Arteries:
• Arteries carry blood from the heart to the rest of the body.
• They’re thick-walled, muscular and have elastic tissue in the walls to cope
with the high pressure caused by the heartbeat.
• The inner lining (endothelium) is folded, allowing the artery to expand – this
also helps top cope with high pressure.
Capillaries:
• Capillaries are the smallest of the blood vessels.
• They are where metabolic exchange occurs – substances are exchanged
between cells and the capillaries.
• There are networks of capillaries in tissue (called capillary beds), which
increase the surface area for exchange.
• Capillary walls are only one cell thick, which speeds up diffusion of
substances (e.g. glucose and oxygen) into and out of cells.
Veins:
• Veins take blood back to the heart.
• They’re wider than equivalent arteries, with very little elastic or muscle
tissue as the blood is under lower pressure.
• Veins contain valves to stop the blood flowing backwards.
• Blood flow through the veins is helped by contraction of the body muscles
surrounding them.
Features of Blood Vessels:
Why do arteries have thick walls with collagen (a rough, fibrous protein)?
In the arteries, the blood travels at a high pressure, so the collagen makes the walls
strong and durable.
Why do arteries have thick layer of smooth muscle and elastic tissue?
Smooth muscle layer to withstand high pressure; elastic tissue to maintain high pressure.
Why do capillaries have a narrow lumen?
Blood moves slower through the capillaries, allowing plenty of time for diffusion.
Why do veins have valves?
To prevent the backflow of blood.
Feature Arteries Veins
Lumen Narrower Wider
Walls Thicker Thinner
Collagen More Less
Smooth Muscle More Less
Elastic Layer More Less
Valves No Yes
, Junaid Ali Topic 1 Revision Notes 4
Atrial Systole Ventricular Systole Diastole
Atria Contracting Relaxing Relaxing
Ventricles Relaxing Contracting Relaxing
AV Valves Open Closed Open
Semi-Lunar Valves Closed Open Closed
Atrial Systole:
• Ventricles relax.
• Atria contract, increasing pressure.
• AV valves open (pressure higher in atria than
ventricles).
• Blood pushed into ventricles.
• Ventricular pressure and volume increase.
Ventricular Systole:
• Atria relax.
• Ventricles contract, increasing pressure.
• AV valves shut to prevent backflow (pressure
higher in ventricles than atria).
• SL valves open (pressure higher in ventricles than
arteries)
• Blood pushed into arteries.
Diastole:
• Atria and ventricles relax.
• SL valves shut to prevent backflow (pressure
higher in arteries than ventricles).
• Blood returns to heart and fills atria (pressure
higher in veins than atria).
• Pressure increases in atria.
• Ventricles relax and pressure decreases; AV
valves open.
• Blood flows passively into ventricles from atria.
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