Circulatory System in Humans
Senior 3 · Biology
Circulatory System in Humans
Lesson 4 of 5
25 min
When a nurse at a health centre takes a blood sample for testing, a laboratory worker may spin it in a centrifuge, a machine that whirls tubes round very fast. The blood separates into layers: a straw-coloured liquid on top, a thin pale band, and a dark red layer at the bottom. Blood looks like a red liquid, but it is really a liquid carrying billions of cells, and each part has its own job.
Definition
Plasma: the pale yellow liquid part of blood, about 90% water, in which cells and dissolved substances are carried.
Red blood cell (erythrocyte): a cell that carries oxygen, using the red protein haemoglobin.
White blood cell (leucocyte): a cell that defends the body against pathogens. The two main kinds are phagocytes, which engulf and digest pathogens, and lymphocytes, which make antibodies.
Platelet: a tiny fragment of a cell that helps blood to clot.
Figure 1: The parts of blood. Left: the layers in a spun blood sample. Right: the cells, all drawn to the same scale.
| Component | Typical number in 1 mm³ of blood (adult) | Main job |
|---|---|---|
| Red blood cells | About 4.5 to 5.5 million | Carry oxygen from the lungs to the tissues |
| White blood cells | About 4000 to 11 000 | Defend the body against pathogens |
| Platelets | About 150 000 to 400 000 | Start blood clotting |
| Plasma | (the liquid, about 55% of blood) | Transports dissolved substances and heat |
Plasma carries: digested food (glucose, amino acids, fatty acids) from the gut to the cells; carbon dioxide, mostly as hydrogencarbonate ions, from the cells to the lungs; urea from the liver to the kidneys; hormones from glands to their target organs; plasma proteins such as fibrinogen (for clotting) and antibodies; mineral ions; and heat from active organs such as muscles and the liver to the rest of the body.
Red blood cells are made in the red bone marrow and live about 120 days. They are well suited to carrying oxygen:
White blood cells are larger, have a nucleus and are far fewer. As you saw in Senior 2, phagocytes engulf bacteria and digest them (phagocytosis), and lymphocytes make antibodies that recognise particular pathogens. Their number rises during an infection.
Platelets are fragments of large cells in the bone marrow. They gather at a wound and start the clotting process you will meet in the next lesson.
Worked example 1: how many red cells?
An adult has 5 litres of blood, with 5 million red cells in each mm³. (1 litre = 1 000 000 mm³.) How many red blood cells does the adult have?
Worked example 2: measuring a red cell
In a photograph of a blood smear taken at a magnification of , a red cell measures 15 mm across. What is its real diameter in micrometres? (1 mm = 1000 µm.)
Worked example 3: reading a blood test
A blood test gives these results for a girl of 16. (Illustrative results.) Which results are outside the typical normal range, and what might they suggest?
| Test | Result | Typical normal range |
|---|---|---|
| Haemoglobin | 98 g per litre | 120 to 160 g per litre |
| White cells | 7500 per mm³ | 4000 to 11 000 per mm³ |
| Platelets | 260 000 per mm³ | 150 000 to 400 000 per mm³ |
Warning
Common mistakes. (1) Saying red blood cells carry oxygen "in the nucleus": they have no nucleus; haemoglobin carries the oxygen. (2) Saying plasma carries most of the oxygen: red cells carry almost all of it; plasma carries carbon dioxide, food, urea and hormones. (3) Calling platelets cells: they are cell fragments, with no nucleus.
The Components of Blood and Their Importance, American Society of Hematology. Watch for what each component does, and why blood donation needs people with healthy blood.
Tip
Memory hook: "Red carries, White defends, Platelets plug, Plasma transports."
Practice: the components of blood
Key points
Sources and credits
Written by Ganzaa: the text, Figure 1, the tables, the worked examples (Worked example 3 uses illustrative results; normal ranges are typical textbook values) and the practice questions.
