AQA GCSE Biology: Blood

Blood is more than a red liquid; it is a tissue made of several components that each do a different job. This guide explains why blood counts as a tissue, then works through plasma, red blood cells, white blood cells and platelets, and how each is adapted to its role. The theme, again, is structure matching function.

Why blood is a tissue

Blood is a tissue because it is a group of similar cells and components that work together to carry out related functions. It is an unusual tissue in being mostly liquid, but the definition still fits, and saying so is what earns the mark.

Plasma

Plasma is the pale yellow liquid part of blood, and everything else is carried in it. Its job is transport. What it carries is worth learning as a list, because questions often name one substance and ask where it is going:

• glucose and amino acids, absorbed from the small intestine and carried to the body's cells;
• urea, the waste made in the liver from excess amino acids, carried to the kidneys to be removed in urine;
• carbon dioxide, made by respiring cells, carried to the lungs to be breathed out;
• hormones, from the glands that make them to the organs they act on;
• antibodies, and the blood cells themselves, held in suspension.

Red blood cells

Red blood cells carry oxygen from the lungs to every respiring cell. Three adaptations do that work:

• they are packed with haemoglobin, the red protein that binds oxygen;
• they are shaped like a biconcave disc, which gives a large surface area for oxygen to diffuse across, and lets them bend to squeeze through capillaries;
• they have no nucleus, which leaves more room for haemoglobin.

The binding works both ways, and you need both. In the lungs, where the oxygen concentration is high, haemoglobin binds oxygen to form oxyhaemoglobin. In the respiring tissues, where the concentration is low, the reaction reverses: oxyhaemoglobin splits back into haemoglobin and oxygen, and the oxygen diffuses into the cells. An answer that gives only the loading half is only half an answer.

White blood cells

White blood cells are part of the immune system, and unlike red blood cells they do have a nucleus. They defend the body in three ways:

• phagocytosis, where the cell engulfs a pathogen and digests it;
• antibodies, produced to lock onto a particular pathogen so that it can be destroyed;
• antitoxins, produced to neutralise the poisons some bacteria release.

Platelets

Platelets are small cell fragments, with no nucleus. At a wound they make the blood clot, forming a mesh that traps blood cells and dries into a scab. That scab does two jobs, and questions usually want both: it stops blood loss, and it stops pathogens getting in.

Bringing it together

Each component is a different answer to the same problem, which is getting something to where it is needed. Plasma moves dissolved substances, red blood cells move oxygen, white blood cells move to wherever a pathogen is, and platelets stay put and seal a leak. When a question asks you to explain an adaptation, it is really asking which of those jobs the feature serves.

How this fits together

Blood keeps every cell supplied, so anything that reduces its numbers or blocks its vessels causes problems. One of the most important of these is coronary heart disease, the subject of the coronary heart disease guide.