Blood vessels

Arteries and Veins shared structures

• Tough outer layer: resists pressure changes (in and out)

• Muscle layer: contracts to control blood flow

• Elastic layer: maintains blood pressure by stretching and springing back (recoiling)

• Endothelium; smooth layer of cells reduce friction and allow diffusion

Capillaries

• Endothelial layer only

Arteries and arterioles

Outer wall is thicker than veins

• Prevents vessel bursting under high pressure

Muscle Layer is thicker than veins

• Can Constrict and dilate to control the volume of blood passing through them

Elastic layer is thicker than veins

• Maintains high blood pressure, stretches at each beat (systole). Then springs back (recoils) during diastole.

Lumen is thinner than veins

• Maintains high pressure

No valves

Examples of when an artery may increase or decrease in blow flow to specific parts of the body:

• Regulate temperature

• Digestion

• Exercise

Veins and Venule

Outer wall is thinner than arteries

• Low pressure and allows for more flexibility

Muscle Layer is thinner than arteries

• Constriction and dilation doesn’t control blood flow to the tissues

Elastic layer is thinner than arteries

• Low pressure doesn’t need protection against bursting

Valves

• Due to low pressure there is a risk of blood flowing in the wrong direction

• Muscles contracting nearby could force blood in the wrong direction, valves prevent this

Capillaries

Epithelial layer is one cell thick

• Short diffusion pathway

Highly Branched

• Large surface area for exchange

Narrow diameter

• Able to permeate tissues, shorter diffusion pathway

Narrow lumen

• Red blood cells are squeezed flat against lumen wall, shortening diffusion pathway

Gaps between endothelial cells

• Allow white blood cells to pass between them

Comparison of structure of veins and arteries

Tissue Fluid

Watery liquid

Containing: Water, glucose, amino acids, fatty acids, ions and oxygen

Moist

Liquid between cells within a tissue

Secretion

• Higher (hydrostatic) pressure at the arterial end of capillaries causes liquid in blood plasma to seep out into the surrounding tissues

• Only enough pressure to force small molecules into the surrounding tissues

Ultrafiltration: Filtration assisted by blood pressure

Return of the Tissue fluid

Tissue fluid re-enters the blood via these mechanisms

• As fluid leaves the capillaries, pressure reaches equilibrium

• Low pressure at venous (veins) end of the capillaries causes net movement back into the capillaries

• Blood plasma has lost water (by ultrafiltration), but not proteins and cells, capillaries then have a lower water potential, so water moves back in by osmosis

Fluid composition

• Higher CO₂ concentration, Urea and other waste materials, 

• Low O₂, Glucose, amino acid etc

Lymphatic system

Function

• Fluid balance

• Immune response

• Fat absorption

• Waste removal

• Protein transport

Lymphatic capillaries

• Take in excess tissue fluid

• Similar to circulatory capillaries, but has a dead end.

• Capillaries combine and re enter the bloodstream via two ducts near the heart