Capillaries and veins
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What does blood flow through between the aorta and vena cava?
Aorta Large artery Small artery Arteriole Capillaries Venule Vein Vena Cava
Describe the structure and function of a capillary
No elastin No smooth muscle Brings blood within 30 micrometres of every cell Velocity 1mm/s Single endothelial cell wall, 0.5micrometres thick Diameter of 5-10micreometres
What is the law of diffusion?
Rate = permeability coefficient x concentration gradient x area
What drives the diffusion of O2?
The concentration gradient
What increases the area of capillaries compared to arteries?
Extensive branching x10,000,000,000
What is a benefit of the low flow rate in capillaries?
Increases time for exchange
What factors determine flow in the capillaries?
No innervation, cannot constrict or dilate Only 25% open at any time Increased tissues VO2 opens more capillaries Flow determine by arterioles
What is exchanged via vesicular transport?
Large charged molecules e.g. proteins, antibodies
What is exchanged via bulk flow?
H20 Electrolytes Small molecules
What allows for bulk flow?
Junctions between endothelial cells Fenestrations
What is the hydrostatic pressure at the start of the capillary?
35mmHg
How do we calculate filtration?
Filtration = reabsorption + lymph flow
What is the starling equilibrium?
Pressure is higher = filtration Pressure is lower = reabsorption
What removes excess fluid?
The lymphatic system
Describe the structure of the lymphatic system
Thin walled vessels (like capillaries) One way valves prevent back flow (like veins) Returns to venous circulation via subclavian veins in chest
What is a result of changing starling forces?
Oedema
Describe veins
Less smooth muscle in tunica media Capable of dilating and constricting Floppy rather than rigid Capacity to hold large volumes Valves prevent backflow
Which vessel is the capacitance vessel?
The veins They hold 60-80% of blood volume
What determines cardiac filling?
Venous return
What determines venous return?
Compliance
What controls venous compliance?
The sympathetic nervous system
What factors affect central venous pressure?
Total blood volume Skeletal muscle pump Respiratory pump Gravity (lying down) Venous capacitance