8.6Blood
Blood is a tissue consisting of plasma, in which the red blood cells, white blood cells and platelets are suspended. You should know the function of each component, and be able to recognise the different cells in a photograph or diagram and explain how they are adapted to their functions.
Practical skill
Observing and drawing blood cells under a microscope
- Look at a prepared slide of a blood smear, starting on a low power objective and moving up.
- Find a field of view with all three: many small red blood cells, occasional larger white blood cells with a visible nucleus, and tiny platelet fragments.
- Draw a representative sample, not the whole field, with clear unbroken lines and no shading.
- Label each cell type and include a magnification scale.
Watch out: Red blood cells look paler in the middle. That is not bad staining — it is the biconcave shape, thinner at the centre, and saying so in a question about adaptation earns the mark.
Blood products — transfused blood, plasma or platelets — save lives, and they carry risks worth being able to evaluate: an infection can be passed on if donated blood is not screened, and a patient given the wrong blood group can have a serious immune reaction. Screening and cross-matching reduce both, which is why the risk in practice is very small. An answer that weighs the benefit against the managed risk is what an evaluation question is asking for.
- Blood
- a tissue consisting of plasma, in which the red blood cells, white blood cells and platelets are suspended
- Plasma
- the liquid part of the blood, in which the blood cells and platelets are suspended and dissolved substances are transported
8.7–8.8, 8.12The heart and blood vessels
The heart is an organ that pumps blood around the body in a double circulatory system. The right ventricle pumps blood to the lungs, where gas exchange takes place; the left ventricle pumps blood around the rest of the body.
"Double" means blood passes through the heart twice on one full circuit. The advantage is pressure: blood leaving the lungs is slowed by the capillaries there, so returning it to the heart lets it be pumped out to the body at high pressure again.
The named vessels are a fixed list: the aorta, vena cava, pulmonary artery, pulmonary vein and coronary arteries. The names of the heart valves are not required.
Knowledge of the lungs is restricted to the trachea, bronchi, alveoli and the capillary network surrounding the alveoli. The alveoli are the exchange surface — millions of them, each with a wall one cell thick, surrounded by capillaries, and ventilated by breathing. Those are the four features of an effective exchange surface described under transport in cells, in one organ.
The natural resting heart rate is controlled by a group of cells in the right atrium that act as a pacemaker. Artificial pacemakers are electrical devices used to correct irregularities in the heart rate.
The body contains three types of blood vessel — arteries, veins and capillaries — and you should be able to explain how the structure of each relates to its function.
Arteries carry blood away from the heart at high pressure, so they have thick muscular and elastic walls and a narrow lumen. Veins return blood at low pressure, so the wall is thin and the lumen wide, and they contain valves to stop blood flowing backwards. Capillaries are where exchange happens, so their walls are one cell thick and permeable, giving a short diffusion path.
Worked example
A heart pumps 70 cm³ of blood per beat at 72 beats per minute. Calculate the rate of blood flow in dm³ per minute.
- Blood flow per minute = volume per beat × beats per minute.
- 70 × 72 = 5040 cm³ per minute.
- Convert: 1 dm³ = 1000 cm³, so 5040 ÷ 1000.
Answer: 5.04 dm³ per minute.
Common mistake
Arteries carry blood away from the heart and veins carry it back — that is the definition, and it has nothing to do with oxygen. The pulmonary artery carries deoxygenated blood, and the pulmonary vein carries oxygenated blood. Both are still correctly named.
- Double circulatory system
- a system in which blood passes through the heart twice on one full circuit: the right ventricle pumps blood to the lungs and the left ventricle pumps it around the rest of the body
- Artery
- a blood vessel that carries blood away from the heart at high pressure, with thick muscular and elastic walls and a narrow lumen
- Vein
- a blood vessel that returns blood to the heart at low pressure, with a thin wall, a wide lumen and valves to stop blood flowing backwards
- Capillary
- a blood vessel where exchange happens, with a permeable wall one cell thick giving a short diffusion path
- Pacemaker
- a group of cells in the right atrium that controls the natural resting heart rate; artificial pacemakers are electrical devices used to correct irregularities in the heart rate
Before you practise
- Plasma carries everything dissolved; red blood cells carry oxygen; white blood cells defend against pathogens; platelets clot.
- A red blood cell is a biconcave disc with no nucleus — both features are there to carry more haemoglobin and increase surface area.
- Arteries carry blood away from the heart at high pressure, so thick muscular walls and a narrow lumen; veins return it at low pressure, so thin walls, a wide lumen and valves.
- Capillaries are one cell thick, which is what allows substances to diffuse in and out.
- The right side of the heart pumps to the lungs and the left side to the rest of the body — the left wall is thicker because the distance is greater.
- cardiac output = stroke volume × heart rate, and watch the units before you multiply.