11.3 – Blood Vessels

Table of Contents

IGCSE Biology  |  Transport in Humans

Blood does not flow freely through the body — it travels through a network of tubes called blood vessels. There are three types: arteries, veins, and capillaries. Each type has a different structure, and that structure is perfectly matched to its job and the pressure of the blood inside it.

1. The Main Named Blood Vessels #

Several important blood vessels connect the heart to specific organs. You need to know their names and what they carry.

Quick rule: In general, arteries carry blood away from the heart and veins carry blood back to the heart. The pulmonary vessels are the main exception — the pulmonary artery carries deoxygenated blood and the pulmonary vein carries oxygenated blood.

Heart and Lungs #

Vessel Connects Carries
Aorta Left ventricle → body Oxygenated blood to the whole body
Vena cava Body → right atrium Deoxygenated blood back from the body
Pulmonary artery Right ventricle → lungs Deoxygenated blood to the lungs
Pulmonary vein Lungs → left atrium Oxygenated blood back from the lungs

Liver #

Vessel Connects Carries
Hepatic artery Aorta → liver Oxygenated blood to the liver
Hepatic vein Liver → vena cava Deoxygenated blood from the liver back to the heart
Hepatic portal vein Small intestine → liver Blood rich in digested food (glucose, amino acids) from the gut to the liver for processing
Note: The hepatic portal vein is unusual — it is a vein that connects two sets of capillaries (in the gut and in the liver) rather than returning blood directly to the heart. It carries nutrients absorbed after a meal.

Kidneys #

Vessel Connects Carries
Renal artery Aorta → kidney Oxygenated blood (containing urea and other waste) to the kidneys for filtering
Renal vein Kidney → vena cava Filtered blood (urea removed) back to the heart

2. Structure of Arteries, Veins, and Capillaries #

The three types of blood vessel all have different structures. The differences in wall thickness, wall composition, lumen size, and valves all match the function and blood pressure of each vessel.

Comparison Table #

Feature Artery Vein Capillary
Wall thickness Very thick Thinner than artery One cell thick (extremely thin)
Muscle in wall Lots of smooth muscle Some smooth muscle (less than artery) None
Elastic tissue in wall Lots of elastic tissue Some elastic tissue (less than artery) None
Lumen diameter Narrow (relative to wall) Wide (relative to wall) Very narrow — just wide enough for one red blood cell
Valves present? No Yes — valves along the length No

3. How Structure Relates to Blood Pressure #

Blood pressure is not the same throughout the circulatory system. It is highest when blood leaves the heart and drops as blood moves further away. The structure of each vessel matches the pressure it must handle.

Vessel Blood Pressure How Structure Matches
Arteries High — and varies (pulses) with each heartbeat
  • Thick walls of muscle and elastic tissue withstand high pressure
  • Elastic tissue stretches during the pressure surge of a heartbeat, then recoils to smooth out the flow
Veins Low — steady, no pulse
  • Thinner walls are enough because there is little pressure to withstand
  • Wide lumen allows blood to flow with minimal resistance
  • Valves prevent backflow where pressure is too low to keep blood moving on its own
Capillaries Very low
  • One-cell-thick walls are sufficient — blood pressure here is very low, so thin walls are not at risk of bursting
Summary: High pressure → thick, muscular, elastic walls (arteries). Low pressure → thinner walls, wide lumen, valves to help (veins). Very low pressure → one-cell-thick walls are sufficient because there is no risk of bursting (capillaries).

Syllabus Reference — Section 11.3: Blood Vessels #

  1. Name the main blood vessels that carry blood to and from the heart, lungs, liver and kidneys, limited to: aorta, vena cava, pulmonary artery, pulmonary vein, hepatic vein, hepatic artery, hepatic portal vein, renal artery and renal vein
  2. Describe, and identify on diagrams and photomicrographs, the structure of arteries, veins and capillaries, limited to:
    (a) relative thickness of wall
    (b) composition of wall (muscle and elastic tissue)
    (c) diameter of lumen
    (d) presence of valves
  3. Explain how the structure of arteries, veins and capillaries is related to the pressure of the blood that they transport
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