Unit 9 Excretion in humans
Key unit competence: Explain the need for excretion and how the human urinary system removes wastes and keeps the body healthy.
Key ideas
- Excretion is the removal from the body of waste products of metabolism (made by cells) and of substances in excess. It is different from egestion, the removal of undigested food (faeces) that never entered body cells.
- Excretory products and organs: lungs (carbon dioxide, water vapour), kidneys (urea, excess water and salts in urine), skin (water, salts and a little urea in sweat), liver (makes urea; bile pigments from broken-down red blood cells).
- Wastes must be removed because they are toxic (urea, ammonia), change the pH of body fluids (CO₂ forms acid) or upset the water and salt balance.
- Deamination in the liver: excess amino acids cannot be stored, so the liver removes their nitrogen-containing amino group, which is converted to urea; the rest is used for energy or stored as fat. Urea travels in the blood to the kidneys.
- The urinary system: two kidneys (filter blood), two ureters (carry urine to the bladder), the urinary bladder (stores urine), the urethra (carries urine out), with a sphincter muscle controlling release. The renal artery brings blood to each kidney; the renal vein takes filtered blood away.
- Each kidney has an outer cortex, inner medulla and pelvis. It contains about a million nephrons (kidney tubules), each with a Bowman's capsule around a glomerulus (a knot of capillaries), a proximal convoluted tubule, a loop of Henle, a distal convoluted tubule and a collecting duct.
- Urine formation: (1) Ultrafiltration: high blood pressure in the glomerulus forces water, glucose, salts, urea and amino acids into Bowman's capsule; blood cells and large proteins stay in the blood. (2) Selective reabsorption: all the glucose, amino acids, most water and needed salts are taken back into the blood in the tubules. (3) The remaining fluid, urine (water, urea, excess salts), flows to the collecting duct and pelvis.
- The kidneys also osmoregulate: when the body is short of water (hot day, sweating), more water is reabsorbed and urine is small in volume and concentrated (dark); after drinking a lot, urine is large and dilute (pale).
- Healthy urinary system: drink enough clean water, avoid too much salt, treat urinary infections early, avoid holding urine too long, practise hygiene, control blood pressure and diabetes, avoid misuse of painkillers and alcohol. Problems: kidney stones, infections, kidney failure (treated by dialysis or a kidney transplant).
Common misconceptions
- "Faeces are excretory products." No: faeces are mainly undigested food; their removal is egestion. Only bile pigments in them are excretory.
- "Urea is made in the kidneys." No: urea is made in the liver by deamination; the kidneys remove it from the blood.
- "Glucose in urine is normal." No: all glucose is reabsorbed; glucose in urine is a sign of diabetes.
- "The bladder makes urine." No: the kidneys make urine; the bladder only stores it.
Worked examples
Example 1. Explain why blood cells and proteins are not normally found in urine.
Solution
In ultrafiltration, the walls of the glomerulus and Bowman's capsule act as a filter. Small molecules (water, glucose, urea, salts) pass through, but blood cells and large protein molecules are too big, so they stay in the blood. Protein or blood in urine shows kidney damage.
Example 2. Explain why a learner's urine is dark and small in volume after a long football match on a hot day.
Solution
The learner lost a lot of water in sweat. To save water, the kidneys reabsorb more water from the tubules back into the blood. So less water is left in the urine, which becomes concentrated (dark) and small in volume.
Practice questions
Distinguish between excretion and egestion, giving an example of each.
Show answer
Excretion removes waste made by cell metabolism, e.g. urea in urine or CO₂ from the lungs. Egestion removes undigested food that never entered cells, e.g. faeces.
Common mistake: Calling faeces an excretory product.
Name four excretory organs and the waste each removes.
Show answer
Lungs: carbon dioxide and water vapour. Kidneys: urea, excess water and salts. Skin: water, salts, some urea in sweat. Liver: bile pigments (and makes urea).
Common mistake: Naming the bladder as an excretory organ.
Explain how urea is formed in the body.
Show answer
Excess amino acids from protein digestion cannot be stored; in the liver their amino group is removed (deamination) and converted into urea; the rest is used for energy.
Common mistake: Saying urea is made in the kidney.
Describe ultrafiltration in the kidney.
Show answer
Blood enters the glomerulus at high pressure (the efferent vessel is narrower); water, glucose, salts, urea and amino acids are forced into Bowman's capsule; blood cells and proteins remain.
Common mistake: Saying blood cells are filtered.
Explain selective reabsorption.
Show answer
In the tubules, useful substances (all glucose, amino acids, most water and some salts) are taken back into the blood capillaries, some by active transport and water by osmosis; urea and excess salts stay in the tubule.
Common mistake: Saying urea is reabsorbed completely.
State the functions of: (a) ureter; (b) bladder; (c) urethra.
Show answer
(a) Carries urine from kidney to bladder. (b) Stores urine. (c) Carries urine out of the body.
Common mistake: Mixing up ureter and urethra.
Give four practices that keep the urinary system healthy.
Show answer
Drink enough clean water; eat less salt; empty the bladder regularly; keep good personal hygiene; treat infections early; avoid misuse of drugs and alcohol; control diabetes and blood pressure.
Common mistake: Giving general advice unrelated to the kidneys.
Explain why urine is concentrated on a hot day.
Show answer
Much water is lost in sweat; the kidneys reabsorb more water into the blood, so little water remains in urine, making it small in volume and dark.
Common mistake: Saying the kidneys make more urea on hot days.
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. The excretory product of the lungs is: a. carbon dioxide 2. The organ that stores urine is the: b. urinary bladder 3. Deamination is the removal of the amino group from: c. glomerulus 4. The knot of capillaries inside Bowman's capsule is the: d. excess amino acids Show answer
1 → a (carbon dioxide) · 2 → b (urinary bladder) · 3 → d (excess amino acids) · 4 → c (glomerulus)
- Water vapour is also excreted.
- It stretches as it fills.
- The amino group forms urea.
- Ultrafiltration happens here.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
skin · dialysis · vaccination · old red blood cells- Sweat is produced by the .
- Bile pigments are made from the breakdown of .
- A machine that cleans the blood of a person whose kidneys have failed is used in .
Show answer
1: skin · 2: old red blood cells · 3: dialysis
- Sweat contains water, salts and some urea.
- The liver breaks down haemoglobin.
- Waste diffuses out of blood into dialysis fluid.
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. The outer region of the kidney is the: a. excretion 2. The removal of metabolic waste from the body is: b. nephron 3. The process that forces small molecules out of the glomerulus is: c. ultrafiltration 4. The functional unit of the kidney is the: d. cortex Show answer
1 → d (cortex) · 2 → a (excretion) · 3 → c (ultrafiltration) · 4 → b (nephron)
- Glomeruli are in the cortex.
- Metabolic wastes are made by cells.
- High blood pressure drives it.
- Each kidney has about a million nephrons.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
egestion · ureters · osmoregulation · absorption- The removal of undigested food from the body is .
- The control of water and salt balance by the kidneys is .
- Urine is carried from the kidneys to the bladder by the .
Show answer
1: egestion · 2: osmoregulation · 3: ureters
- Faeces never entered body cells.
- Urine volume changes with water intake.
- There is one ureter for each kidney.