Blood Vessels and Tissue Fluid

Different types of blood vessels:

• ARTERIES: Carry blood AWAY from the heart and into the arterioles
• ARTERIOLES: Smaller arteriesthat control blood flowfrom arteries to capillaries
• CAPILLARIES: Tiny Vessels thatlink arterioles to veins
• VEINS: Carryblood from capillaries back to the heart

All of these blood vessels EXCEPT CAPILLARIES have the same basic layer structure:

• TOUGH OUTER LAYER - resists pressure changes from both within and outside the vessel
• MUSCLE LAYER - contract and control blood flow
• ELASTIC LAYER - helps maintain blood pressure by stretching and springing back
• THIN INNER LINING (ENDOTHELIUM) - smooth, prevents friction + thin - allow diffusion
• LUMEN - NOT a layer but the central cavity of the blood vessel through which the blood flows

FUNCTION:

• To transport blood rapidly under high pressure from the heart to tissues 

STRUCTURE:

• Muscle layer is THICK compared to veins: Smaller arteries can be constricted and dilated to control volume of blood passing through them

• Elastic layer is THICK compared to veins: Important as blood pressure needs to be kept high if blood is to reach the extremities of the body. Elastic wall is STRETCHED at each beat of the heart (SYSTOLE), SPINGS BACK when heart relaxes (DISTOLE). This helps maintain high pressure and smooth pressure surges created by the beating of the heart

• Thickness of the wall is LARGE: Resists the vessel bursting under pressure

• NO VALVES: Except in arteries LEAVING THE HEART - blood is under constant high pressure, tends not to flow backwards

FUNCTION:

• Carry blood (under lower pressure than arteries) from arteries to capillaries

• Controls the blood flow between them

STRUCTURE:

• Muscle layer is THICKER than arteries: This contracts to allow constriction of the lumen, restricting the blood flow so controls its movement into the capillaries that supply the tissues with blood

• Elastic layer is THINNER than arteries: Blood pressure is lower

FUNCTION:

• Transport blood slowly, under low pressure, from tissues to the heart

STRUCTURE:

• Muscle layer is THIN compaired to arteries: Veins carry blood AWAY from tissues, so their constriction and dilation cannot control the flow of blood to the tissues

• Elastic layer is THIN comparied to arteries: Low pressure of blood within the veins won't cause them to burst and pressure is too low to create recoil action

• Overall thickness is THIN: No need for a thick wall, pressure within veins is too low to create risk of bursting. Also allows them to flattern easily, aiding blood flow in them.

• VALVES THROUGHOUT: Ensure blood flow is one way, doesn't flow backwards. When body muscles contract, veins are compressed, pressurising the blood within them

FUNCTION:

• To exchange metabolic materials eg. oxygen, carbon dioxide, glucose, between blood and cells
• Blood flow is SLOWER - allows more time for exchange of materials

STRUCTURE:

• Walls consist of only the LINING layer: Extremely THIN, so distance over which diffusion takes place is SHORT. RAPID diffusion of materials between blood and cells

• NUMEROUS and HIGHLY BRANCHED: So has a LARGE SURFACE AREA

• NARROW diameter: Permeate tissues, so no cell is far away from a capillary

• NARROW LUMEN: Red blood cells are squeezed flat against the side of the capillary, brings them closer to the cell so SHORT diffusion pathway

• Spaces inbetween the lining (endothelial) cells: Allow white blood cells to escape in order to deal with infections within tissues

FORMATION:

• Blood pumped through the arteries, arterioles and then narrower capillaries creates HIGH HYDROSTATIC PRESSURE at the ARTERIOLE end of the capillary. HIGH WATER POTENTIAL
• This forces the tissue fluid OUT OF THE BLOOD PLASMA, into a gap between the capillary and the cells. Some goes into the cells
• This force is only big enough to force water out, leaving all cells and proteins in the blood. Type of filtration is called ULTRAFILTRATION

RETURINING BACK TO THE CIRCULATORY SYSTEM: 

• Needs to be returned when exchanged metabolic materials with the cells
• SOME return via the capillaries 
• Loss of water creates LOW HYDROSTATIC PRESSURE + LOW WATER POTENTIAL in capillary
• HIGHER in the tissue fluid gap, so is FORCED BACK into the capillaries
• OSMOTIC forces from the proteins and blood plasma pull water back into capillaries
• MOST go through cells, to LYMPH vessel - DRAINAGE, returns tissue fluid to vein in NECK