2i Excretion¶
Part of 2 Structure and Functions in Living Organisms.
Excretion removes the waste made by the chemical reactions inside cells, and in humans the kidneys also keep the water content of the blood steady. Plants excrete simple waste gases, while the human kidney is a much more detailed system that filters the blood and then takes back what the body needs.
What You Need to Learn¶
Further detail: Pearson Edexcel International GCSE Biology specification.
What excretion is and which organs carry it out. Biology-only students also learn the structure of the urinary system and the nephron, how urine is made by ultrafiltration and selective reabsorption, and how ADH (antidiuretic hormone) controls the water content of the blood.
Excretion Overview¶
Excretion is the removal of the waste products of metabolism, and of other toxic or excess substances, from the body. Metabolic waste is made by reactions inside cells. Egestion is different: it is the removal of undigested food from the gut, which was never part of a cell's chemistry.
| Organism | Organ | What it excretes |
|---|---|---|
| Flowering plant | Stomata in the leaves | Carbon dioxide from respiration and oxygen from photosynthesis, which diffuse out |
| Human | Lungs | Carbon dioxide and water vapour |
| Human | Kidneys | Urea, excess water and excess salts, as urine |
| Human | Skin | Water and salts, in sweat |
The lungs are easy to overlook, but carbon dioxide is a waste product of respiration, so the lungs are organs of excretion.
Urea is made in the liver when excess amino acids are broken down. The amino group is removed and combined with carbon dioxide to form urea, which enters the bloodstream and is filtered out by the kidneys.
Exam technique
Define excretion with the word metabolic in it. "Removal of waste" could describe egestion as well, and the lungs are a useful example to quote because they show that excretion is not only about urine.
Biology-Only Content¶
This content is required for Biology-only students and is not required for Combined Science students.
The three sections that follow cover the structure of the kidney, how urine is made and how its water content is controlled. All three are Biology-only.
The Urinary System and the Nephron¶
Each kidney has an outer region, the cortex, and an inner region, the medulla. The renal artery brings oxygenated blood to the kidney, and the renal vein carries blood away. Urine leaves each kidney through a ureter, is stored in the bladder, and leaves the body through the urethra. The ureter and urethra are easily confused: the ureter joins a kidney to the bladder, and the urethra carries urine from the bladder to the outside.
Each kidney contains millions of filtering tubes called nephrons, which are its functional units.
| Part of the nephron | Role |
|---|---|
| Glomerulus | A tight ball of capillaries where blood is filtered |
| Bowman's capsule | A cup around the glomerulus that collects the filtrate |
| Proximal convoluted tubule | Where all the glucose is reabsorbed |
| Loop of Henle | Where salts are reabsorbed |
| Collecting duct | Where water is reabsorbed, in an amount controlled by ADH |
The kidney has two roles. It is an organ of excretion, because it removes urea, and it is an organ of homeostasis, because it controls the water content of the blood.
Ultrafiltration and Selective Reabsorption¶
Ultrafiltration. Blood reaches the glomerulus at high pressure, and the filtrate is forced out into Bowman's capsule. The pressure is high for two reasons: the blood has just come from the heart, and the afferent arteriole bringing blood in is wider than the efferent arteriole taking it away, so the blood is squeezed. The wall of the capillary has small gaps, and a basement membrane acts as a mesh beneath it, so filtering is by size. Small molecules pass through: water, urea, ions and glucose. Blood cells and large proteins are too large and stay in the blood.
Selective reabsorption. Ultrafiltration is not selective, because it forces out everything small, including glucose that the body needs. Selective reabsorption takes the useful substances back into the blood:
- All the glucose is reabsorbed in the proximal convoluted tubule, by active transport. The cells there have many mitochondria to supply the energy.
- Salts are reabsorbed in the loop of Henle.
- Water is reabsorbed in the collecting duct, in an amount that depends on ADH.
What remains, water, salts and urea, is urine. It flows down the ureters to the bladder.
Worked example: comparing blood, filtrate and urine
| Substance | In blood plasma | In the filtrate | In urine |
|---|---|---|---|
| Protein | Yes | No | No |
| Glucose | Yes | Yes | No |
| Urea | Yes | Yes | Yes, more concentrated |
Protein is missing from the filtrate because it is too large to pass through the filter. Glucose is in the filtrate but not in urine because it is all reabsorbed. Urea is not reabsorbed, so it stays in the urine. If protein appears in urine, the filter has been damaged. If glucose appears, reabsorption has not recovered all of it.
Exam technique
Describe selective reabsorption as substances being taken back into the blood, and name where each one is reabsorbed. For ultrafiltration, name both causes of the high pressure and the filter that keeps back cells and proteins. A common slip is to say that the glucose is "filtered out" and so lost.
Osmoregulation and ADH¶
Osmoregulation is the control of the water content of the blood and body fluids. ADH (antidiuretic hormone) is released by the pituitary gland and controls how much water the kidney tubules reabsorb.
| Blood water content | ADH released | Collecting duct | Urine |
|---|---|---|---|
| Too low: blood too concentrated, for example after sweating | More | More permeable to water, so more water is reabsorbed | Small volume, concentrated |
| Too high: blood too dilute | Less | Less permeable to water, so less water is reabsorbed | Large volume, dilute |
This is negative feedback. A rise in blood concentration triggers more ADH, which restores the concentration towards normal. In hot weather, sweating can dehydrate the body and lose salts. The brain detects the rise in blood concentration and produces the sensation of thirst, which encourages drinking.
Urine is water, urea and ions, and its concentration varies with how hydrated the body is and with the temperature.
Common Confusions¶
- Excretion vs egestion: Excretion removes metabolic waste products such as urea and carbon dioxide. Egestion removes undigested food from the gut.
- Ureter vs urethra: The ureter carries urine from a kidney to the bladder. The urethra carries urine from the bladder out of the body.
- Kidney roles: The kidney is involved in both excretion and homeostasis, not just one of them.
- Ultrafiltration vs selective reabsorption: Ultrafiltration is not selective, because it forces out all small molecules. Selective reabsorption then returns the useful substances to the blood.
- ADH effects: More ADH means more water reabsorbed, less urine and more concentrated urine. Less ADH means less water reabsorbed and a larger volume of more dilute urine.
Check Yourself¶
- State why the lungs are classed as organs of excretion.
- State the difference between excretion and egestion.
- (Biology only) Explain why the pressure in the glomerulus is high.
- (Biology only) A patient's urine contains glucose. Suggest which stage of urine production is not working properly, and explain.
- (Biology only) A person has been running in hot weather and has not drunk anything. Explain how ADH changes the volume and concentration of their urine.
- (Biology only) A student says: "The ureter carries urine out of the body." Explain what is wrong and correct it.
Answers
- The lungs remove carbon dioxide, which is a waste product of respiration, so they are organs of excretion.
- Excretion removes the waste products of metabolism made inside cells, whereas egestion removes undigested food from the gut.
- The blood arrives from the heart under pressure, and the afferent arteriole is wider than the efferent arteriole, so the blood in the glomerulus is squeezed at high pressure.
- Selective reabsorption. Glucose is filtered into the nephron and should all be reabsorbed by active transport in the proximal convoluted tubule, so glucose in urine means this reabsorption has not recovered all of it.
- The blood becomes more concentrated, so the pituitary gland releases more ADH. ADH makes the collecting duct more permeable to water, so more water is reabsorbed into the blood. The person produces a smaller volume of more concentrated urine.
- The ureter carries urine from the kidney to the bladder. The urethra carries urine from the bladder out of the body.
Key Terms¶
- Excretion: removal of waste products of metabolism from the body.
- Urea: a nitrogen-containing waste product formed from excess amino acids in the liver.
- Nephron: the microscopic functional unit of the kidney that filters blood and produces urine.
- Ultrafiltration: pressure filtration of small molecules from the blood into the Bowman's capsule.
- Selective reabsorption: the return of useful substances from filtrate to the blood.
- Osmoregulation: the regulation of water content in the body's fluids.
- ADH (antidiuretic hormone): a hormone that increases collecting-duct permeability to water.
- Glomerulus: the knot of capillaries inside Bowman's capsule where ultrafiltration occurs.
- Bowman's capsule: the cup-shaped structure at the start of the nephron that collects the filtrate.
- Loop of Henle: the part of the nephron tubule in which salt reabsorption occurs.
- Collecting duct: the final section of the nephron where water reabsorption is regulated by ADH.
- Ureter: the tube carrying urine from a kidney to the bladder.
- Urethra: the tube carrying urine from the bladder out of the body.