Welcome to Your Guide to Excretion!

Excretion is one of the essential characteristics of living organisms. It is how living things clean up their internal environment by removing waste products generated by chemical reactions inside cells (metabolism).

If metabolic wastes are not removed, they accumulate and become toxic to cells. Let’s explore how both plants and humans excrete metabolic wastes, and take a detailed look at how the kidneys function in excretion and osmoregulation.

1. What is Excretion?

Excretion is the removal of waste products of metabolism, toxic substances, and substances in excess of requirements from the body.

Excretion vs. Egestion

It is vital not to confuse excretion with egestion:

  • Excretion: Removal of metabolic wastes produced inside cells (e.g. carbon dioxide, urea, excess mineral ions).
  • Egestion: The passing out of undigested food as faeces through the anus. Because this food never entered body cells or participated in metabolic reactions, it is not metabolic waste.

2. Excretion in Flowering Plants

Plants also produce metabolic waste products through chemical reactions:

  • Oxygen ( ext{O}_2): Produced during photosynthesis during the day. When the rate of photosynthesis is higher than the rate of respiration, oxygen is in excess and is excreted as a waste product via diffusion through the stomata.
  • Carbon Dioxide ( ext{CO}_2): Produced during cellular respiration. In darkness (or when respiration exceeds photosynthesis), carbon dioxide is excreted via diffusion through the stomata.

3. Human Excretory Organs and Products

In humans, three main organs carry out excretion:

  1. Lungs: Excrete carbon dioxide (and water vapour) produced during aerobic respiration. Carbon dioxide diffuses from blood into alveoli and is exhaled.
  2. Skin: Excretes water and mineral ions (and a small amount of urea) through sweat produced by sweat glands.
  3. Kidneys: Excrete urea, excess water, and excess mineral ions as urine.

4. The Urinary System

The human urinary system consists of four main structures:

  • Kidneys: Two bean-shaped organs that filter blood, remove urea, and control water balance.
  • Ureters: Tubes that carry urine from each kidney to the bladder.
  • Bladder: A muscular sac that stores urine.
  • Urethra: The tube through which urine passes out of the body.

5. Nephron Structure and Kidney Function

Each kidney contains hundreds of thousands of microscopic filtering units called nephrons. The nephron performs two crucial jobs: excretion and osmoregulation.

Stage 1: Ultrafiltration in the Bowman's Capsule

  • Blood arrives at the nephron via the renal artery into a knot of capillaries called the glomerulus.
  • High pressure forces water, urea, glucose, and mineral ions out of the glomerulus capillaries and into the Bowman's capsule.
  • Large molecules like plasma proteins and red blood cells cannot pass through the capillary basement membrane because they are too large; they remain in the blood.
  • The liquid collected in the Bowman's capsule is called the glomerular filtrate.

Stage 2: Selective Reabsorption of Glucose in the Proximal Convoluted Tubule

  • The filtrate flows from the Bowman's capsule into the proximal convoluted tubule (PCT).
  • All glucose is selectively reabsorbed back into the surrounding blood capillaries by active transport, so that it can be used for respiration.
  • In a healthy individual, no glucose remains in the urine.

Stage 3: Water Reabsorption in the Collecting Duct

  • As the filtrate moves through the tubule and down the collecting duct, water is reabsorbed by osmosis back into the blood capillaries according to the body's hydration needs.
  • The remaining fluid, now containing water, urea, and ions, is called urine.
  • Urine flows into the renal pelvis, down the ureter, and into the bladder.

6. Osmoregulation and the Role of ADH

Osmoregulation is the control of the water content and ion concentration of the blood (homeostasis).

The hormone ADH (anti-diuretic hormone) is released by the pituitary gland to regulate blood water content:

When Blood Water Content is Low (Dehydration):

  1. The hypothalamus detects low water content in the blood.
  2. The pituitary gland releases more ADH into the blood.
  3. ADH makes the walls of the collecting ducts more permeable to water.
  4. More water is reabsorbed from the collecting duct back into the blood.
  5. The kidneys produce a small volume of concentrated (darker) urine.

When Blood Water Content is High (Over-hydrated):

  1. The hypothalamus detects high water content in the blood.
  2. The pituitary gland releases less ADH.
  3. The walls of the collecting ducts become less permeable to water.
  4. Less water is reabsorbed into the blood.
  5. The kidneys produce a large volume of dilute (pale) urine.

Summary Table

  • Lungs: Excrete carbon dioxide from respiration.
  • Skin: Excretes water and mineral ions via sweat.
  • Kidneys: Excrete urea, excess water, and ions via urine; regulate blood water content using ADH.
  • Plants: Excrete oxygen (photosynthesis waste) and carbon dioxide (respiration waste) through stomata.