Excretion in Humans
Definition of excretion and its distinction from egestion; the three organs of excretion; urea production by deamination in the liver; and Extended: kidney structure and nephron, ultrafiltration mechanism, selective reabsorption of glucose and amino acids, ADH and osmoregulation feedback loop, and kidney dialysis.
Excretion in Humans
CORE EXTENDEDWhat is Excretion?
Excretion is the removal from the body of the waste products of metabolism.
Excretion removes metabolic waste — substances produced inside body cells as by-products of chemical reactions: CO₂ from respiration, urea from amino acid breakdown, water from metabolic reactions.
Egestion removes undigested food (faeces) through the anus — this material has never been metabolised inside cells and is therefore not excretion.
Organs of Excretion
| Organ | Waste excreted | How it is produced |
|---|---|---|
| Lungs | Carbon dioxide; water vapour | CO₂ and H₂O are produced by aerobic respiration in every body cell; transported in blood to lungs and exhaled |
| Kidneys | Urea; excess water; excess mineral ions | Urea is produced in the liver from excess amino acids; kidneys filter blood and produce urine |
| Skin | Water; mineral ions; small amount of urea | Released in sweat by sweat glands; primarily for temperature regulation, but the products are also metabolic waste |
Urea Production — Deamination
Proteins cannot be stored in the body. When more amino acids are consumed than needed for protein synthesis, the excess must be broken down. This happens in the liver:
Step 1 — Deamination: The amino group (–NH₂) is removed from excess amino acids, producing ammonia (NH₃) and an organic acid (keto acid). This occurs in liver cells.
Step 2 — Urea formation: Ammonia is highly toxic, so it is immediately converted to the less toxic compound urea (CO(NH₂)₂) in the liver via the urea cycle.
Urea is then released into the blood, transported to the kidneys, filtered out, and excreted in urine.
| Substance | Where produced | Why dangerous if not removed | How excreted |
|---|---|---|---|
| Urea | Liver (from deamination of excess amino acids) | Toxic at high concentrations; disrupts cell function | Dissolved in urine via kidneys |
| Carbon dioxide | All body cells (aerobic respiration) | Forms carbonic acid in blood → lowers pH → enzyme denaturation | Exhaled via lungs |
Kidney Structure — Extended
| Structure | Location | Function |
|---|---|---|
| Cortex | Outer region of kidney | Contains the Bowman's capsules and glomeruli (site of ultrafiltration); also contains proximal and distal convoluted tubules |
| Medulla | Inner region of kidney | Contains the loops of Henlé and collecting ducts; involved in concentrating urine |
| Pelvis | Central funnel-shaped cavity | Collects urine from all nephrons and channels it into the ureter |
| Ureter | Tube from kidney to bladder | Carries urine from kidney to bladder |
| Nephron | Microscopic tubule (each kidney has ~1 million) | Functional unit of the kidney — filters blood and produces urine by ultrafiltration and selective reabsorption |
Ultrafiltration — Extended
Location: Bowman's capsule (in the cortex)
Mechanism:
1. Blood enters the glomerulus (a knot of capillaries) via the afferent arteriole. The afferent arteriole is wider than the efferent arteriole — this creates high pressure in the glomerulus.
2. The high pressure forces small molecules from the blood plasma through the capillary walls and into the Bowman's capsule, forming the glomerular filtrate.
3. Molecules small enough to be filtered: water, glucose, amino acids, urea, mineral ions, small drugs.
4. Molecules too large to be filtered (remain in blood): red blood cells, white blood cells, platelets, plasma proteins (e.g. albumin).
Their presence in urine (haematuria, proteinuria) indicates damage to the glomerular filtration barrier — a sign of kidney disease. This is tested in Paper 4 data questions.
Selective Reabsorption — Extended
The glomerular filtrate flows along the nephron tubule. Useful substances are reclaimed back into the blood by selective reabsorption:
| Substance | Reabsorbed? | Where & how |
|---|---|---|
| All glucose | ✓ Completely (normally) | Proximal convoluted tubule — by active transport (against concentration gradient; requires ATP) |
| All amino acids | ✓ Completely | Proximal convoluted tubule — active transport |
| Water | ✓ Most (~99%) | Proximal tubule, loop of Henlé, collecting duct — by osmosis; amount regulated by ADH |
| Mineral ions | ✓ Most (variable) | Proximal and distal tubules — active transport; regulated by hormones |
| Urea | ✗ Not reabsorbed (remains in filtrate) | Passes along the entire nephron and is excreted in urine |
By the time filtrate reaches the proximal convoluted tubule, the glucose concentration in the tubule lumen may still be high. Active transport (using ATP from mitochondria-rich tubule cells) moves glucose against its concentration gradient back into the surrounding capillaries. This is why tubule cells are packed with mitochondria.
Osmoregulation and ADH — Extended
Osmoregulation = the regulation of water content (and therefore solute concentration) of body fluids.
When blood water content is too LOW (e.g. dehydration, sweating):
→ Osmoreceptors in the hypothalamus detect the lower water potential
→ The pituitary gland releases more ADH (antidiuretic hormone) into the blood
→ ADH makes the collecting duct walls more permeable to water
→ More water is reabsorbed from the collecting duct back into blood by osmosis
→ Small volume of concentrated (dark) urine is produced
→ Blood water content rises back to normal (negative feedback)
When blood water content is too HIGH (e.g. drinking large amounts of water):
→ Less ADH released → collecting duct less permeable → less water reabsorbed → large volume of dilute (pale) urine
| Condition | ADH level | Collecting duct permeability | Urine produced |
|---|---|---|---|
| Dehydrated / hot / exercising | High | High (more permeable) | Small volume, concentrated, dark |
| Well-hydrated / cold / at rest | Low | Low (less permeable) | Large volume, dilute, pale |
Kidney Failure and Dialysis — Extended
If the kidneys fail, urea and other waste products accumulate in the blood to dangerous levels. Two treatment options:
Blood is passed through a machine containing a partially permeable membrane surrounded by dialysis fluid. Urea and excess ions diffuse out of the blood across the membrane into the dialysis fluid (down concentration gradient). Glucose and useful ions are kept at the correct concentration in the dialysis fluid so they are not lost from the blood. The patient requires dialysis several times a week for several hours.
A donor kidney (from a living or deceased donor) is surgically implanted. It must be closely matched in tissue type to reduce the risk of rejection. The patient takes immunosuppressant drugs for life to prevent the immune system attacking the donor kidney. A successful transplant restores normal kidney function and removes the need for dialysis.
Dialysis fluid contains glucose at normal blood concentration. If the fluid had no glucose, glucose would diffuse out of the blood down its concentration gradient — the patient would lose vital glucose. By matching blood glucose concentration in the dialysis fluid, no net movement of glucose occurs, so it is conserved.
On a hot day, a person exercises heavily and does not drink. Describe and explain the changes in ADH level and urine production that would occur. [4 marks]
- Sweating causes loss of water from blood → blood water potential decreases (blood becomes more concentrated) [1 mark]
- Osmoreceptors in the hypothalamus detect this → more ADH is released from the pituitary gland [1 mark]
- ADH makes the collecting duct walls more permeable to water → more water is reabsorbed by osmosis back into blood [1 mark]
- Result: a small volume of concentrated (dark) urine is produced; blood water content is restored toward normal [1 mark]
Which row correctly identifies where urea is produced and where it is excreted?
| Produced in | Excreted by | |
|---|---|---|
| A | Kidneys | Liver |
| B | Liver | Lungs |
| C | Liver | Kidneys |
| D | Kidneys | Skin |
Comprehensive Practice Questions
Mixed questions across Topic 13.
Which of the following is an example of excretion?
- A. Undigested food passing out of the anus as faeces
- B. Carbon dioxide leaving the body through the lungs during exhalation
- C. Food being broken down by digestive enzymes in the small intestine
- D. Water being absorbed from the large intestine into the blood
(a) Define excretion. [1 mark]
(b) Explain why the body must remove urea. Include how urea is produced. [3 marks]
(c) State two other substances excreted by the kidneys. [2 marks]
- (a) Excretion is the removal from the body of the waste products of metabolism [1 mark]
- (b) Urea is produced in the liver by deamination of excess amino acids [1 mark]; urea is toxic at high concentrations [1 mark]; if not removed it would accumulate in the blood and disrupt cell function / enzyme activity [1 mark]
- (c) Any two of: excess water / excess mineral ions [2 marks]
(a) Explain what happens during ultrafiltration in the Bowman's capsule. State which substances are filtered and which are not. [4 marks]
(b) Explain why dialysis fluid used in kidney machines contains glucose at normal blood concentration but no urea. [3 marks]
- The afferent arteriole is wider than the efferent arteriole, creating high blood pressure in the glomerulus [1 mark]
- High pressure forces small molecules through the capillary wall into the Bowman's capsule, forming the glomerular filtrate [1 mark]
- Filtered substances: water, glucose, amino acids, urea, mineral ions [1 mark]
- Not filtered (too large): red blood cells, plasma proteins (e.g. albumin), white blood cells [1 mark]
- Dialysis fluid contains no urea so that urea in the blood diffuses out across the partially permeable membrane down its concentration gradient — removing it from the blood [1 mark]
- Dialysis fluid contains glucose at normal blood concentration so that there is no net diffusion of glucose from blood into the fluid [1 mark]
- This ensures the patient does not lose glucose (which is needed for respiration) during dialysis treatment [1 mark]
High-Frequency Mistakes — Topic 13 Overall
- 🔄Confusing excretion and egestionExcretion removes metabolic waste produced inside body cells (CO₂, urea, water). Egestion removes undigested food (faeces) that has never been metabolised. Faeces is NOT excretion — it was never inside the cells.
- 🏠Saying urea is produced in the kidneysUrea is produced in the liver by deamination of excess amino acids. The kidneys only filter urea out of the blood and excrete it in urine. Production site = liver; excretion site = kidneys. Always state both.
- 💧Ext: Saying glucose is filtered out and excreted in urineGlucose IS filtered at the glomerulus but is completely reabsorbed by active transport in the proximal convoluted tubule. Healthy urine contains no glucose. Glucose in urine (glycosuria) indicates diabetes or kidney damage. Only urea, excess water, and excess ions appear in normal urine.
- 🔢Ext: Saying blood cells are filtered at the glomerulusBlood cells and plasma proteins are too large to pass through the filtration barrier and remain in the blood. Their presence in urine indicates kidney damage (glomerulonephritis). Only small molecules (water, glucose, urea, ions, amino acids) are filtered.
- 💧Ext: Confusing ADH effects — high ADH = dilute urine (WRONG)High ADH = concentrated urine (small volume, dark). High ADH makes the collecting duct MORE permeable, so MORE water is reabsorbed, leaving LESS water in the urine. Many students reverse this: remember — ADH prevents you losing water, so high ADH = water retained = concentrated urine.
- 🏭Ext: Saying dialysis removes glucose from the bloodDialysis fluid contains glucose at normal blood concentration, so there is no concentration gradient and no net diffusion of glucose out. If the fluid had no glucose, patients would lose vital glucose every session. The fluid is carefully formulated to remove only waste (urea) and excess ions.
- 🌡Saying the skin is a major excretory organ for ureaThe skin excretes only a small amount of urea in sweat — the kidneys are the primary route for urea excretion. The skin's primary excretory products are water and mineral ions in sweat, and its primary function is temperature regulation, not excretion.
Highest-yield Core items: the definition of excretion (metabolic waste — learn word for word); three organs of excretion with their specific products; urea production by deamination in the liver and excretion by kidneys. For Extended: ultrafiltration mechanism (high pressure → small molecules filtered; large molecules retained); selective reabsorption (glucose and amino acids completely reabsorbed by active transport; urea not reabsorbed); the ADH feedback loop (dehydration → more ADH → more permeable collecting duct → concentrated urine); and dialysis fluid composition with reasoning. The ADH mechanism and dialysis fluid question are near-certain Paper 4 targets.