Human Excretion & Kidneys

IGCSE Edexcel Biology
2.70–2.75 Excretion, kidney structure, nephron function and ADH
Key Concepts: Excretion is the removal of metabolic waste products from the body. The kidneys filter blood by ultrafiltration and selectively reabsorb useful substances. ADH controls water reabsorption, regulating blood water potential.

Section A — Excretion and Its Organs

1. Define excretion and state how it differs from egestion. [2]
2. Complete the table for the organs of excretion in humans. [6]
OrganMetabolic waste excretedProcess involved
Lungs
Kidneys
Skin
3. Explain why urea is produced and state where in the body it is made. [3]

Section B — Kidney Structure

4. Name the two regions of the kidney and state which contains the nephrons. [2]
5. State the function of each of the following kidney structures. [4]
StructureFunction
Renal artery
Renal vein
Ureter
Bladder

Section C — Nephron Function

6. Describe the process of ultrafiltration in the Bowman's capsule. Include the role of high blood pressure and the size of molecules that pass through. [4]
7. Explain why glucose is not normally found in urine, even though it passes into the Bowman's capsule during filtration. [3]
8. Name two other useful substances (besides glucose) that are selectively reabsorbed from the filtrate. [2]
9. State which substances remain in the filtrate and are excreted in urine. [2]

Section D — ADH and Water Regulation

10. State where ADH is produced and where it acts. [2]
11. A person becomes dehydrated after exercising in hot weather. Describe how their kidneys respond to restore normal blood water potential. Include the role of ADH in your answer. [4]
12. Compare the urine produced by a dehydrated person with the urine produced by a person who has drunk a large amount of water. [2]

Total marks: 36

Mark Scheme

1. Excretion is the removal of metabolic waste products (produced by chemical reactions in cells) [1]; egestion is the removal of undigested food from the gut (not produced by cells, not metabolic waste) [1] [2]
2. Lungs: CO₂ (and water vapour) / diffusion (gas exchange); Kidneys: urea (and water, salts) / filtration and selective reabsorption; Skin: urea and water (in sweat) / sweating [6 — 2 per row]
3. Excess amino acids cannot be stored [1]; they are deaminated in the liver — the amine group is removed and converted to ammonia, then to the less toxic urea [1]; urea is produced in the liver [1] [3]
4. Cortex and medulla [1]; the nephrons are found in the cortex (and medulla) [1] [2]
5. Renal artery: carries oxygenated blood (with urea) to the kidney; Renal vein: carries filtered blood (with less urea) away from the kidney; Ureter: carries urine from the kidney to the bladder; Bladder: stores urine until it is released [4 — 1 per row]
6. High blood pressure forces fluid out of the capillary (glomerulus) into the Bowman's capsule [1]; small molecules (water, urea, glucose, ions) pass through [1]; large molecules (proteins, red blood cells) are too large to pass through [1]; the fluid that enters the capsule is called the filtrate [1] [4]
7. All glucose in the filtrate is selectively reabsorbed [1]; reabsorption occurs in the tubule (proximal convoluted tubule) [1]; glucose is reabsorbed by active transport back into the blood [1] [3]
8. Any two: water; mineral ions (salts); amino acids [2 — 1 each]
9. Urea [1]; excess water and ions [1] [2]
10. ADH is produced in the hypothalamus (and released from the pituitary gland) [1]; it acts on the collecting duct (and distal tubule) of the nephron [1] [2]
11. Blood water potential falls (blood becomes more concentrated) [1]; the hypothalamus detects this and signals the pituitary to release more ADH [1]; ADH increases the permeability of the collecting duct to water [1]; more water is reabsorbed into the blood, restoring water potential; a small volume of concentrated urine is produced [1] [4]
12. Dehydrated person: small volume of concentrated (dark) urine; person who drank a lot: large volume of dilute (pale) urine [2 — 1 per comparison]