P: capillaries + veins Flashcards
Arterioles function
regulate blood flow to capillary beds
Venules function
collecting channels and storage vessels
Internal diameters of capillaries?
4-9µm
Continuous capillary structure
- Adjacent endothelial cells tightly joined together
- Intercellular clefts permit passage of ions & small molecules
- Not permeable to proteins
- Muscle, lungs and adipose tissue
Fenestrated capillary structure:
- Wider intercellular pores provide greater permeability
- Permeable to small proteins
- Kidney, endocrine gland & intestinal capillaries have numerous fenestrations
Discontinuous capillary structure
- Large, leaky capillaries with pores
- Allow red and white blood cells and various serum proteins to pass
- Liver, spleen and bone marrow.
What are true capillaries?
smooth muscle at origin of capillary (called precapillary sphincter). Regulates entry of blood to capillary. Also participate in delivery of. Nutrients + removal of wastes from cells and deliver blood into venules - nutritional flow.
What do arteriovenous shunts do?
bypass capillaries - nonnutritional flow
Capillary blood pressure (hydrostatic pressure) increases from:
- Dilation of arterioles –> increased inflow
- Increased venous pressure –> reduced outflow
Transmural pressure:
intravascular pressure - extravascular pressure.
Effect of an increase in transmural pressure:
triggers contraction of terminal arterioles
Effect of a decrease in transmural pressure:
triggers relaxation of terminal arterioles
Ways of transcapillary exchange:
- Diffusion
- Filtration
- pinocytosis
Capillary flow-limited transport:
- Small molecules pass rapidly through pores and accumulate at arterial end of capillary if blood flow is slow
- If flow is rapid, diffusion happens further down capillary, increasing supply to tissues
- Larger molecules diffuse at slower rate + diffuse further down capillary
- Increase in blood flow/ increase in capillary density increases supply of diffusible solutes.
How do lipid-soluble molecules (O2, CO2) diffuse in capillaries?
Through endothelial walls, not limited to pores.
Filtration at capillaries:
- Bulk movement of fluid through pores in capillaries into interstitial area forms tissue gel due to proteoglycan content of extracellular environment
- Rivulets of free-moving flid adjacent to cells and collagen fibres
- Direction and magnitude of movement of fluid between capillaries and interstitium is determined by relative strength of hydrostatic and osmotic forces.
Hydrostatic forces:
- Hydrostatic pressure moves fluid out of capillaries into interstitial fluid, driving capillary filtration
- Dependent on capillary bp, which is dependent on arterial and venous pressures
- Increase in arterial or venous pressure elevates capillary hydrostatic pressure
- Increases in arteriolar resistance reduces capillary hydrostatic pressure
- Vasodilation of arterioles and constriction of venules raises hydrostatic pressure
Hydrostatic forces when moving to standing position:
- Initial movement of blood to lower extremities raises arterial and venous pressure, increasing capillary hydrostatic pressure
Increased filtration and oedema results if not corrected by myogenic regulation.
Average capillary hydrostatic pressure:
32mmHg at arterial end, 15mmHg at venous end.
Oncotic pressure/ colloid osmotic pressure:
Plasma has a higher [solute] –> this creates an osmotic pressure called colloid osmotic pressure or oncotic pressure. This promotes movement of water out of tissue fluid into capillaries (absorption). Stays at abot 25mmHg
Starling forces:
opposing hydrostatic and oncotic pressures
Determining factors if capillaries filter or absorb?
- Magnitude of capillary hydrostatic pressure
- If capillary hydrostatic pressure is very high: 100% filtration. If it is very low: 100% absorption.
What is pinocytosis?
Movement of fluid across endothelial cells by vesicles
How does pinocytosis work?
- Section of cell membrane engulfs plasma
- Membrane buds off (endocytosis) to form intracellular vesicle
- Vesicle is transported across cell + fuses with cell membrane (exocytosis), releasing contents into interstitial fluid
Responsible for exchange of large molecules.
Total lymph production rate:
120ml/hr
[Protein] of lymph from most tissues:
around 2g/dl
[Protein] of lymph from liver:
around 6g/dl
[Protein] of lymph from intestines:
3-4g/dl
What is oedema?
Excess fluid accumulation in extracellular spaces caused by excessive capillary filtration and/or poor lymphatic drainage
Causes of oedema?
- Increases capillary filtration coefficient
- Increased capillary hydrostatic pressure
- Decreased plasma colloid osmotic pressure
How does increased blood pressure/ venous obstruction cause oedema?:
increases capillary filtration pressure –> more tissue fluid formed at arteriolar ends of capillaries
How does increased tissue protein concentration cause oedema?:
Decreased osmosis of water into venular ends of capillaries –> localized tissue edema due to leakage of plasma proteins through capillaries through inflammation + allergic reactions.
Pressure of blood in the right atrium. How does decreased plasma protein concentration cause oedema?
Decreases osmosis of water into venular ends of capillaries.
What is central venous return?
Pressure of blood in the right atrium.
Effect of increased pressure in smaller veins (venous pressure):
increased venous return to right atrium when CVP is unchanged.
Increase in blood volume and reduction in compliance of veins increases overall venous pressure.
Effect of reduced CVP?
Reduction in CVP increases venous return to right atrium
Effect of gravity on venous pressure:
- Pooling of blood in lower extremities of arterial and venous systems
- Higher compliance of veins –> greater pooling + distension
Reduces venous return and cardiac output
- Higher compliance of veins –> greater pooling + distension
Orthostatic hypotension:
when moving from lying to standing, blood pooled in thoracic cavity shifts towards lower extremities, causing an immediate drop in venous return + cardiac output. Resultant drop in arterial pressure.
