Blood Vessels and their Functions

Blood Vessels

Arteries: Carry blood away from the heartArterioles: Control blood flow from arteries to capillariesCapillaries: Link arterioles to venulesVenules: Link capillaries to veinsVeins: Carry blood towards the heart

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Guide to blood vessels

Structure of Blood Vessels• Tough outer layer – resists

pressure• Muscle layer – can contract

and control the flow of blood

• Elastic layer – can stretch and recoil to maintain blood pressure

• Endothelium – smooth layer to prevent friction

• Lumen – not a layer – a cavity

Structure of Arteries

• Thick muscle layer – control the flow of blood

• Thick elastic layer – maintains blood pressure by stretching and recoiling

• Narrow lumen

Structure of Veins

• Thin muscle layer• Thin elastic layer• Wide lumen• Valves to prevent

backflow of blood

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Blood flow in veins

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Varicose veins

If a vein wall becomes weakened, valves may no longer close properly. This allows backflow of blood, causing the vein to become enlarged and bumpy, and become varicose.

This usually happens in superficial veins, near the skin surface in the lower legs, as opposed to deep veins, which lie underneath muscles.

Varicose veins can be surgically removed without affecting blood flow, as most blood is returned to the heart by deep veins.

Structure of Capillaries

• No muscle• No elastic

• Thin layer of cells only

Capillary Structure and Function

• Thin layer of cells – short diffusion distance.

• Numerous and highly branched – large SA for diffusion.

• Narrow diameter – keep all cells close by.

• Narrow lumen – bring red blood cells close to cells = short diffusion distance.

• Spaces between cells – allow white blood cells to escape.

Structure of Arterioles

• Thicker muscle layer than arteries

• Thinner elastic layer than arteries

• Can constrict to control blood flow through capillaries

• Muscle and elastic layers

• Similar to veins but lumen is smaller

• Can constrict to control blood flow through capillaries

Structure of Venules

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What is blood?

Blood is a specialized transport medium that is also considered a special type of connective tissue. An average adult has 4–6 litres of blood.

Blood has a range of functions such as:

transport

defence

thermoregulation

maintaining pH of body fluids.

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The composition of blood

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Features of erythrocytes

What are the specialized features of an erythrocyte?

flattened, biconcave disc shape: ensures large surface area to volume

ratio for efficient gas exchange

diameter (6–8 µm) larger than capillary diameter: slows blood flow to enable diffusion of oxygen

no nucleus or organelles:

maximises space for haemoglobin, so

more oxygen can be transported

large amount of haemoglobin: for

transporting oxygen

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Blood clotting