2i Excretion¶

Part of 2 Structure and Functions in Living Organisms.

Excretion is about removing waste products of metabolism and keeping water and ion balance under control. In this course that means linking plant waste gases with the much more detailed kidney story in humans.

Learning Objectives¶

Excretion Overview¶

Excretion is the removal of waste products and toxic substances produced by metabolic reactions in the body.

• In flowering plants, carbon dioxide and oxygen are metabolic waste products that diffuse out through the stomata.
• In humans, there are three main organs of excretion:
• The lungs excrete carbon dioxide and water vapour.
• The kidneys excrete urea, excess water and excess salts as urine.
• The skin excretes water and salts in sweat.

Urea is produced in the liver when excess amino acids are broken down. The amino group is removed and combined with carbon dioxide to form urea, which is released into the bloodstream and filtered out by the kidneys.

Biology-Only Content: The Urinary System and the Nephron¶

This content is required for Biology-only students and is not required for Combined Science students.

Structure of the urinary system: - The kidneys have an outer region (the cortex) and an inner region (the medulla). - Urine leaves each kidney via the ureter, is stored in the bladder and passes out of the body through the urethra. - Each kidney is supplied with oxygenated blood via the renal artery and deoxygenated blood is returned by the renal vein. - Each kidney contains millions of tiny filtering tubes called nephrons.

The nephron — stages of urine production:

1. The renal artery carries blood to the glomerulus — a tight ball of capillaries within the Bowman's capsule at the start of the nephron.
2. Blood pressure in the glomerulus is high (the capillary leaving, the efferent arteriole, is narrower than the one entering, the afferent arteriole), forcing small molecules out of the blood into the Bowman's capsule in a process called ultrafiltration.
3. The filtrate contains water, salts, glucose and urea. Large proteins and blood cells cannot pass through and remain in the blood.
4. All glucose is reabsorbed back into the blood by active transport in the proximal convoluted tubule. Many mitochondria in these cells provide the energy needed.
5. Salts are reabsorbed in the loop of Henle.
6. Water is reabsorbed in the collecting duct (the amount depends on ADH levels — see below).
7. The remaining filtrate — water, salts and urea — becomes urine, which flows through the ureters to the bladder.

The Urinary System and the Nephron¶

• The urinary system includes the kidneys, ureters, bladder and urethra. Each kidney contains many nephrons.
• A nephron includes Bowman's capsule and glomerulus, convoluted tubules, the loop of Henle and the collecting duct.
• This layout allows blood to be filtered and then selectively adjusted so useful substances are kept while wastes are removed.

Ultrafiltration and Selective Reabsorption¶

• Ultrafiltration happens in Bowman's capsule when high pressure forces small molecules such as water, urea, ions and glucose out of the blood into the filtrate.
• Large molecules such as proteins and blood cells remain in the bloodstream because they are too large to pass through the filter.
• All glucose is selectively reabsorbed by active transport in the proximal convoluted tubule; salts are reabsorbed in the loop of Henle; and water is reabsorbed in the collecting duct according to the body's needs.

Osmoregulation and ADH¶

Osmoregulation is the control of water content in blood and body fluids.

ADH (antidiuretic hormone) is produced by the pituitary gland. It regulates how much water is reabsorbed by the kidney tubules.

• When the brain detects that the blood is too concentrated (e.g. after sweating or insufficient fluid intake), the pituitary gland releases more ADH.
• ADH travels in the bloodstream to the kidney tubules and increases the permeability of the collecting duct to water.
• More water moves out of the tubule back into the blood.
• The result is a smaller volume of more concentrated urine and blood becomes less concentrated.

This is a negative feedback mechanism: a rise in blood concentration triggers increased ADH release, which restores the concentration back towards normal.

In high temperatures, increased sweating can lead to dehydration and loss of salts. The brain detects the resulting blood concentration rise and generates a feeling of thirst, encouraging the individual to drink.

Urine composition: water + urea + ions. Its concentration varies with hydration state and temperature.

Common Confusions¶

• Excretion vs egestion: Excretion removes metabolic waste products such as urea and carbon dioxide. Egestion removes undigested food material from the gut.
• Ultrafiltration vs selective reabsorption: Ultrafiltration is non-selective — all small molecules are forced out. Selective reabsorption then returns only the useful substances to the blood.
• ADH effects: More ADH = more water reabsorbed = less urine produced = more concentrated urine. Less ADH = less water reabsorbed = more dilute urine in larger volume.

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 (all glucose, some water, some ions) from the filtrate back into the blood.
• Osmoregulation: the regulation of water content in the body's fluids.
• ADH (antidiuretic hormone): a hormone from the pituitary gland that increases the permeability of the collecting duct, causing more water to be reabsorbed into the blood.
• 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.

Connected Pages¶

• 2h Transport
• 2j Co-ordination and Response
• IGCSE Biology Key Terms Glossary