Peripheral Circulation: Capillaries and Veins
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What are the major types of vessels within systemic circulation?
Aorta, Larger artery, small artery, arteriole, capillaries, venule, vein and vena cava.
Capillary structure and function
Single endothelial cell wall (0.5um thick), no elastin, no smooth muscle (so cannot dilate or constrict), bring blood to within 30um of virtually every cell, slow velocity (good for diffusion) Width 5-10um, slightly wider at the venous end than the arteriole end
What are the mechanisms of exchange in capillaries?
• Diffusion • Vesicular Transport • Bulk Flow
Diffusion mechanism of exchange in capillaries
O2, CO2 and lipid soluble substances (e.g. hormones) can diffuse across through the endothelial cells
What is Ficks Law of Diffusion
Rate = permeability coefficient x concentration gradient x area. Rate of diffusion: size, charge, area, permeability, concentration gradient (CO2 binds 20x more readily than O2 = greater permeability coefficient.
Why is low flow rate in capillaries important?
Ensures enough time for exchange. The larger the area the lower the velocity flow = good for diffusion.
Why must there be flow for diffusion to occur in the capillaries?
The is no innervation of nerves = no constriction or dilation (constant resistance) Only 25% open at any time Increased tissue VO2 opens more capillaries Flow determined by arterioles.
Flow equation
Delta P (pressure difference mmHg) / Resistance to flow.
The effects of capillary pressure when arterioles dilates
Pressure increases.
The effects of capillary pressure when arterioles constrict
Pressure decreases.
Explain vesicular transport in capillaries
Transport of large charged molecules (e.g. proteins, antibodies) through the endothelial cells using vesicles. Not a commonly used mechanism
Explain bulk flow in capillaries
Transport of H20, electrolytes, small molecules (Me <70,000). They move through the fenestrations of fenestrated capillaries.
When arterioles contract what happens
Contraction = constriction = increase resistance = decrease flow
When arterioles dilate what happens
Relaxation = dilation = decrease resistance = increase flow
What is the interstitial?
Interstitial fluid is a solution that bathes and surrounds the tissue cells of multicellular animals.
What pressure does blood enter a capillary?
35mmHg. This forces fluid into the interstitium via fenestrations. (Pc)
Why aren’t large volumes lost from the system due to filtration from the capillaries
Filtration = Reabsorption + Lymph flow. Forces cause reabsorption back into the capillary and lymph. We filter slightly more than we reabsorb.
What does starlings force/ equation tell us?
Illustrates the role of hydrostatic and oncotic force in the movement of fluid across the capillary membrane.
What is the hydrostatic pressure in the intersitium?
0mmHg (Pif)
What is the oncotic pressure in interstitium?
Very Low. pi if
What is the hydrostatic pressure when the blood leaves the venule?
15 mmHg. Pressure drop inside the vessel.
What is the capillary oncotic pressure
25 mmHg (pi C) osmotic pressure is due to proteins
Any change in starlings forces will cause what?
A change in blood flow
At the arteriole end of a capillary why is there filtration out of the capillary?
Because the hydrostatic pressure is greater than the oncotic pressure