Animal Organ Systems

Describe asks for the features and characteristics of each system — which means naming the system, naming its organs, and saying what it does. The final clause adds explain for the circulatory system, which needs a cause-and-effect account rather than a name.

6 marks. Take the four needs in the order the question lists them, giving each a system, at least one organ and a function, then explain the circulatory system as the link.

Nutrients are managed by the digestive system, comprising the mouth, oesophagus, stomach, small intestine and large intestine, together with the liver and pancreas as accessory organs. It breaks food down mechanically and chemically into small soluble molecules and absorbs them, mainly across the villi of the small intestine, into the blood.

Water is managed by two systems working together. The digestive system absorbs water from food and drink, principally in the large intestine, where water and salts are reabsorbed from the remaining material. The excretory system then regulates how much is retained, the kidneys adjusting the volume of water excreted in the urine according to the body's needs.

Gases are managed by the respiratory system — the lungs together with the trachea and bronchi in mammals, or gills in fish. It takes in oxygen from the air for cellular respiration and removes the carbon dioxide that respiration produces, by diffusion down concentration gradients across a large, thin, moist exchange surface.

Wastes are managed by the excretory system — the kidneys, ureters and bladder — which removes nitrogenous waste, chiefly urea, along with excess salts and water. Carbon dioxide, itself a metabolic waste, is removed by the respiratory system instead, so waste management is shared between the two.

None of these systems reaches the body's cells directly; each exchanges its materials with the blood, and the circulatory system — the heart, blood vessels and blood — carries them between the systems and the cells. It collects absorbed nutrients and water from the digestive system and oxygen from the respiratory system and delivers them to every cell, then collects carbon dioxide and carries it to the lungs, and urea and carries it to the kidneys. Because the systems are linked in this way they are interdependent: if the kidneys fail, urea accumulates in the blood that is delivered to every cell, so the whole body is affected rather than the kidneys alone.

Analyse asks you to identify the components of the problem and the relationships between them. There are three: the geometry of a large body, the distance materials must travel, and the rate at which an active animal consumes them.

5 marks. Take the three components in turn, showing how each makes diffusion insufficient, then draw the conclusion about what organ systems provide instead.

As a body increases in size its volume grows faster than its surface area, because volume scales with the cube of length and surface area only with the square, so the surface-area-to-volume ratio falls. A large animal therefore has proportionally far less body surface across which to exchange materials, while containing far more living tissue that must be supplied.

Diffusion is only rapid over very short distances, because its rate falls sharply as the distance increases. In a large animal, cells deep within the body may be many centimetres from the nearest external surface, so even if enough material could cross that surface it would take far too long to reach them.

Animals are heterotrophs, so they must take in ready-made organic food rather than manufacturing it, and they are typically active, which means their cells respire rapidly and demand a fast, steady supply of oxygen and nutrients while generating carbon dioxide and urea that must be cleared just as quickly. The requirement is therefore not merely for exchange but for exchange at a high and sustained rate.

The three combine into a single mismatch: supply capacity falls with size just as demand rises with activity, and the distance over which the supply must act increases at the same time. Diffusion across the body surface cannot close that gap.

Organ systems resolve each failure in turn. Specialised exchange surfaces restore the area: the alveoli of the lungs and the villi of the small intestine are both extensively folded to provide enormous surface area within a compact organ. The circulatory system removes the distance problem by carrying materials in bulk flow through vessels to within a short diffusion distance of every cell, so diffusion still does the final step but only over micrometres. And dedicated systems handle each need at a rate matched to demand — digestive for nutrients, respiratory for gases, excretory for wastes. Diffusion is not abandoned; it is confined to the last, shortest stage.