Human Exchange Surfaces

The Lungs

The main role of the lungs is the exchange of gases: oxygen and carbon dioxide.
As we breathe in, oxygen enters the lungs and moves via diffusion across the thin walls of the alveoli into the bloodstream.
Carbon dioxide, a waste product of cellular metabolism, diffuses out of the blood and into the alveoli, ready to be exhaled.
The lungs contain millions of alveoli, providing a large surface area for gas exchange.
The moist lining in the alveoli aids in dissolving gases and the rich supply of capillaries maintains a high concentration gradient.

The Gut

The gut is responsible for the exchange of nutrients from the food we eat into our blood.
Most absorption of nutrients occurs in the small intestine.
The walls of the small intestines are folded into millions of tiny finger-like projections called villi, which massively increase the surface area.
The villi have a network of capillaries to carry away the nutrients via the bloodstream.
The villi themselves are lined with single layer of cells to allow quick and efficient absorption of nutrients.

The Skin

Your skin is an important exchange surface in your body.
It acts as a barrier to prevent the loss of water and also as a defence against microbes.
The skin allows the passage of heat and small amounts of waste products, such as urea and salts, through sweat.
The skin’s sweat glands regulate body temperature by releasing sweat, which evaporates from the skin surface, removing heat.

The Kidneys

They’re responsible for filtering waste products out of your bloodstream to form urine.
The functional units of the kidneys are called nephrons, each made up of a glomerulus and a tubule.
The glomerulus filters the blood, and the filtered fluid then passes along the tubule, where useful substances are reabsorbed back into the blood.
The remaining waste products, including urea, ions, and water, form urine and are removed from the body.

Always remember that the efficiency of these exchange surfaces is determined by their large surface area, thinness of surface, and good blood supply. These features help in maximising diffusion rates.