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Topic 9 · 9.3–9.4

Blood vessels and blood

Core and Extended · Papers 1–6

🎯What you need to be able to do

  • Describe the structure of arteries, veins and capillaries; state the functions of capillaries; explain how structure relates to pressure and function EXTENDED.
  • Identify the main blood vessels to and from the heart, lungs and kidneys; and the liver EXTENDED.
  • List the components of blood, identify red and white blood cells, and state their functions; identify lymphocytes and phagocytes EXTENDED.
  • State the roles of clotting; describe it as fibrinogen converted to fibrin EXTENDED.

📚The biology

Arteries, veins and capillaries

Cross-sections of blood vessels. An artery with a thick muscular, elastic wall and a narrow lumen. A vein with a thin wall and a wide lumen. A tiny capillary with a wall one cell thick. A lengthways view of a vein showing a valve whose flaps let blood flow towards the heart only.
Artery: thick wall, narrow lumen. Vein: thin wall, wide lumen, valves. Capillary: wall one cell thick.

Capillaries exchange substances with the tissues: oxygen and nutrients pass out to the cells; carbon dioxide and other wastes pass in.

  • EXTENDED Arteries carry blood at high pressure: their thick walls withstand it, the muscle and elastic tissue stretch and recoil to keep the blood flowing (you feel this as the pulse), and the narrow lumen maintains the pressure.
  • Veins carry blood at low pressure: thin walls are enough, the wide lumen reduces resistance to flow, and valves prevent backflow.
  • Capillaries: the wall is one cell thick, so the diffusion distance is short; they are narrow and form huge networks, so every cell is close to one; their walls are leaky, letting some plasma out to bathe the cells.

The main blood vessels

A flow map of the main blood vessels. The pulmonary artery carries deoxygenated blood from the heart to the lungs and the pulmonary vein brings oxygenated blood back. The aorta carries oxygenated blood from the heart to the body, with branches: the hepatic artery to the liver, the renal artery to the kidneys, and an artery to the gut. The hepatic portal vein carries blood from the gut to the liver. The hepatic vein from the liver and the renal vein from the kidneys join the vena cava, which returns deoxygenated blood to the heart.
Heart: vena cava, aorta, pulmonary artery, pulmonary vein. Kidneys: renal artery and vein. Liver (Extended): hepatic artery, hepatic vein, hepatic portal vein.

The pulmonary artery is the only artery carrying deoxygenated blood, and the pulmonary vein the only vein carrying oxygenated blood. EXTENDED The hepatic portal vein carries blood rich in absorbed nutrients from the gut straight to the liver.

Blood

Blood components. Red blood cells: biconcave discs with no nucleus, containing haemoglobin. A phagocyte with a lobed nucleus, which engulfs pathogens. A lymphocyte with a large round nucleus filling most of the cell, which makes antibodies. Platelets: small cell fragments involved in clotting. Plasma: the liquid part.
Phagocytes and lymphocytes are both white blood cells. EXTENDED Tell them apart by the nucleus: lobed in phagocytes, large and round in lymphocytes.
red blood cells: transport oxygen, carried by the haemoglobin they contain
white blood cells: phagocytosis and antibody production
platelets: clotting
plasma: transports blood cells, ions, nutrients (such as glucose), urea, hormones and carbon dioxide

EXTENDED Lymphocytes produce antibodies; phagocytes engulf pathogens by phagocytosis.

Blood clotting prevents blood loss and stops pathogens entering through a wound. EXTENDED At a wound, the soluble protein fibrinogen is converted to insoluble fibrin, which forms a mesh that traps red blood cells and forms a clot.

Short separate strands of soluble fibrinogen, then, at a wound, a mesh of long fibrin threads trapping red blood cells.
EXTENDED Fibrinogen → fibrin mesh → clot, which dries into a scab.

✏️Worked example

(a) A blood vessel has a wide lumen, a thin wall and valves. Identify it and state the pressure of the blood it carries. [2] (b) Name the blood vessel that carries blood (i) from the kidneys, (ii) from the heart to the lungs. [2] (c) EXTENDED Explain how the structure of capillaries is related to their function. [3]

(a) A vein; low pressure.

(b) (i) Renal vein. (ii) Pulmonary artery.

(c) Their wall is one cell thick, so substances diffuse a short distance; they form large networks, giving a large surface area and bringing every cell close to a capillary; they are leaky, letting plasma out to supply the cells.

Check it. Artery or vein is decided by direction relative to the heart, not by oxygen: the pulmonary artery carries deoxygenated blood, but it carries it away from the heart.
“Arteries carry oxygenated blood.” True for all except the pulmonary artery. Define them as vessels that carry blood away from the heart.

📝Practise

In the style of the multiple-choice and theory papers. EXTENDED marks Supplement content.

1. (Multiple choice.) Which component of blood carries urea to the kidneys? A: red blood cells. B: white blood cells. C: platelets. D: plasma.
D.
2. (Theory.) State two differences in structure between an artery and a vein. [2]
Any two: artery has a thicker wall; artery has a narrower lumen; veins have valves; artery wall has more muscle and elastic tissue.
3. (Theory.) State the function of haemoglobin, and name the cells that contain it. [2]
It carries (binds) oxygen. Red blood cells.
4. (Theory.) State two roles of blood clotting. [2]
Prevents blood loss; prevents the entry of pathogens.
5. (Theory.) Name the vessel that (a) brings oxygenated blood from the lungs to the heart, (b) carries blood to the kidneys. [2]
(a) Pulmonary vein. (b) Renal artery.
6. (Theory.) EXTENDED Explain why arteries have thick, elastic walls. [2]
They carry blood at high pressure from the heart; the thick wall withstands the pressure, and the elastic tissue stretches and recoils, keeping the blood flowing smoothly.
7. (Theory.) EXTENDED Describe how a phagocyte and a lymphocyte can be distinguished in a blood smear, and state the function of each. [4]
Phagocyte: lobed nucleus, more cytoplasm; engulfs pathogens (phagocytosis). Lymphocyte: large round nucleus filling most of the cell; produces antibodies.
8. (Theory.) EXTENDED Name the blood vessel that carries absorbed glucose from the small intestine to the liver. [1]
The hepatic portal vein.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • Royal Society of Biology — examining blood smears under the microscope
  • NHS Blood and Transplant — what blood is made of