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.

What You Need to Learn¶

Further detail: Pearson Edexcel International GCSE Biology specification.

On this page you'll learn about excretion overview. If you're taking Biology-only, you'll also learn about the urinary system and the nephron, ultrafiltration and selective reabsorption, and osmoregulation and ADH (antidiuretic hormone). The notes bring these ideas together into one clear overview of excretion.

Excretion Overview¶

Excretion is the removal of waste products of metabolism and other toxic or excess substances from the body. It is different from egestion, which is the removal of undigested food from the gut.

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.

Students often overlook the lungs here, but carbon dioxide is a metabolic waste product of respiration, so the lungs are organs of excretion.

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, which are the functional units of the kidney.

The nephron — stages of urine production:

The renal artery carries blood to the glomerulus — a tight ball of capillaries within the Bowman's capsule at the start of the nephron.
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.
The high pressure, combined with gaps in the capillary wall and a basement membrane acting as a filter, means small molecules pass through but large proteins and blood cells remain in the blood.
The filtrate contains water, salts, glucose and urea.
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.
Salts are reabsorbed in the loop of Henle.
Water is reabsorbed in the collecting duct (the amount depends on ADH levels — see below).
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.
The kidney therefore has both an excretory role (removing urea) and a homeostatic role (controlling water content in the blood).

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 filtration barrier.
The high pressure comes from blood arriving from the heart and from the afferent arteriole being wider than the efferent arteriole.
All glucose is selectively reabsorbed by active transport in the proximal convoluted tubule because it is useful and needed for respiration.
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.
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 or the other.
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 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.