Blood

Biology · Transport in humans. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

Test this topic free All Biology topics

The idea

Blood has four components: red blood cells, white blood cells, platelets and the liquid plasma in which they float. Each has its own job, so a question usually gives a job or a symptom and asks for the component.

Under the microscope, red blood cells are small, very many, and have no nucleus. White blood cells are larger and fewer. A lymphocyte has a large round nucleus that almost fills the cell. A phagocyte has a lobed nucleus.

Blood does not touch the body cells directly. At the capillaries, part of the plasma leaks out and becomes tissue fluid. Substances move between the blood and the cells through this fluid.

Rules to remember

Red blood cells: transport oxygen (contain haemoglobin, no nucleus)
Lymphocytes: produce antibodies
Phagocytes: engulf and digest pathogens
Platelets: clotting; soluble fibrinogen is converted to insoluble fibrin, which forms a mesh that traps blood cells
A clot prevents blood loss and stops pathogens entering
Plasma transports blood cells, ions, glucose, amino acids, hormones, carbon dioxide, urea, vitamins and plasma proteins
Oxygen and glucose diffuse from blood → tissue fluid → cells; carbon dioxide and urea diffuse from cells → tissue fluid → blood

Common mistake

Students say white blood cells "eat" pathogens or that all white cells make antibodies. Use the right words: phagocytes engulf pathogens; lymphocytes produce antibodies.

Worked example

A blood test shows that a patient has a normal number of red and white blood cells but very few platelets. What problem is this patient most likely to have?

Red cells are normal, so oxygen transport is normal. White cells are normal, so antibodies and phagocytosis are normal.
Platelets are needed to start the change of soluble fibrinogen into insoluble fibrin.
With few platelets, little fibrin forms, so a clot forms slowly over a wound.

Answer: Cuts bleed for a long time, so more blood is lost and pathogens can enter more easily.

Ask about this

Type what you are stuck on. The helper finds the explanation, worked examples and the notes to read, free and instant.

Read the full notes

Transport in Humans

Keep going

← Blood vessels Disease →Take a Biology test