1. Excretion and excretory organs
Excretion is the removal of harmful metabolic wastes formed by the chemical activities of body cells. Removing undigested food (faeces) is egestion, not excretion.
| Organ | Wastes removed |
|---|---|
| Kidneys | Urea, excess water and salts (urine) |
| Lungs | Carbon dioxide, water vapour |
| Skin | Water, salts, a little urea (sweat) |
| Liver | Bile pigments from broken-down haemoglobin (in bile); also makes urea |
Urea is formed in the liver by deamination: the nitrogen-containing part of excess amino acids is removed and converted into urea, which the blood carries to the kidneys.
| Organ | Main waste |
|---|---|
| Kidneys | Urea |
| Lungs | Carbon dioxide |
| Skin | Sweat |
| Liver | Bile pigments |
Key terms
- Excretion
Removal of metabolic wastes formed in body cells.
- Egestion
Removal of undigested food as faeces.
- Deamination
Removal of the nitrogen part of excess amino acids in the liver, forming urea.
2. The urinary system
- Kidneys (a pair): bean-shaped, against the back wall of the abdomen on either side of the backbone; they filter blood and form urine.
- Renal artery: a branch of the aorta that brings blood (with urea) to each kidney. Renal vein: takes cleaned blood to the inferior vena cava.
- Ureters: a muscular tube from each kidney to the bladder.
- Urinary bladder: a muscular, stretchable bag that stores urine; a ring of muscle (sphincter) at its neck controls release.
- Urethra: carries urine from the bladder to the outside.
Key terms
- Ureter
Tube carrying urine from a kidney to the bladder.
- Urethra
Tube carrying urine from the bladder to the outside.
- Urinary bladder
Muscular bag that stores urine.
3. External and internal structure of the kidney
External: bean-shaped, dark red, about 10 cm long; outer side convex, inner side concave with a notch, the hilum, where the renal artery enters and the renal vein and ureter leave; covered by a thin, tough capsule.
Longitudinal section:
- Cortex — outer, dark, dotted; contains the Malpighian capsules and convoluted tubules.
- Medulla — inner, made of striped, cone-shaped renal pyramids containing the loops of Henle and collecting ducts.
- Pelvis — funnel-shaped cavity into which the pyramids open; it leads into the ureter.
Key terms
- Cortex
Outer region of the kidney with the Malpighian capsules.
- Medulla
Inner region made of renal pyramids.
- Renal pelvis
Funnel-shaped cavity that collects urine and leads to the ureter.
- Hilum
Notch where the renal vessels and ureter join the kidney.
4. The nephron and the Malpighian capsule
The nephron (kidney tubule) is the structural and functional unit of the kidney; each kidney has about one million.
- Malpighian capsule (renal corpuscle) = Bowman's capsule (a double-walled cup) + glomerulus (a knot of capillaries inside it).
- The afferent arteriole (wider) brings blood into the glomerulus; the efferent arteriole (narrower) carries it away, so the pressure in the glomerulus is high.
- The efferent arteriole forms a network of capillaries round the tubule, which drains into a branch of the renal vein.
- Tubule: proximal convoluted tubule (cortex) → loop of Henle (dips into the medulla) → distal convoluted tubule (cortex) → collecting duct → pelvis.
Key terms
- Nephron
Structural and functional unit of the kidney.
- Glomerulus
Knot of capillaries inside Bowman's capsule.
- Bowman's capsule
Double-walled cup that receives the filtrate.
- Afferent arteriole
The wider vessel bringing blood into the glomerulus.
5. Formation of urine
- Ultrafiltration (in the Malpighian capsule): high pressure forces water, glucose, amino acids, salts and urea out of the glomerulus into Bowman's capsule. Blood cells and plasma proteins are too large and stay in the blood. About 180 litres of filtrate form each day.
- Selective reabsorption (mainly the proximal tubule; also the loop of Henle and distal tubule): all glucose and amino acids and most water and salts return to the blood in the capillaries round the tubule. Over 99% of the filtrate is reabsorbed.
- Tubular secretion (in the distal tubule): extra potassium and hydrogen ions, some drugs and other wastes pass from the blood into the tubule.
About 1.5 litres of urine are passed a day.
| Substance | Plasma | Filtrate | Urine |
|---|---|---|---|
| Proteins | Present | Absent | Absent |
| Glucose | Present | Present | Absent |
| Urea | Low | Low | About 67 times higher |
Glucose in urine suggests very high blood glucose (diabetes); protein or blood cells in urine suggest damage to the glomerular filter.
| Stage | Site |
|---|---|
| Ultrafiltration | Malpighian capsule |
| Selective reabsorption | Proximal tubule (mainly) |
| Tubular secretion | Distal tubule |
Key terms
- Ultrafiltration
Filtration of blood under pressure in the Malpighian capsule.
- Selective reabsorption
Return of useful substances from the tubule to the blood.
- Tubular secretion
Active addition of extra substances from the blood into the tubule.
- Glomerular filtrate
Fluid in Bowman's capsule: plasma without proteins.
Exam tips
Common mistakes
- Calling the removal of faeces excretion (it is egestion).
- Saying urea is made in the kidneys (it is made in the liver).
- Swapping the ureter (kidney → bladder) and the urethra (bladder → outside).
- Drawing the efferent arteriole wider than the afferent.
- Saying glucose is not filtered: it is filtered, then fully reabsorbed.
How marks are usually awarded
- 1 mark: a term (ultrafiltration, deamination, nephron), an odd one out with its category, a part named.
- 2 marks: two differences (cortex and medulla, filtrate and urine), a feature with its reason.
- 3 marks: the three stages with their sites; a labelled urinary system, kidney section or nephron.