Topic 3: Exchange: Blood Vessels

Artery structure

Lumen → endothelium → elastic fibres (thick layer of elastin) → smooth muscle (thick in a layer of muscle) → thick outer layer of collagen fibres

Vein structure

Lumen → endothelium → elastic fibres → smooth muscle

Capillary structure

Lumen → endothelium

Smooth muscle layer

Contracts to control the flow of blood in arteries, arterioles and veins

Elastic layer

Allows the vessel to stretch and recoil (spring back) in arteries, arterioles and veins

Endothelium

Thin inner lining which is smooth to reduce friction in all vessels

Arteries

• Carry blood away from heart

• Narrow lumen to maintain pressure (compared to vein)

• Thick muscular walls and elastic layer allows stretch and recoil with each ventricular contraction to maintain high blood pressure

• Endothelium is folded to allow stretching to maintain high pressure

• Collagen provides strength to prevent vessel from bursting and to maintain vessel shape

• Elastic fibres (elastin allows) stretch and recoil to minimise changes in pressure

• Thick smooth muscular layer contracts and relaxes to constrict or dilate lumen and control blood flow

Pressure decreases along blood vessel due to friction

Structures to cope with high-pressure and recoils under low pressure

Most veins carry deoxygenated blood. Veins into heart.

Except pulmonary vein (lungs to heart – oxygenated)

Most arteries carry oxygenated blood. Arteries out of heart.

Except for pulmonary artery (from heart to lungs – deoxygenated)

Arterioles

• Branch of arteries into capillaries (smaller branches)

• Blood is directed to different areas of the body by muscles inside the arterioles which contract to restrict blood flow and relax to allow full blood flow (allows blood to be directed to areas according to demand)

• Smaller than arteries

• Larger lumen than arteries

• More smooth muscle and less elastin than capillaries as do not need to withstand such high pressure

• Relatively thicker layer of muscle than arteries

Capillaries

• smallest diameter of all blood vessels – same as red blood cells – narrow lumen allows red blood cells to be close to body

• Short diffusion pathway as very close to cells and walls are one cell thick

• Large number of capillaries means that is an increase in surface area for gas exchange

• Site of substance exchange with cells

Continuous capillaries

• Uninterrupted endothelial layer

• Small ions through only e.g. oxygen

• Most common type

• E.g. form the blood brain barrier

Fenestrated capillaries

• Increase permeability due to pores

• Water, small molecules and some proteins can diffuse across

• E.g. villi in small intestine and kidneys

Sinusoid capillaries

• Irregular endothelial layer, intracellular gaps

• Incomplete basement membrane

• Allows blood cells to move out of it

• E.g. liver, spleen, and parts of lymphoid system

Veins

• Carry blood towards the heart

• Wider lumen than arteries as blood has lower pressure

• Collagen provides strength to prevent the vessel bursting and maintain vessel shape

• Little smooth muscle and elastic fibre - not much is needed due to low blood pressure and thinner walls allows veins to be easily compressed aiding flow of blood

• Valves – pocket valves shot to prevent backflow of blood when veins are squeezed by surrounding skeletal muscle

Valves

Pocket valves in veins have similar structure to valves in heart, but are the only type of valve controlled by skeletal muscle.

Skeletal muscle ensures blood flow is in the right direction. Contract and compress the vein, helping to increase pressure and push blood along against gravity. Valve above the muscle opens.

Muscles are relaxed = valves closed

How the system works

Arteries → arterioles → capillaries (form network and tissue called capillary bed) → venules → veins (back to heart)

Venules

• smaller than veins

• Very thin walls, very little smooth muscle

• Also have valves

What is the BP like in arteries?

High

What is BP like in arterioles?

Low

What is BP like in capillaries?

Low = tissue fluid

What is BP like in venules?

Low

What is BP like in the veins?

Lowest BP = values (vena cava)

Blood vessel pressure

Blood always hows from high to low pressure. The vena cava is the final blood vessel to return to the heart, so it has the lowest pressure. Blood pressure is highest in the aorta immediately after a ventricular contraction.

Blood loses pressure as it moves along the vessels from the aorta due to it branching into many vessels, the increasing of lumen size of the vessels and friction.

There are fluctuations in the blood pressure of arteries and arterioles as elastic tissue stretches when blood moves through at high pressure (when the ventricles contracts/ systole) then recoils (springs back) as blood pressure drop (when the ventricle relaxes / diastole).

This helps to even out the pressure of blood flowing through the artery

Systole

Part of the cardiac cycle during which chambers of the heart contract after refilling with blood

Diastole

The relaxed phase of the cardio cycle when the chambers of the heart are refilling with blood