Unit 9 Excretion and homeostasis
Key unit competence: Explain the principles of excretion and osmoregulation and interpret good health as a result of coordinated homeostatic mechanisms by feedback.
Key ideas
- Excretion is the removal of metabolic waste made by cells (CO₂, urea, excess water and salts, bile pigments). It differs from egestion (removal of undigested food). Excretory organs: lungs (CO₂, water vapour), kidneys (urea, salts, water), skin (sweat: water, salts, some urea), liver (bile pigments from haemoglobin breakdown; makes urea by deamination). Unicellular organisms excrete by diffusion across the cell membrane; freshwater protists remove water with contractile vacuoles.
- Homeostasis is the maintenance of a constant internal environment (tissue fluid) within narrow limits: temperature, blood glucose, water potential, pH, CO₂ and O₂. A control system has a receptor, a coordinator (control centre), and an effector. Negative feedback reverses a change back to the set point (most homeostasis). Positive feedback increases a change (oxytocin in birth, blood clotting).
- Blood glucose (about 4–6 mmol dm⁻³, ~90 mg/100 cm³) is controlled by the islets of Langerhans in the pancreas. A rise is detected by β cells, which secrete insulin: cells take up more glucose (more GLUT channels), liver and muscles convert glucose to glycogen (glycogenesis), respiration increases, fat is made. A fall is detected by α cells, which secrete glucagon: liver breaks down glycogen (glycogenolysis) and makes glucose from amino acids and glycerol (gluconeogenesis). Adrenaline also raises blood glucose.
- Diabetes mellitus: type 1 (insulin-dependent) — β cells destroyed (autoimmune), usually starts young, treated by insulin injections; type 2 — cells become resistant to insulin, linked to obesity, low exercise, age; treated by diet, exercise, drugs. Symptoms: high blood glucose, glucose in urine (glycosuria), thirst, frequent urination, tiredness, weight loss. Glucose is tested in urine with Benedict's or test strips (glucose oxidase).
- Temperature regulation: endotherms (mammals, birds) produce heat metabolically and keep a constant body temperature; ectotherms (reptiles, fish, amphibians) rely on the environment and use behaviour (basking, burrowing). Enzymes work best at an optimum; too hot → denaturation; too cold → slow reactions.
- The hypothalamus is the thermoregulatory centre: it detects blood temperature and receives impulses from skin receptors. Too hot: vasodilation of skin arterioles, sweating (evaporation removes latent heat), hairs lie flat, lower metabolic rate. Too cold: vasoconstriction, shivering, hair erector muscles contract (trap air), increased thyroxine and adrenaline raise metabolism, behaviour (warm clothes).
- Kidney structure: cortex, medulla (pyramids), pelvis, ureter; renal artery and vein. The nephron: Bowman's capsule around the glomerulus, proximal convoluted tubule (PCT), loop of Henle, distal convoluted tubule (DCT), collecting duct.
- Urine formation: (1) Ultrafiltration: high pressure in the glomerulus (afferent arteriole wider than efferent) forces small molecules (water, glucose, urea, salts, amino acids) through the capillary endothelium, basement membrane (the filter) and podocytes; proteins and cells stay in blood. (2) Selective reabsorption in the PCT: all glucose and amino acids, most salts and water return to blood (microvilli, many mitochondria, active transport and co-transport). (3) Loop of Henle: a counter-current multiplier builds a high salt concentration in the medulla. (4) Collecting duct: water reabsorbed by osmosis under ADH control.
- Osmoregulation: osmoreceptors in the hypothalamus detect low blood water potential (dehydration); the posterior pituitary releases ADH; ADH makes the DCT and collecting duct more permeable (aquaporins) so more water is reabsorbed → small volume of concentrated urine. When water potential is high, less ADH → large volume of dilute urine. Lack of ADH causes diabetes insipidus.
- Kidney failure is treated by dialysis (blood passes along a partially permeable membrane against dialysis fluid with normal glucose and salt levels but no urea, flowing counter-current) or a kidney transplant (needs tissue matching and immunosuppressant drugs). Animals and environment: freshwater fish gain water by osmosis, produce much dilute urine, absorb salts actively by gills; marine bony fish lose water, drink sea water, excrete salts by gills, little urine; terrestrial animals conserve water (long loops of Henle in desert mammals, e.g. kangaroo rat; uric acid in birds and insects).
Common misconceptions
- "Excretion and egestion are the same." No: egestion removes undigested food that never entered cells; excretion removes metabolic waste.
- "Insulin turns glucose into glucagon." No: insulin promotes conversion of glucose into glycogen; glucagon is a separate hormone.
- "Sweating cools you because sweat is cold." No: cooling happens when sweat evaporates, taking latent heat from the skin.
- "ADH makes more urine." No: ADH (anti-diuretic) reduces urine volume by increasing water reabsorption.
Worked examples
Example 1. Glomerular filtrate forms at 125 cm³ per minute and urine at 1 cm³ per minute. Calculate the percentage of filtrate reabsorbed.
Solution
Reabsorbed = 125 − 1 = 124 cm³ min⁻¹. Percentage = 124 ÷ 125 × 100 = 99.2%.
Example 2. After a meal, blood glucose rises from 5 to 8 mmol dm⁻³ and returns to 5 within two hours. Explain this.
Solution
β cells of the islets detect the rise and secrete insulin. Insulin increases glucose uptake into cells, stimulates glycogenesis in liver and muscle and increases respiration. Blood glucose falls back to the set point; insulin secretion then falls (negative feedback).
Practice questions
Distinguish between excretion and egestion and name four excretory products.
Show answer
Excretion removes metabolic waste made by cells; egestion removes undigested food. Products: CO₂, urea, excess water, salts, bile pigments, uric acid.
Common mistake: Calling faeces an excretory product.
Explain the principle of negative feedback with an example.
Show answer
A change from the set point is detected by receptors; the control centre causes effectors to reverse the change; when normal is restored, the corrective action stops. E.g. rise in blood glucose → insulin → glucose falls.
Common mistake: Describing positive feedback instead.
Describe how blood glucose is lowered after a meal.
Show answer
β cells detect the rise; insulin secreted; more glucose enters cells; liver and muscle make glycogen; more respiration; fat synthesis; glucose returns to normal.
Common mistake: Saying glucagon lowers blood glucose.
Compare type 1 and type 2 diabetes.
Show answer
Type 1: no insulin made, β cells destroyed, often early onset, insulin injections. Type 2: insulin made but cells resistant, linked to obesity and inactivity, later onset, diet, exercise and drugs.
Common mistake: Saying type 2 is caused by eating sugar alone.
Explain how the body responds to a cold environment.
Show answer
Skin receptors and hypothalamus detect fall; vasoconstriction reduces heat loss; shivering produces heat; hair erector muscles contract; more thyroxine and adrenaline raise metabolism; less sweating.
Common mistake: Saying blood vessels move away from the skin.
Describe ultrafiltration in the nephron.
Show answer
Afferent arteriole wider than efferent gives high pressure; water and small solutes forced through capillary endothelium, basement membrane and podocytes into Bowman's capsule; cells and proteins remain in blood.
Common mistake: Saying ultrafiltration uses active transport.
Explain how ADH controls the water content of blood.
Show answer
Low water potential detected by osmoreceptors in hypothalamus; posterior pituitary releases ADH; collecting duct more permeable; more water reabsorbed; small volume of concentrated urine; negative feedback.
Common mistake: Saying ADH makes more urine.
Compare osmoregulation in freshwater and marine bony fish.
Show answer
Freshwater: gain water by osmosis, little drinking, large volume of dilute urine, actively absorb salts at gills. Marine: lose water by osmosis, drink sea water, excrete salts at gills, small volume of urine.
Common mistake: Swapping the two environments.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. ADH makes the collecting duct: a. more permeable to water 2. All glucose in the filtrate is normally reabsorbed in the: b. proximal convoluted tubule 3. Type 1 diabetes is treated mainly by: c. contractile vacuoles 4. Freshwater protists remove excess water using: d. insulin injections Show answer
1 → a (more permeable to water) · 2 → b (proximal convoluted tubule) · 3 → d (insulin injections) · 4 → c (contractile vacuoles)
- Aquaporins are inserted.
- By co-transport.
- β cells are destroyed.
- Water enters by osmosis.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
hypothalamus · medulla · break down glycogen to glucose · cerebellum- Glucagon causes the liver to .
- The part of the kidney containing the loops of Henle is mainly the .
- The thermoregulatory centre is the .
Show answer
1: break down glycogen to glucose · 2: medulla · 3: hypothalamus
- Glycogenolysis.
- It has a high salt concentration.
- It detects blood temperature.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Which is a positive feedback example? a. glomerulus and Bowman's capsule 2. Urea is made in the: b. oxytocin during childbirth 3. Insulin is secreted by: c. β cells of the pancreas 4. Ultrafiltration takes place in the: d. liver Show answer
1 → b (oxytocin during childbirth) · 2 → d (liver) · 3 → c (β cells of the pancreas) · 4 → a (glomerulus and Bowman's capsule)
- The change is increased.
- By deamination of amino acids.
- In the islets of Langerhans.
- High blood pressure forces filtrate out.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
drinking sea water and excreting salt · producing large amounts of dilute urine · posterior pituitary gland · 99%- Marine bony fish keep their water balance by .
- ADH is released from the .
- Filtrate forms at 120 cm³ min⁻¹ and urine at 1.2 cm³ min⁻¹. The percentage reabsorbed is .
Show answer
1: drinking sea water and excreting salt · 2: posterior pituitary gland · 3: 99%
- They lose water by osmosis.
- It is made in the hypothalamus.
- 118.8 ÷ 120 × 100.