Lecture 30 - Cardiovascular System: Capillary Exchange Flashcards
Describe capillary beds
Networks that connect between arterioles and venules
What is the function of arterioles and anastomoses?
They control the total flow of blood through the capillary bed. How?
Arterioles - can reduce blood supply to entire capillary bed through the contraction of smooth muscle
Arteriovenous anastomoses - they can dilate, diverting blood away from the higher resistance in the rest of the capillary bed
Function of precapillary sphincters
They contract and dilate to create variable, pulsatile flow through different capillaries within a bed
T/F? Capillaries in many tissues have a continuous (complete) and tight endothelial layer
False - They have a continuous, but leaky, endothelial layer
Paracellular transport can occur
Describe fenestrated capillaries and where they can be found
Have a complete endothelium, but have small pores along the endothelial cell
Often found in areas involved in absorption (intestine) or filtration (kidneys), and many neuroendocrine organs
Describe sinusoidal capillaries and where they can be found
Have a discontinuous epithelium, allowing for the movement of very large molecules
Found in liver, bone marrow, spleen, and some endocrine organs. Produces/recycles plasma proteins and blood cells
What is the purpose of slow blood flow in capillaries?
It allows large amounts of time for exchange between the contents of blood and tissue
What is capillary exchange and what are the three components to capillary exchange?
Describes the bidirectional movements of substances into or out of the blood to/from body tissues
3 components:
1. Diffusion
2. Filtration
3. Osmosis
What is diffusion?
The passive movement of substances due to concentration differences
Water, ions, and small organic molecules diffuse through fenestrations. Large molecules move through sinusoids
What does rate of diffusion across capillary walls depend on?
The chemical properties of the substance
What is filtration and hydrostatic pressure?
Filtration is the movement of fluid through small pores in response to pressure differences. Hydrostatic pressure is the force exerted on the vessel wall by the fluid inside
What is osmosis and what is osmotic pressure?
Diffusion of water molecules across a selectively permeable membrane to equalize osmolarity. Osmotic pressure is a force that is pushing water to flow by osmosis
What are the two types of pressures that net filtration pressure (NFP) is dependent on? What is NFP?
- Capillary Hydrostatic Pressure (CHP) - pressure of the blood contents inside the capillary on the capillary walls
- Blood Colloid Osmotic Pressure (BCOP) - pressure driving water from the ISF into the blood plasma due to the presence of large suspended molecules in the plasma ([molecs.]_plasma > [molecs.]_ISF) that cannot cross the capillary wall
NFP = CHP + BCOP
NFP tells us the pressure gradient available to produce filtration
Is filtration or reabsorption favoured near an ateriole?
There is positive NFP, favouring filtration
CHP>BCOP = +ve NFP, meaning a pressure gradient that favours fluid movement into the ISF
What happens if filtration occurs along a capillary?
What is the NFP in the middle of the capillary bed?
If filtration occurs, CHP will decrease because of resistance and there is less fluid remaining inside the capillary. BCOP will remain the same (all the large particles are still present)
In the middle, CHP=BCOP=0 NFP, therefore no more fluid moves into the ISF, but solute diffusion still occurs
