Capillary Circulation Flashcards
Capillary functions
- Nutrient Exchange: • Glucose • Oxygen • Free Fatty Acids • Other Nutrients
- Waste Exchange: • CO2 • Lactate & Ammonia • Other metabolic byproducts
Characteristics of capillaries that facilitate simple diffusion:
- Capillaries have thin walls - one cell thick
- Therefore, the distance traveled by diffusion particles (i.e. O2 & CO2) is short
- Capillaries have a high total surface area
- Flow velocity through capillaries is slow because of the large cross sectional area of capillary beds in the circulation. This will facilitate diffusion.
major factor that dictates open/closed precapillary sphincters
-metabolites generated in local tissues/environment
Primary local factors that affect precapillary shincters in skeletal muscle?
- dec P_O2 ***( hypoxia)
- inc P_CO2
- metabolic acids
Primary local factors that affect precapillary shincters in brain?
- P_CO2 and O2
- adenosine
Primary local factors that affect precapillary shincters in heart?
- *adenosine
- pH
- P_O2 and CO2
The effect of epinephrine contraction vs. dilation
will depend on the relative concentration of alpha1 and beta2 receptors within a given capillary bed.
The primary response of norepinephrine will be to (on capillaries)
cause a constriction since it will activate predominantly alpha1 receptors.
Dopaminergic receptors are abundant in the kidney & gut. At low doses, dopamine will cause a (on periperal resistance and kidney). At higher doses, dopamine will (on receptors and effect)
- will cause a drop in peripheral resistance and a selective perfusion of the kidney.
- dopamine will activate alpha1 receptors causing vasoconstriction.
Angiotensin II is a physiologically important
vasoconstrictor
Kinins, such as Bradykinin, are all
vasodialators
In the heart and most other places, adenosine acts to
vasodilate (A2 receptors) while in the kidney, adenosine acts to vasoconstrict (A1 receptors).
Hypoxia, Increased CO2, K+ & H+ : when present systemically, all tend to produce
vasodialation
Krebs Cycle Intermediates: presence of these indicate increased metabolism will cause
vasodialation
Endothelin (constriction) & Endothelium Derived Relaxing Factor do what?
vasodialation - all released from endothelium
Starling Forces that Favor Filtration:
- Capillary Hydrostatic Pressure: is simply the force that occurs due to blood pressure within the capillary that tends to drive fluid outward and through the endothelial pores.
- Interstitial Fluid Oncotic Pressure: The word oncotic is used to describe the osmotic effect due to capillary protein. Oncotic pressure, then is simply a colligative property. In this case - we are describing the influence of proteins which are present in the interstitium - their osmotic tendency to pull fluid out to the interstitium from within the capillaries.
Starling Forces that Favor Reabsorption:
- Capillary Oncotic Pressure: capillary oncotic pressure is the osmotic driving force - due to the presence of proteins inside the capillary - and tends to draw fluids from outside the capillary to the inside of the capillary.
- Interstitial Fluid Hydrostatic Pressure: This force tends to drive fluid back into the capillary. (For this discussion, we are assuming that interstitial fluid pressure is positive.)
conditions that may lead to edema and reasons
1) Liver disease=low serum production - not enough stuff in the blood= net effect more filtration out of capillary and less reabsorption
2) Elevated Capillary Blood Pressure: This can occur via two mechanisms - vasodilation & obstruction in the circulation. —>Increase in capillary hydrostatic pressure - will give rise to tendency towards edema.
3) Vasodilation - recall that in vasodilation, blood will enter the capillary at higher pressure. This will create a situation in which capillary blood pressure is excessively elevated with resultant edema. —> perhaps due to vasodilator drugs (i.e. some antihypertensives) - may produce edema.
4) Obstruction - such as a venous blood clot will cause the back up of pressure and hence an increase in pressure.—> will effectively increase capillary blood pressure due to the backing up of blood.
8) Congestive Heart Failure - Since the left ventricle is not efficiently pumping blood out of the heart, pulmonary pressure will increase and pulmonary capillaries will tend to leak due to elevated hydrostatic pressure. There will be potential towards pulmonary congestion and possibly pulmonary edema.
10) Post Capillary Venule permeability to proteins is increased. Proteins leaking out from venules will create an imbalance of Starlings Forces. Again, increased filtration and decreased absorption will be favored.
Effective Capillary Blood Pressure represents:
line that represents the forces that favor filtration.
Function of Lymph Capillaries:
- Removes excess fluid from interstitial space.
2. Removes excess protein from interstitial space.
