OCR A-Level Biology: Communication, Homeostasis and Excretion

Communication, homeostasis and excretion is the first of three Module 5 courses on OCR A-Level Biology A (H420), and it lays down the integrative physiology that the rest of the module — and several Module 6 topics — will draw on repeatedly. Why does a mammal maintain a core temperature so close to 37 °C across a 60 °C swing in ambient temperature? Why does a liver lobule have hexagonal cross-sectional geometry and centripetal sinusoidal flow? Why does the loop of Henle have to be long, narrow and counter-current to generate the medullary salt gradient that drives water reclamation? Every answer is grounded in a single transferable principle: control systems with negative feedback, executed by tightly integrated tissues whose ultrastructure mirrors their function.

Course 7 of 12 on the LearningBro OCR A-Level Biology learning path, this course pairs the conceptual scaffolding of homeostasis with the anatomical detail of the liver and the kidney. It sits between the cellular content of Module 2 — particularly biological membranes and cell division, whose carrier proteins and tight junctions reappear in the nephron — and the rest of Module 5, especially neuronal and hormonal communication, which supplies the ADH cascade and the hypothalamic detectors that drive osmoregulation. The biochemistry of urea synthesis and detoxification also routes back through biological molecules and forward into photosynthesis and respiration. Get the feedback grammar fluent here and the rest of Module 5 slots into place.

Guide Overview

The Communication, Homeostasis and Excretion course is structured as ten lessons that move from the general principles of control systems through thermoregulation to the dual workhorses of excretion — the liver and the kidney — and finally to clinical applications.

OCR H420 Specification Coverage

This course covers Module 5 sub-modules 5.1.1 (communication and homeostasis) and 5.1.2 (excretion as an example of homeostatic control) in full. Each spec strand is mapped to one or more lessons below (refer to the official OCR specification document for exact wording).

Sub-topic	Spec area	Primary lesson(s)
The need for communication systems	5.1.1	Communication and Homeostasis Principles
Principles of homeostasis; receptors, effectors and feedback	5.1.1	Communication and Homeostasis Principles; Negative and Positive Feedback
Temperature control in ectotherms and endotherms	5.1.1	Thermoregulation
Excretion as the removal of metabolic waste	5.1.2	Excretion Overview
Liver histology and the hepatic blood supply	5.1.2	Liver Structure
Liver functions: detoxification, urea synthesis, storage	5.1.2	Liver Functions
Kidney gross anatomy and the nephron	5.1.2	Kidney Structure and the Nephron
Ultrafiltration at the renal capsule and selective reabsorption in the PCT	5.1.2	Ultrafiltration and Selective Reabsorption
Osmoregulation, counter-current multiplier and the ADH cascade	5.1.2	Osmoregulation
Kidney failure, dialysis, transplantation and urine analysis	5.1.2	Kidney Failure and Detection in Urine

Module 5 content is examined across all three H420 papers, but communication and excretion are particularly heavy on Paper 2 (Biological diversity, food and health) and on the synoptic Paper 3 (Unified biology), where homeostatic reasoning is routinely combined with cell biology, biochemistry or ecology. Quantitative work on glomerular filtration rate, water potential gradients in the medulla, and Q10-style temperature curves in ectotherms appears reliably on Paper 3.

Communication and Homeostasis Principles

The communication and homeostasis principles lesson opens the course with the rationale for cell-to-cell signalling in multicellular organisms: tissue specialisation creates interdependence, and interdependence requires coordination. The lesson develops the canonical control-system block diagram — stimulus, receptor, communication pathway (nervous or hormonal), effector, response — and contrasts the speed and specificity of neuronal signalling with the slower, broader reach of hormonal signalling. The vocabulary established here is reused in every subsequent lesson on the course and in the whole of neuronal and hormonal communication.

The historical anchor is Claude Bernard's 1865 framing of the internal environment — the milieu intérieur — as the medium whose constancy is the precondition for autonomous physiological life, refined by Walter Cannon in 1929 into the term "homeostasis" we use today. A common mark-loss pattern is to describe homeostasis as "keeping the internal environment constant" without qualifying that the regulated variable oscillates around a set point within a narrow physiological range; the system is dynamic, not static. Another is to omit the receptor or the communication pathway from a control diagram, jumping from stimulus to effector.

Negative and Positive Feedback

The negative and positive feedback lesson develops the formal logic of feedback loops. Negative feedback opposes the direction of change and is by far the dominant mode in mammalian physiology — body temperature, blood glucose, blood water potential, blood pH and blood pressure are all under negative-feedback control. Positive feedback amplifies an initial change and is rare but functionally critical where a rapid, committed response is needed: the depolarisation phase of an action potential (covered in neuronal and hormonal communication), the oxytocin-driven uterine contractions of childbirth, and the clotting cascade.

Examiners reliably test the ability to label the components of a negative-feedback loop and to predict the direction of effector activity given a perturbation. A common pitfall is to describe a response without identifying the corrective direction — for example, writing that "insulin is released" when blood glucose rises, without stating that insulin lowers blood glucose back towards the set point. Another is to conflate negative feedback with feedforward control or with simple homeostatic stability.

Thermoregulation

The thermoregulation lesson contrasts ectothermic and endothermic strategies for body-temperature control. Ectotherms depend largely on behavioural thermoregulation (basking, shade-seeking, body orientation, burrowing) and on physiological tolerances; their metabolic rate climbs and falls with ambient temperature, giving a Q10 of around two across the rise-phase. Endotherms invest metabolic energy in maintaining a near-constant core temperature, using the hypothalamic thermoregulatory centre as the integrator and a coordinated suite of effectors: vasoconstriction or vasodilation of skin arterioles, sweating, piloerection, shivering thermogenesis and non-shivering thermogenesis in brown adipose tissue.

The endothermic strategy gives behavioural and ecological flexibility — sustained activity at low ambient temperatures, nocturnal foraging, latitudinal range — at the metabolic cost of a much higher resting energy demand. A common mark-loss pattern is to say that "blood is moved closer to the surface" during vasodilation; the arterioles dilate and a larger volume of blood flows through the superficial capillary beds, but no vessel physically migrates. Another is to confuse vasoconstriction with the closure of arteriovenous shunts, or to claim that sweating cools by "removing heat" rather than by latent heat of vaporisation as water evaporates from the skin.

Excretion Overview

The excretion overview lesson establishes excretion as the removal of metabolic waste — the nitrogenous waste produced by amino-acid catabolism, the carbon dioxide produced by respiration, and the bile pigments produced by haemoglobin breakdown. The lesson contrasts excretion with egestion (the removal of undigested material that was never absorbed) and with secretion (the active release of useful substances such as digestive enzymes or hormones). The distinction is heavily examined and routinely lost on faecal material, which is egested rather than excreted because most of its bulk has never crossed the gut epithelium.

The lesson also sets up the molecular rationale for urea production: ammonia generated from deamination of excess amino acids (introduced in biological molecules) is too toxic to circulate at physiological concentrations, so terrestrial mammals condense it with carbon dioxide via the urea cycle in the liver — first formalised by Krebs in 1932 — to produce a less-toxic, water-soluble nitrogenous waste suitable for renal excretion.

Liver Structure

The liver structure lesson develops the hexagonal lobule as the functional histological unit of the liver. The hepatic artery (oxygenated) and the hepatic portal vein (nutrient-rich blood drained from the gut) deliver blood at the periphery of each lobule into specialised sinusoidal capillaries, which carry mixed blood centripetally past hepatocytes towards the central vein. Kupffer cells (resident macrophages) line the sinusoids and phagocytose bacteria and worn-out red blood cells; bile canaliculi run between hepatocyte cords in the opposite direction and drain into the bile duct.

The architectural geometry serves the function: every hepatocyte sits within one cell diameter of a sinusoid, so exchange of metabolites with the blood is fast; the centripetal flow means oxygen tension drops across the lobule, creating metabolic zonation; and the counter-current arrangement of bile and blood maximises efficiency. A common mark-loss pattern is to describe the lobule as receiving blood from a "vein" without distinguishing the hepatic portal vein from the hepatic vein (which carries blood out of the liver to the inferior vena cava). Another is to confuse Kupffer cells with hepatocytes.

Liver Functions

The liver functions lesson develops three pillars: deamination and urea synthesis, detoxification, and metabolic storage. Deamination removes the amine group from excess amino acids to yield ammonia plus a keto acid; the keto acid feeds into respiration via the link reaction or gluconeogenesis (revisited in photosynthesis and respiration), while ammonia is shunted into the ornithine cycle (the urea cycle) to be condensed with carbon dioxide. The cycle's net stoichiometry — two ammonia plus one carbon dioxide yielding one urea plus water — is examined explicitly.

Detoxification covers alcohol metabolism (alcohol dehydrogenase converts ethanol to ethanal, then aldehyde dehydrogenase converts ethanal to ethanoate), the cytochrome P450 system for xenobiotic processing, and the conversion of hydrogen peroxide by catalase. The metabolic-storage role spans glycogen (synthesised under insulin control and broken down under glucagon and adrenaline control — covered in blood glucose control), iron from haemoglobin breakdown, and fat-soluble vitamins. A common pitfall is to describe deamination as "removing nitrogen" without naming the amine group, or to claim that the liver "produces urine" — the liver produces urea, which is then excreted by the kidney.

Kidney Structure and the Nephron

The kidney and nephron structure lesson develops the gross anatomy of the kidney — cortex, medulla, renal pelvis, ureter — and the microanatomy of the nephron: renal (Bowman's) capsule with its enclosed glomerulus, proximal convoluted tubule, descending and ascending limbs of the loop of Henle, distal convoluted tubule and collecting duct. Roughly one million nephrons per kidney process around 180 litres of filtrate per day, of which only one to two litres are excreted as urine.

The histological detail matters. The PCT epithelium is densely packed with microvilli on its luminal face and packed with mitochondria, both characteristic of high-throughput active transport — the carrier proteins and tight junctions developed in biological membranes and cell division reappear here. The descending limb is permeable to water but not to ions; the ascending limb is impermeable to water but actively pumps sodium and chloride out into the medullary interstitium. The collecting duct is the site of ADH-regulated water reabsorption developed two lessons later. A common mark-loss pattern is to mis-locate the glomerulus (it sits inside the Bowman's capsule, not separately) or to confuse the loop of Henle's two limbs and their differential permeabilities.

Ultrafiltration and Selective Reabsorption

The ultrafiltration and selective reabsorption lesson develops the physical basis of glomerular filtration. The afferent arteriole is wider than the efferent arteriole, generating a hydrostatic pressure within the glomerular capillary loop high enough to drive plasma across three barriers — the fenestrated capillary endothelium, the basement membrane (the actual size-selective filter, retaining proteins above around 69 kDa), and the slit diaphragms between podocyte foot processes. The result is a filtrate similar in composition to plasma but essentially protein-free.

Selective reabsorption in the PCT then reclaims essentially all of the glucose, all of the amino acids, around 80 percent of the water and a large fraction of the salt. Sodium is pumped out of the PCT cells into the peritubular blood by basolateral sodium-potassium ATPases (introduced in biological membranes and cell division); this lowers intracellular sodium, allowing apical co-transport of glucose and amino acids from the lumen down their concentration gradients. Water follows by osmosis through aquaporins. A common pitfall is to describe the basement membrane as the "main filter for ions" — it filters by size, not charge specificity, and small ions pass freely. Another is to claim glucose is "filtered out" of the blood — it is filtered into the filtrate and then reabsorbed; glucose appearing in urine indicates either pathology (diabetes mellitus, covered in neuronal and hormonal communication) or saturation of the sodium-glucose co-transporters.

Osmoregulation

The osmoregulation lesson develops the counter-current multiplier in the loop of Henle and the ADH cascade. The loop's hairpin geometry, combined with the differential permeability of its two limbs, generates a salt-concentration gradient that increases from cortex to deep medulla. The descending limb loses water by osmosis into the increasingly concentrated interstitium; the ascending limb actively pumps out sodium and chloride. By the time filtrate enters the collecting duct, it can be exposed to this medullary gradient over its full length, allowing variable water reclamation depending on the permeability of the collecting-duct wall.

That permeability is set by antidiuretic hormone (ADH, also called vasopressin), released from the posterior pituitary under the control of osmoreceptor neurones in the hypothalamus (revisited in the mammalian nervous system and brain). When blood water potential falls (after sweating, after a salty meal), ADH release rises; ADH binds basolateral receptors on collecting-duct principal cells and triggers insertion of aquaporin-2 channels into the apical membrane, raising water permeability and producing a smaller volume of more concentrated urine. When blood water potential rises, ADH release falls, aquaporins are withdrawn, and a larger volume of dilute urine is produced. A common mark-loss pattern is to describe ADH as "making water move" — ADH does not move water; it sets the permeability of the collecting duct, and water then follows the pre-existing osmotic gradient.

Quantitative Skills: Q10, GFR and Renal Clearance

Module 5 is not a purely descriptive module. Paper 3 in particular sets calculation items on temperature coefficients, glomerular filtration rate and renal clearance, and these are marks that reward method as much as the final number. Work through each of the following as a template you can reproduce under exam conditions.

The temperature coefficient Q10

The temperature coefficient $Q_{10}$ quantifies how much a rate changes for a 10 °C rise in temperature. It is defined as

$Q_{10} = \frac{R_2}{R_1}$

where $R_2$ is the rate at the higher temperature and $R_1$ is the rate 10 °C lower. For most enzyme-controlled reactions across the physiological range, $Q_{10} \approx 2$ — the rate roughly doubles for every 10 °C. This is exactly why an ectotherm's metabolic rate, and hence its activity, climbs steeply as the environment warms, and why an endotherm invests so heavily in holding its core temperature stable: a wandering core temperature would mean a wandering metabolic rate.

Worked example. A lizard's oxygen consumption is measured at 15 °C as 0.8 cm³ g⁻¹ h⁻¹ and at 25 °C as 1.7 cm³ g⁻¹ h⁻¹. Calculate $Q_{10}$ .

$Q_{10} = \frac{R_2}{R_1} = \frac{1.7}{0.8} = 2.1$

A $Q_{10}$ of 2.1 confirms the near-doubling expected of an enzyme-limited process, and tells the examiner you understand why ectotherm activity tracks ambient temperature. Where the two temperatures are not exactly 10 °C apart, the general form is $Q_{10} = (R_2/R_1)^{10/(T_2 - T_1)}$ — but OCR items almost always give you a clean 10 °C interval, so check the temperature difference before reaching for the exponent.

Glomerular filtration rate

Glomerular filtration rate (GFR) is the volume of filtrate formed by both kidneys per unit time — around 120 cm³ min⁻¹, or roughly 180 dm³ per day, in a healthy adult. Because almost all of that filtrate is reabsorbed, only about 1–2 dm³ leaves the body as urine. A falling GFR is the single most important clinical marker of declining kidney function, which is why it recurs in the kidney-failure lesson.

Worked example. If GFR is 120 cm³ min⁻¹, what volume of filtrate is produced in 24 hours?

$120 \text{ cm}^3\text{ min}^{-1} \times 60 \text{ min} \times 24 \text{ h} = 172{,}800 \text{ cm}^3 = 172.8 \text{ dm}^3$

Notice the unit discipline: convert minutes to hours before you touch the volume, and convert cm³ to dm³ only at the end by dividing by 1000. Premature unit conversion is one of the commonest ways to drop the accuracy mark on this question type.

Renal clearance

Renal clearance measures how effectively the kidney removes a given substance from the blood — the volume of plasma completely cleared of that substance per minute. It is the quantitative bridge between the filtration you meet in this course and the clinical assessment of kidney function, and it is calculated as

$C = \frac{U \times V}{P}$

where $C$ is clearance (cm³ min⁻¹), $U$ is the urine concentration of the substance, $V$ is the rate of urine production (cm³ min⁻¹) and $P$ is the plasma concentration of the substance. Both concentrations must be in the same units for them to cancel.

Worked example. For a substance with urine concentration 70 mg cm⁻³, plasma concentration 1.0 mg cm⁻³ and urine production of 1.5 cm³ min⁻¹:

$C = \frac{U \times V}{P} = \frac{70 \times 1.5}{1.0} = 105 \text{ cm}^3\text{ min}^{-1}$

The plasma clearance is 105 cm³ min⁻¹. A high clearance relative to GFR indicates a substance that is both filtered and secreted into the tubule; a clearance close to zero indicates a substance that is almost completely reabsorbed (glucose in a healthy person clears at nearly zero because the sodium-glucose co-transporters reclaim it all). This is why the clearance of a marker that is freely filtered but neither reabsorbed nor secreted gives an estimate of GFR itself — a synoptic point examiners reward at the top band.

Exam tip — units are half the marks. On every Module 5 calculation, write the unit at every stage, not just at the end. A correct numerical answer with the wrong or missing unit routinely loses the final accuracy mark, and a wrong number carried forward with correct method and units can still earn the method marks. Show the substitution line ( $C = U \times V / P = \ldots$ ) before the arithmetic so the examiner can award M1 even if you slip on the calculator.

Kidney Failure and Detection in Urine

The kidney failure and urine detection lesson develops the clinical and analytical applications. Chronic kidney failure (glomerulonephritis, diabetic nephropathy, hypertension-driven nephrosclerosis) eventually drops glomerular filtration rate below the threshold compatible with metabolic stability. Treatment options include haemodialysis (blood pumped across a semipermeable membrane against a dialysate of carefully controlled composition, allowing waste solutes to diffuse out and useful solutes to be retained), peritoneal dialysis (using the peritoneal membrane as the exchange surface) and renal transplantation.

Urine analysis is examined in two distinct contexts: detection of protein or glucose as markers of glomerular damage or hyperglycaemia respectively, and detection of human chorionic gonadotrophin (hCG) in pregnancy testing via monoclonal antibodies (revisited synoptically with the antibody content of Module 4). A common pitfall is to describe dialysis as "filtering the blood like a kidney" without specifying that the exchange is driven by concentration gradients across the dialyser membrane rather than by hydrostatic pressure.

Linking to the Other Courses

Communication, homeostasis and excretion is one of the most synoptic courses on H420. The urea cycle that drives detoxification routes back through the amino-acid biochemistry of biological molecules and forwards into the link reaction and Krebs cycle of photosynthesis and respiration, because the carbon skeletons left after deamination feed directly into central metabolism. The sodium-potassium ATPase that drives reabsorption in the PCT is the same pump introduced in biological membranes and cell division and reused in neuronal and hormonal communication to maintain the resting potential. The hypothalamic-pituitary axis that controls ADH release is developed in detail in neuronal and hormonal communication, and the negative-feedback grammar established here recurs in blood glucose control and in the hormonal aspects of the menstrual cycle. Even the Kupffer-cell phagocytosis introduced in liver structure connects forward to the innate immune responses of Module 4.

Required Practicals / PAGs

This course anchors three OCR Practical Activity Groups (PAGs):

PAG 2 (Dissection) is anchored by the kidney structure lesson, where a mammalian kidney dissection allows direct observation of the cortex-medulla boundary, the renal pelvis, and the calyces draining into the ureter.
PAG 10 (Data logger and computer-modelling) is anchored by the thermoregulation lesson (temperature traces in ectotherm and endotherm simulations) and by the negative and positive feedback lesson (modelled feedback-loop dynamics).
PAG 11 (Research skills) is reinforced across the course through the integration of physiological, anatomical and clinical literature in the kidney failure lesson.

Exam Technique for Module 5

Module 5 questions cluster around a small number of predictable command words, and reading the command word correctly is the difference between a full-mark and a half-mark response.

Describe asks only for observable detail — the sequence of events in the cardiac-independent nephron, the segments of the loop of Henle and their permeabilities. No causal explanation is required, but the detail must be complete and in order.
Explain asks for the mechanism and the because. "Explain how ADH increases water reabsorption" is not answered by "ADH is released" — it is answered by tracing the cascade: fall in blood water potential → osmoreceptor stimulation → ADH release from the posterior pituitary → binding to collecting-duct receptors → aquaporin-2 insertion → raised water permeability → water follows the medullary osmotic gradient → smaller volume of more concentrated urine.
Suggest flags that the exact scenario is not in the specification and the examiner wants you to apply principles to an unfamiliar case — a novel diuretic, an unusual animal, a disease that alters one variable. Marks are for reasoning from what you know, not for recall.
Compare requires linked points ("whereas", "in contrast to") that address both items in the same sentence — ectotherm versus endotherm, negative versus positive feedback, the descending versus the ascending limb.

A high-yield habit for this module specifically: whenever a question involves a regulated variable, name the receptor, the coordination centre, the effector and the corrective direction explicitly. Examiners hand out marks for each component of the control loop, and the most common way to lose them is to jump from stimulus to response without naming the intervening machinery.

The highest-yield common mistakes, gathered

Across the ten lessons, the same handful of errors recur in mark schemes. Rehearse the correct version of each:

Common mistake	Why it loses marks	The mark-earning version
"Homeostasis keeps the internal environment constant"	Implies a static value	The variable oscillates around a set point within a narrow range; the system is dynamic
"Blood moves closer to the skin surface" in vasodilation	No vessel migrates	Superficial arterioles dilate, so more blood flows through the surface capillary beds
"Sweating removes heat"	Vague, no mechanism	Heat is lost as the latent heat of vaporisation when water evaporates from the skin
"The liver produces urine"	Confuses two organs	The liver produces urea; the kidney produces urine
"ADH makes water move"	ADH is not a pump	ADH sets collecting-duct permeability; water follows the pre-existing osmotic gradient
"Glucose is filtered out of the blood"	Ignores reabsorption	Glucose is filtered into the filtrate then reabsorbed; urinary glucose signals pathology or co-transporter saturation
"The basement membrane filters ions"	Confuses size and charge	The basement membrane is a size filter (~69 kDa cut-off); small ions pass freely
Faeces described as "excreted"	Never crossed the gut epithelium	Faeces are egested; only bile pigments in faeces are genuinely excretory

Going further. Undergraduate physiology develops the medullary counter-current mechanism with the concept of single-effect multiplication and the role of the vasa recta as a counter-current exchanger that preserves the gradient the loop builds. Reading around the vasa recta, and the way urea recycling contributes up to half of the deep-medullary osmolarity, is excellent preparation for a biomedical or physiology admissions interview and directly deepens the collecting-duct reasoning examined here.

Mini-FAQ

Is the loop of Henle a counter-current multiplier or a counter-current exchanger? The loop itself is the multiplier — it actively builds the medullary gradient using ATP-driven salt pumping in the ascending limb. The vasa recta (the capillaries running alongside) act as a counter-current exchanger that supplies the medulla with blood without washing the gradient away. OCR expects the multiplier terminology for the loop.

Why is the ascending limb impermeable to water? If water could follow the salt out of the ascending limb, the salt gradient would immediately dissipate by osmosis and no concentration could build. Impermeability to water is precisely what lets the limb pump salt into the interstitium and leave the tubular fluid dilute — the mechanistic heart of the whole system.

Does the liver "detoxify" or "excrete" alcohol? The liver detoxifies it — alcohol dehydrogenase and aldehyde dehydrogenase convert ethanol to ethanoate, which enters respiration. Excretion (removal from the body) is a separate concept; the liver's role here is chemical modification, not removal.

Is fever an example of negative or positive feedback? Fever is a regulated shift of the set point driven by cytokines acting on the hypothalamus, defended by ordinary negative feedback around the new higher set point. It is a favourite discriminator because students often mislabel it as a feedback failure.

How is GFR different from renal clearance? GFR is the total volume of filtrate formed per minute. Clearance is substance-specific — the volume of plasma cleared of one particular solute per minute — and for a marker that is freely filtered but neither reabsorbed nor secreted, clearance equals GFR. That equivalence is the top-band synoptic point.

Closing

Communication, homeostasis and excretion is the conceptual hub of Module 5: the negative-feedback grammar built here returns in neuronal and hormonal communication and again in plant signalling, and the histological reasoning around the liver lobule and the nephron is the highest-yield anatomical content on the entire H420 specification. Start with the course landing page and work through all ten lessons in sequence. A quick-win revision tip: sketch a labelled nephron diagram from memory each day for a week, marking the permeability of every segment, and you will lock in the marks examiners hand out most reliably on Paper 2. From there, the rest of Module 5 is a series of variations on the same control-system theme rather than a list of disconnected facts.

OCR A-Level Biology: Communication, Homeostasis and Excretion

Guide Overview

OCR H420 Specification Coverage

Sub-topic	Spec area	Primary lesson(s)
The need for communication systems	5.1.1	Communication and Homeostasis Principles
Principles of homeostasis; receptors, effectors and feedback	5.1.1	Communication and Homeostasis Principles; Negative and Positive Feedback
Temperature control in ectotherms and endotherms	5.1.1	Thermoregulation
Excretion as the removal of metabolic waste	5.1.2	Excretion Overview
Liver histology and the hepatic blood supply	5.1.2	Liver Structure
Liver functions: detoxification, urea synthesis, storage	5.1.2	Liver Functions
Kidney gross anatomy and the nephron	5.1.2	Kidney Structure and the Nephron
Ultrafiltration at the renal capsule and selective reabsorption in the PCT	5.1.2	Ultrafiltration and Selective Reabsorption
Osmoregulation, counter-current multiplier and the ADH cascade	5.1.2	Osmoregulation
Kidney failure, dialysis, transplantation and urine analysis	5.1.2	Kidney Failure and Detection in Urine

Communication and Homeostasis Principles

Negative and Positive Feedback

Thermoregulation

Excretion Overview

Liver Structure

Liver Functions

Kidney Structure and the Nephron

Ultrafiltration and Selective Reabsorption

Osmoregulation

Quantitative Skills: Q10, GFR and Renal Clearance

The temperature coefficient Q10

The temperature coefficient $Q_{10}$ quantifies how much a rate changes for a 10 °C rise in temperature. It is defined as

$Q_{10} = \frac{R_2}{R_1}$

Worked example. A lizard's oxygen consumption is measured at 15 °C as 0.8 cm³ g⁻¹ h⁻¹ and at 25 °C as 1.7 cm³ g⁻¹ h⁻¹. Calculate $Q_{10}$ .

$Q_{10} = \frac{R_2}{R_1} = \frac{1.7}{0.8} = 2.1$

Glomerular filtration rate

Worked example. If GFR is 120 cm³ min⁻¹, what volume of filtrate is produced in 24 hours?

$120 \text{ cm}^3\text{ min}^{-1} \times 60 \text{ min} \times 24 \text{ h} = 172{,}800 \text{ cm}^3 = 172.8 \text{ dm}^3$

Renal clearance

$C = \frac{U \times V}{P}$

Worked example. For a substance with urine concentration 70 mg cm⁻³, plasma concentration 1.0 mg cm⁻³ and urine production of 1.5 cm³ min⁻¹:

$C = \frac{U \times V}{P} = \frac{70 \times 1.5}{1.0} = 105 \text{ cm}^3\text{ min}^{-1}$

Exam tip — units are half the marks. On every Module 5 calculation, write the unit at every stage, not just at the end. A correct numerical answer with the wrong or missing unit routinely loses the final accuracy mark, and a wrong number carried forward with correct method and units can still earn the method marks. Show the substitution line ( $C = U \times V / P = \ldots$ ) before the arithmetic so the examiner can award M1 even if you slip on the calculator.

Kidney Failure and Detection in Urine

Linking to the Other Courses

Required Practicals / PAGs

This course anchors three OCR Practical Activity Groups (PAGs):

PAG 2 (Dissection) is anchored by the kidney structure lesson, where a mammalian kidney dissection allows direct observation of the cortex-medulla boundary, the renal pelvis, and the calyces draining into the ureter.
PAG 10 (Data logger and computer-modelling) is anchored by the thermoregulation lesson (temperature traces in ectotherm and endotherm simulations) and by the negative and positive feedback lesson (modelled feedback-loop dynamics).
PAG 11 (Research skills) is reinforced across the course through the integration of physiological, anatomical and clinical literature in the kidney failure lesson.

Exam Technique for Module 5

Module 5 questions cluster around a small number of predictable command words, and reading the command word correctly is the difference between a full-mark and a half-mark response.

Describe asks only for observable detail — the sequence of events in the cardiac-independent nephron, the segments of the loop of Henle and their permeabilities. No causal explanation is required, but the detail must be complete and in order.
Explain asks for the mechanism and the because. "Explain how ADH increases water reabsorption" is not answered by "ADH is released" — it is answered by tracing the cascade: fall in blood water potential → osmoreceptor stimulation → ADH release from the posterior pituitary → binding to collecting-duct receptors → aquaporin-2 insertion → raised water permeability → water follows the medullary osmotic gradient → smaller volume of more concentrated urine.
Suggest flags that the exact scenario is not in the specification and the examiner wants you to apply principles to an unfamiliar case — a novel diuretic, an unusual animal, a disease that alters one variable. Marks are for reasoning from what you know, not for recall.
Compare requires linked points ("whereas", "in contrast to") that address both items in the same sentence — ectotherm versus endotherm, negative versus positive feedback, the descending versus the ascending limb.

The highest-yield common mistakes, gathered

Across the ten lessons, the same handful of errors recur in mark schemes. Rehearse the correct version of each:

Common mistake	Why it loses marks	The mark-earning version
"Homeostasis keeps the internal environment constant"	Implies a static value	The variable oscillates around a set point within a narrow range; the system is dynamic
"Blood moves closer to the skin surface" in vasodilation	No vessel migrates	Superficial arterioles dilate, so more blood flows through the surface capillary beds
"Sweating removes heat"	Vague, no mechanism	Heat is lost as the latent heat of vaporisation when water evaporates from the skin
"The liver produces urine"	Confuses two organs	The liver produces urea; the kidney produces urine
"ADH makes water move"	ADH is not a pump	ADH sets collecting-duct permeability; water follows the pre-existing osmotic gradient
"Glucose is filtered out of the blood"	Ignores reabsorption	Glucose is filtered into the filtrate then reabsorbed; urinary glucose signals pathology or co-transporter saturation
"The basement membrane filters ions"	Confuses size and charge	The basement membrane is a size filter (~69 kDa cut-off); small ions pass freely
Faeces described as "excreted"	Never crossed the gut epithelium	Faeces are egested; only bile pigments in faeces are genuinely excretory

Going further. Undergraduate physiology develops the medullary counter-current mechanism with the concept of single-effect multiplication and the role of the vasa recta as a counter-current exchanger that preserves the gradient the loop builds. Reading around the vasa recta, and the way urea recycling contributes up to half of the deep-medullary osmolarity, is excellent preparation for a biomedical or physiology admissions interview and directly deepens the collecting-duct reasoning examined here.

OCR A-Level Biology: Communication, Homeostasis and Excretion

Guide Overview

OCR H420 Specification Coverage

Communication and Homeostasis Principles

Negative and Positive Feedback

Thermoregulation

Excretion Overview

Liver Structure

Liver Functions

Kidney Structure and the Nephron

Ultrafiltration and Selective Reabsorption

Osmoregulation

Quantitative Skills: Q10, GFR and Renal Clearance

The temperature coefficient Q10

Glomerular filtration rate

Renal clearance

Kidney Failure and Detection in Urine

Linking to the Other Courses

Required Practicals / PAGs

Exam Technique for Module 5

The highest-yield common mistakes, gathered

Mini-FAQ

Closing

Related Reading

Stay in the loop

OCR A-Level Biology: Communication, Homeostasis and Excretion

Guide Overview

OCR H420 Specification Coverage

Communication and Homeostasis Principles

Negative and Positive Feedback

Thermoregulation

Excretion Overview

Liver Structure

Liver Functions

Kidney Structure and the Nephron

Ultrafiltration and Selective Reabsorption

Osmoregulation

Quantitative Skills: Q10, GFR and Renal Clearance

The temperature coefficient Q10

Glomerular filtration rate

Renal clearance

Kidney Failure and Detection in Urine

Linking to the Other Courses

Required Practicals / PAGs

Exam Technique for Module 5

The highest-yield common mistakes, gathered

Mini-FAQ

Closing

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