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Homeostasis and Human Urinary System, worked answers

Fully worked answers for Homeostasis and Human Urinary System, original structured and essay questions with mark-scheme keywords highlighted.

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How this topic is examined

Describing ultrafiltration and reabsorption in the nephron in the correct order.
Explaining osmoregulation, including the role of ADH in adjusting water reabsorption.
Comparing the composition of blood, glomerular filtrate and urine.
Explaining thermoregulation using vasodilation and vasoconstriction of skin blood vessels.
Describing how insulin and glucagon keep blood glucose within a narrow range.
Linking a urinary health issue, such as a kidney stone or infection, to the part of the system it affects.

Model answer structure

Read the command word and answer to the marks, one clear point per mark.
Define the key biological term precisely before you explain it.
Explain the process or reason in the correct sequence, using the right terms.
Where useful, add a labelled diagram or a worked example.
End with the link the question asks for (cause → effect, structure → function).

Fully worked answers

Explain how ultrafiltration occurs in the glomerulus.

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The **afferent arteriole** carrying blood into the glomerulus is **wider** than the **efferent arteriole** carrying it out, which builds up a **high hydrostatic pressure** in the glomerular capillaries. This pressure forces **water, glucose, salts and urea** out of the blood, across the filtration membrane and into the **Bowman capsule** as glomerular filtrate. **Large plasma proteins and blood cells** cannot pass through the membrane, so they stay in the blood.

afferent wider than efferenthigh hydrostatic pressurefiltration membraneBowman capsuleproteins and blood cells too large

Explain why glucose is present in the glomerular filtrate but absent from the urine of a healthy person.

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Glucose molecules are **small enough to be filtered** out of the blood during ultrafiltration, so they appear in the **glomerular filtrate**. As the filtrate flows along the **proximal convoluted tubule**, glucose is completely taken back into the blood by **selective reabsorption**, because it is a useful nutrient. This reabsorption uses **active transport**, so all the glucose is recovered and none is left in the urine. If glucose does appear in urine it suggests the blood glucose level is abnormally high, as in **diabetes**.

small enough to be filteredselective reabsorptionproximal convoluted tubuleactive transportdiabetes

A person loses a lot of water through sweating on a hot day. Explain how the body increases water reabsorption by the kidney.

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Losing water makes the **blood more concentrated**, lowering its **water potential**. **Osmoreceptors in the hypothalamus** detect this change, and the **pituitary gland** releases **more ADH** into the blood. ADH makes the walls of the **distal tubule and collecting duct more permeable to water**, so **more water is reabsorbed** back into the blood. The result is a **small volume of concentrated urine**, which conserves water and returns the blood water potential toward normal by **negative feedback**.

blood more concentratedosmoreceptors in hypothalamusmore ADH releasedcollecting duct more permeablemore water reabsorbedconcentrated urine

Explain how the body cools down when its temperature rises above normal.

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Temperature receptors in the **skin and hypothalamus** detect the rise. The **arterioles near the skin surface dilate (vasodilation)**, so more blood flows close to the surface and more heat is lost by **radiation**. The **sweat glands secrete more sweat**, and as the sweat **evaporates it absorbs heat energy** from the skin, cooling the body. These responses **oppose the rise** in temperature and return it toward the set point, which is **negative feedback**.

hypothalamusvasodilationheat lost by radiationsweating and evaporationnegative feedback

Explain how insulin and glucagon keep the blood glucose level within a narrow range.

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Both hormones are produced by the **pancreas**. After a meal the blood glucose level **rises**, so **insulin** is released; it makes **liver and muscle cells take up glucose and store it as glycogen**, which **lowers** the level back toward normal. Between meals the level **falls**, so **glucagon** is released; it makes the liver **break down glycogen into glucose** and release it, which **raises** the level back toward normal. The two hormones have **opposite effects**, and each response opposes the change, so the level is controlled by **negative feedback**.

pancreasinsulin lowersglycogen storageglucagon raisesglycogen broken downnegative feedback

Describe how haemodialysis removes waste from the blood of a patient with kidney failure.

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The patient blood is passed through a **dialysis machine** on one side of a **partially permeable membrane**, with **dialysis fluid** flowing on the other side. The dialysis fluid has **no urea** and a **normal level of glucose and salts**, so **urea and excess salts diffuse out** of the blood down their **concentration gradients**, while glucose and useful salts do not leave. The cleaned blood is then returned to the patient.

partially permeable membranedialysis fluidno urea in fluiddiffusion down concentration gradientuseful substances retained

Explain what is meant by negative feedback and why it is important in homeostasis.

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Negative feedback is a control mechanism in which a **change away from the set point** is **detected by a receptor**, processed by a **control centre**, and corrected by an **effector** whose response **opposes the change**. Because the response always pushes the condition **back toward normal** rather than further away, the internal environment is kept within **narrow limits**. This keeps temperature, blood glucose and water content stable enough for **enzymes and cells to function normally**.

change detected by receptorcontrol centreeffector opposes changereturns to set pointstable internal environment

Phrasing that earns marks

Homeostasis: Homeostasis is the maintenance of a constant internal environment, temperature, blood glucose, water content and pH, despite changes outside the body. It keeps conditions stable enough for enzymes and cells to function normally, and is controlled through negative feedback involving the nervous and endocrine systems. Without homeostasis, small outside changes could disrupt enzyme activity and cell metabolism throughout the body.
Urinary system: The urinary system removes urea and excess water and salts from the blood. The kidneys filter the blood continuously; ureters carry the urine they produce to the bladder, which stores it; the urethra then carries urine out of the body during urination. This removal of urea, a waste product of protein breakdown, is why the urinary system is described as an excretory organ system.
The nephron: The nephron is the kidney's individual filtering and processing unit, made of the glomerulus, Bowman's capsule and a system of tubules. Each kidney contains a very large number of nephrons working in parallel, which is why kidney function can still be adequate even if some nephrons are damaged. This same arrangement explains why a person can stay healthy with only one functioning kidney.
Ultrafiltration: High blood pressure in the glomerulus forces water, glucose, salts and urea out of the blood and into Bowman's capsule, forming the glomerular filtrate. Large plasma proteins and blood cells are too big to pass through the filter and stay in the blood. This high pressure exists because the vessel carrying blood into the glomerulus is wider than the one carrying it out.
Selective reabsorption: As the filtrate flows along the tubule, useful substances are reabsorbed back into the blood: all of the glucose, most of the water, and some of the salts, depending on the body's needs. What remains becomes urine. This process is described as selective because different substances are reabsorbed to different extents rather than all being treated the same way.
Osmoregulation and kidney health: Antidiuretic hormone (ADH) adjusts how much water the tubules reabsorb, concentrating or diluting urine to keep blood water content steady. When the kidneys fail, waste and excess water build up in the blood, and treatment is by dialysis or a kidney transplant. More ADH is released when the body is short of water, producing a smaller volume of more concentrated urine.
Negative feedback: Negative feedback is the general control pattern behind homeostasis: a receptor detects a change away from the normal level, a control centre triggers a response, and an effector corrects the change so the condition returns toward the set point. Thermoregulation, osmoregulation and blood glucose control all follow this same pattern, which is why learning one example well makes the others easier to explain.
Thermoregulation: The skin and the hypothalamus in the brain work together to keep body temperature constant. When the body is too warm, skin blood vessels dilate and sweat glands become more active to lose heat; when it is too cold, skin blood vessels constrict and muscles may shiver to generate heat. Hair standing on end (goosebumps) is a related response that traps a thin layer of warm air near the skin.

Frequently asked questions

What is homeostasis?

Homeostasis is the maintenance of a constant internal environment in the body, such as steady body temperature, blood glucose level, water content and pH, despite changes outside. It keeps conditions ideal for enzymes and cells to work, and is controlled by organs including the kidneys, liver, skin and the nervous and endocrine systems. Almost every homeostatic mechanism in the body follows the same negative feedback pattern of detecting, responding to and correcting a change.

How is urine formed in the nephron?

First, ultrafiltration: high pressure in the glomerulus forces water, glucose, salts and urea out of the blood into the Bowman's capsule, while large proteins and blood cells stay behind. Then selective reabsorption: as the filtrate flows along the tubule, useful substances such as all the glucose, some salts and much of the water are reabsorbed into the blood. What remains, mainly urea, excess salts and water, becomes urine. This urine then flows through the ureter to the bladder, where it is stored before being released.

Why is there normally no glucose in urine?

Glucose is small enough to be filtered out during ultrafiltration, but it is a useful nutrient, so it is completely reabsorbed back into the blood during selective reabsorption. Healthy urine therefore contains no glucose. If glucose appears in urine, it often means blood glucose is too high, as in diabetes.