posterior pituitary releases more ADH, which makes the collecting duct more permeable to water by inserting more aquaporins into the membranes.
- More water is reabsorbed by osmosis into the blood, so a small volume of concentrated urine is produced and the water potential rises back to normal.
- If the blood is too dilute, less ADH is released, the collecting duct is less permeable, and a large volume of dilute urine is produced.
Examples in context
Example 1. Diabetes insipidus. If ADH is not produced or the collecting duct does not respond to it, the duct stays impermeable to water, so large volumes of dilute urine are produced and the person becomes very thirsty, a direct illustration of ADH's role.
Example 2. Kidney failure and dialysis. When the kidneys cannot filter the blood, dialysis uses a partially permeable membrane and a dialysis fluid to remove urea and excess ions and water down concentration gradients, mimicking the nephron.
Try this
Q1. State what is meant by negative feedback. [2 marks]
- Cue. A change from the set point is detected and triggers a response that reverses the change, returning the factor towards normal.
Q2. Explain why blood cells and plasma proteins are not found in the glomerular filtrate. [2 marks]
- Cue. They are too large to pass through the basement membrane and the gaps between the podocytes, so they remain in the blood.
Q3. Name the hormone that increases the permeability of the collecting duct to water. [1 mark]
- Cue. ADH (antidiuretic hormone).
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR H420/01 20195 marksDescribe how urine is formed in the nephron, including ultrafiltration in the glomerulus and selective reabsorption in the proximal convoluted tubule.Show worked answer →
Take the two named processes in order, with the mechanism of each.
Ultrafiltration: blood enters the glomerulus through a wide afferent arteriole and leaves through a narrower efferent arteriole, creating a high hydrostatic pressure. This forces water, glucose, ions and urea out through the basement membrane (which acts as a molecular filter) and the gaps between the podocytes into the Bowman's capsule, forming the filtrate. Blood cells and large plasma proteins are too big to pass and stay in the blood.
Selective reabsorption (proximal convoluted tubule): all the glucose and amino acids and most ions are reabsorbed into the blood, much by active transport / co-transport with sodium ions; the cells have many microvilli and mitochondria for this. Water then follows by osmosis.
Markers reward high hydrostatic pressure from the arteriole difference, the basement membrane and podocytes as the filter, proteins staying behind, and active reabsorption of glucose with water following by osmosis.
OCR H420/01 20214 marksExplain how the body responds to a fall in the water potential of the blood (for example after sweating heavily) to restore it.Show worked answer →
Run the negative feedback loop through ADH and the collecting duct.
Osmoreceptors in the hypothalamus detect the fall in water potential of the blood. They stimulate the posterior pituitary to release more ADH (antidiuretic hormone) into the blood.
ADH makes the collecting duct more permeable to water by inserting more aquaporins into the cell membranes. More water is reabsorbed by osmosis from the collecting duct into the blood, so a smaller volume of more concentrated urine is produced and the water potential of the blood rises back to normal.
This is negative feedback: the response reverses the original change. Markers reward osmoreceptors detecting the change, more ADH, more aquaporins/permeability, more water reabsorbed, and concentrated urine.
Related dot points
- 5.1.4 Hormonal communication: the principles of hormonal coordination and the contrast with nervous coordination; the structure and function of the adrenal glands and pancreas; the control of blood glucose concentration by insulin and glucagon (glycogenesis, glycogenolysis and gluconeogenesis); the second messenger model of adrenaline and glucagon; and the causes of type 1 and type 2 diabetes.
A focused answer to the OCR H420 5.1.4 dot point on hormonal communication. Covers hormonal versus nervous coordination, the adrenal glands and pancreas, the control of blood glucose by insulin and glucagon, the second messenger model, and the causes of type 1 and type 2 diabetes.
- 5.1.3 Neuronal communication: the structure of a neurone; the establishment of the resting potential by the sodium-potassium pump; the generation of an action potential by voltage-gated channels (depolarisation and repolarisation); the all-or-nothing principle and the refractory period; saltatory conduction in myelinated neurones; and synaptic transmission by acetylcholine at a cholinergic synapse.
A focused answer to the OCR H420 5.1.3 dot point on neuronal communication. Covers neurone structure, the resting potential, the action potential and its ionic basis, the all-or-nothing principle and refractory period, saltatory conduction, and synaptic transmission by acetylcholine.
- 5.1.5 Plant and animal responses: tropisms and the role of auxin (IAA) in phototropism; the structure and function of the mammalian nervous system (central and peripheral, voluntary and autonomic), the reflex arc and the fight-or-flight response; and the structure and the sliding filament mechanism of skeletal muscle contraction.
A focused answer to the OCR H420 5.1.5 dot point on plant and animal responses. Covers tropisms and the role of auxin in phototropism, the organisation of the mammalian nervous system, the reflex arc and fight-or-flight, and the sliding filament mechanism of muscle contraction.
- 3.1.2 Transport in animals: the structure and functions of arteries, arterioles, capillaries, venules and veins; the formation of tissue fluid from plasma at the arterial end of a capillary bed and its return at the venous end and via the lymphatic system, explained in terms of hydrostatic and oncotic (osmotic) pressure.
A focused answer to the OCR H420 3.1.2 dot point on blood vessels and tissue fluid. Covers the structure and function of arteries, arterioles, capillaries, venules and veins, and how hydrostatic and oncotic pressure form tissue fluid at the arterial end and return it at the venous end and via the lymph.
- 5.2.2 Respiration: the four stages of aerobic respiration (glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation); the role of decarboxylation, dehydrogenation, reduced NAD and FAD, the electron transport chain, chemiosmosis and ATP synthase; the synthesis of ATP and the role of oxygen as the final electron acceptor; and anaerobic respiration in animals (lactate) and in yeast (ethanol).
A focused answer to the OCR H420 5.2.2 dot point on respiration. Covers the four stages of aerobic respiration (glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation), chemiosmosis and ATP synthase, the role of oxygen, and anaerobic respiration producing lactate or ethanol.
Sources & how we know this
- OCR A Level Biology A (H420) Specification — OCR (2023)