What's in Blood?

Explain the relationship between structure and function, which means every structural feature you name must be followed by the job it makes possible. A labelled list of components is a Describe answer.

6 marks across four components. Give each a structural feature and the function it enables, and spend the extra detail on the red blood cell, which has three features worth having.

Plasma is the pale yellow liquid component, making up roughly 55 per cent of blood and itself about 90 per cent water, in which the blood cells are suspended. Because water is such a good solvent, an enormous range of substances dissolves in it, so one fluid can transport glucose, amino acids, ions, hormones, carbon dioxide and urea simultaneously. Being a freely flowing liquid, it can pass through even the narrowest capillary, and because water has a high heat capacity it also distributes heat around the body and helps keep body temperature even.

A red blood cell is a biconcave disc, has no nucleus, and is flexible, and its function is to transport oxygen. The biconcave shape increases its surface-area-to-volume ratio, so oxygen is loaded and unloaded across the membrane faster than a sphere of the same volume would allow. The absence of a nucleus frees internal space, so each cell can be packed with more haemoglobin, the iron-containing protein that binds oxygen, and therefore carries more oxygen. Its flexibility lets it deform and squeeze through capillaries narrower than itself, so it can reach every tissue.

A white blood cell retains its nucleus and can change shape, and its function is to defend the body against pathogens. Keeping the nucleus means it retains the DNA needed to direct protein synthesis, which some types use to manufacture antibodies. Being able to change shape lets it squeeze out of capillaries into infected tissue and engulf microbes by surrounding them.

Platelets are not whole cells but small fragments of cells, without nuclei, and their function is to form clots that seal wounds. Being small fragments means enormous numbers can circulate and accumulate rapidly at a damaged site, where they release clotting factors that trigger the formation of a clot. Clotting requires many small triggers arriving quickly rather than a few whole cells, so a fragment is the appropriate structure.

The four components illustrate a single principle: each structure is justified by the one function it serves. Two of them have given something up to specialise — the red cell its nucleus, the platelet its whole-cell status — and in each case the loss is what makes the function possible. This is the same trade of independence for specialisation seen at every level of this course.

Analyse means identifying the components and relating them. The detail that makes this question interesting is that the cell count is normal, so the usual explanation — too few red cells — is unavailable and the answer must go one level deeper.

5 marks. Identify what is actually deficient, follow the consequence to oxygen delivery, then to the tissues, then to the two named symptoms, and close on what the case demonstrates.

Haemoglobin is the iron-containing protein inside the red blood cell that binds oxygen, and iron is an essential part of the molecule. With insufficient iron the body cannot synthesise enough haemoglobin, so although red blood cells are produced in normal numbers, each is underfilled.

The oxygen a red cell carries depends on how much haemoglobin it contains, not on the cell being present. With less haemoglobin per cell and a normal cell count, the total oxygen-carrying capacity of the blood falls, so each unit of blood leaving the lungs is carrying less oxygen than it should even though the lungs themselves are working normally.

Every tissue therefore receives less oxygen per unit of blood delivered. With less oxygen available, cells cannot carry out aerobic respiration at the normal rate, so they release less ATP, and every energy-requiring process in the body — including muscle contraction — is constrained.

Tiredness follows directly: with less ATP available, muscles fatigue quickly and the person feels persistently exhausted even without exertion. Breathlessness is a compensatory response rather than a direct effect — the body detects that tissues are under-oxygenated and increases breathing rate and heart rate to move more blood and air. This cannot fully succeed, because the limitation is the blood's carrying capacity rather than the supply of air, so the person breathes harder and still feels short of breath.

The case shows that a blood component's function depends on its internal structure and not merely on its presence. A red blood cell with the correct shape, the correct flexibility and no nucleus is still, without sufficient haemoglobin, unable to perform the function all those adaptations exist to serve. Structure means the molecular contents as much as the cell's form, which is why treatment targets the iron supply rather than the cell count.