2.7.3 Veins

The blood vessels that bring blood back to the heart are called veins.

• Relatively not deep in the muscles.
• Can be seen as blue vessels under the skin.
• Have valves (in veins > 2 mm diameter).
• Blood pressure is comparatively low.

Wall Structure: A vein also consists of three tunics:

• Diameter of 40 to 50 μm.
• Tubes composed of endothelium.
• Collect blood from capillaries and transport it to small veins.

• Veins having diameters greater than 2 mm contain valves.
• Valves allow blood flow toward the heart but not in the opposite direction.
• Valves are present only in the lower part of the body, especially in the abdomen and hind limbs.
• In the upper region above the heart, there is no valve.

• Blood pressure in veins is comparatively low.
• Flow is helped by:
• Gravity
• Semilunar valves
• Muscular contraction

Comparison of Arteries, Capillaries, and Veins

Intermediate Level

Structure of Blood Vessel Walls

Types of Arteries

Types of Capillaries

Valves in Veins

Factors Aiding Venous Return

Formula / Mathematical Form

Not applicable to this topic.

Solved Example

Not applicable to this topic (descriptive topic).

Important Points

• 1. Three types of blood vessels: arteries, capillaries, veins.
• 2. Arteries carry blood away from the heart.
• 3. Capillaries permit exchange of materials.
• 4. Veins return blood to the heart.
• 5. Arteries have thick walls; veins have thin walls.
• 6. Capillaries are one cell thick.
• 7. Arteries have no valves; veins have valves.
• 8. Tunica adventitia is the outermost layer.
• 9. Tunica media is the middle layer.
• 10. Tunica intima is the innermost layer.
• 11. Aorta is approximately 23 mm in diameter.
• 12. Arterioles are about 0.2 mm in diameter.
• 13. Capillaries are 7–9 μm in diameter.
• 14. Venules are 40–50 μm in diameter.
• 15. Veins > 2 mm have valves.
• 16. Valves prevent backflow in veins.
• 17. Blood flow in veins is aided by gravity, valves, and muscular contraction.
• 18. Red blood cells flow in single file in capillaries.

Remember

• - Arteries = away from heart; thick walls; no valves
• - Capillaries = exchange; one cell thick
• - Veins = toward heart; thin walls; valves
• - Tunica adventitia = outer
• - Tunica media = middle
• - Tunica intima = inner
• - **Aorta** = 23 mm
• - **Arterioles** = 0.2 mm
• - **Capillaries** = 7–9 μm
• - **Venules** = 40–50 μm
• - **Veins > 2 mm** = valves
• - Valves = prevent backflow
• - Venous return = gravity, valves, muscular contraction

Common Mistakes

• 1. Confusing arteries and veins — Arteries carry blood away; veins carry blood toward the heart.
• 2. Thinking arteries always carry oxygenated blood — Pulmonary arteries carry deoxygenated blood.
• 3. Thinking veins always carry deoxygenated blood — Pulmonary veins carry oxygenated blood.
• 4. Mixing up the tunics — Adventitia (outer), media (middle), intima (inner).
• 5. Forgetting that capillaries are one cell thick — This allows exchange.
• 6. Thinking veins have thick walls — Veins have thin walls.
• 7. Confusing valves in veins with semilunar valves — Veins have valves to prevent backflow.
• 8. Mixing up the sizes — Aorta (23 mm), arterioles (0.2 mm), capillaries (7–9 μm), venules (40–50 μm).
• 9. Believing arteries have valves — Arteries have no valves.
• 10. Forgetting the role of muscular contraction in venous return — Muscular contraction helps squeeze veins.

Quick Revision

Blood vessels are tubular structures that carry blood throughout the body. There are three main types: arteries, capillaries, and veins. Arteries carry blood away from the heart; they have thick walls with three tunics (adventitia, media, intima) and no valves. Capillaries are one cell thick and permit exchange of gases, nutrients, and wastes. Veins return blood to the heart; they have thin walls, three tunics, and valves (in veins > 2 mm) to prevent backflow. The aorta is about 23 mm in diameter; arterioles are about 0.2 mm; capillaries are 7–9 μm; venules are 40–50 μm. Blood flow in veins is aided by gravity, semilunar valves, and muscular contraction.

60-Second Revision

• Arteries = away from heart; thick walls; no valves
• Capillaries = exchange; one cell thick
• Veins = toward heart; thin walls; valves
• Tunica adventitia = outer; media = middle; intima = inner
• Aorta = 23 mm; arterioles = 0.2 mm
• Capillaries = 7–9 μm; venules = 40–50 μm
• Veins > 2 mm = valves
• Valves = prevent backflow
• Venous return = gravity, valves, muscular contraction
• Warning: Pulmonary arteries carry deoxygenated blood; pulmonary veins carry oxygenated blood

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• Chapter:
• 2. Blood Circulatory System of Man

• Topic:
• 2.8 ROLE OF ARTERIOLES IN VASOCONSTRICTION AND VASODILATION

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