Lecture 15 - Blood Flow Flashcards
What is tissue perfusion involved in?
Delivery of oxygen and nutrients to, and removal of wastes from:
Tissue cells
Gas exchange in the lungs
Absorption of nutrients from the digestive tract
Urine formation isn’t he kidneys
What does extrinsic control of blood flow involve?
The sympathetic nervous system and hormones to control blood flow through the whole body
Intrinsic control of blood flow
Automatic adjustments of blood flow to each tissue are made in proportion to its need by modifying T diameter of local arterioles
What are Metabolic controls strongly stimulated by?
Shortage of oxygen at the tissues
What do cells lacking oxygen release?
Nitric oxide
A powerful vasodilator, to increase local blood flow.
normally balanced by endothelins
If nitric oxide levels rise, they overcome the effects of the endothelins and blood vessels dilate.
Endothelins
vasoconstrictors released by endothelium that are also present in the bloodstream.
What does Myogenic control involve?
localized response of vascular smooth muscle to passive stretch
Increased arterial pressure stretches vessel walls more than normal, so smooth muscle responds by constricting, causing decreased blood flow to the tissue to avoid damage from elevated pressure.
Long term autoregulation
develops over weeks or months involves an increase in diameter of blood vessels and an increase in number of vessels in a specific area, called angiogenesis
Why does Muscular auto regulation occur
almost entirely in response to decreased oxygen concentrations
What happens in active or exercise hyperemia?
blood flow increases in direct proportion to muscle or metabolic activity. Local controls override sympathetic vasoconstriction and blood flow can increase tenfold compared to when at rest.
Blood flow to brain
Cerebral blood flow is tightly regulated to meet neuronal needs, because neurons cannot tolerate periods of ischemia (inadequate blood supply), and increased blood carbon dioxide causes marked vasodilation.
Brain is vulnerable to extreme systemic pressure changes; mean arterial pressure below 60 mmHg can cause syncope (fainting) while mean arterial pressure above 160 mmHg can result in cerebral edema.
What does low pulmonary oxygen cause?
Vasoconstriction
What does high pulmonary oxygen cause?
Vasodilation
Local vasoconstriction
Tightening blood vessels in one area
Decreases local blood pressure
Systemic vasoconstriction
Tightening blood vessels everywhere
Increases blood pressure
What does slow blood flow through capillaries promote?
Diffusion of nutrients and gases and bulk flow of fluids
Bulk flow
Fluid forced out through intercellular clefts int he capillaries at the arterial end and most of the fluid returns to the blood at venous end
Causes continuous mixing between plasma and interstitial or extracellular fluid.
Hydrostatic pressure
Force of a fluid against a membrane
Capillary hydrostatic pressure
capillary blood pressure that tends to force fluids through capillary walls.
pressure is greater at arterial end of the capillary bed than at the venule end.
Interstitial fluid hydrostatic pressure
the pressure pushing fluid back into the blood vessel.
usually assumed to be zero because lymphatic vessels drain interstitial fluid.
Colloid osmotic pressure
the force opposing hydrostatic pressure
created by the presence of large, non-diffusible molecules that are prevented from moving through the capillary membrane.
Capillary colloid osmotic pressure
the “sucking” pressure created by non-diffusible plasma proteins pulling water back into the capillary.
Interstitial fluid colloid osmotic pressure
is inconsequential because interstitial fluid has a very
low protein content.
When do fluids leave capillaries?
if net hydrostatic exceeds net colloid osmotic pressure.
This occurs at the arterial end of the capillary bed.
When do fluids enter the capillaries?
if net colloid osmotic pressure exceeds net hydrostatic pressure.
This occurs at the venous end of the capillary bed.
Edema
an abnormal increase in the amount of interstitial fluid.
caused by either an increase in outward pressure (driving fluid out of the capillaries) or a decrease in inward pressure (failure to return fluid back to the capillaries or to drain into the lymphatic system).
What can cause an increase in interstitial fluid colloids osmotic pressure?
Inflammatory response
Inflammation increases capillary permeability and allows proteins to leak into interstitial fluid, which causes fluid to be pulled into the interstitial space.
