Factors affecting blood flow Flashcards
how does blood flow vary
from one organ to another
what does blood flow between organs depend on
the metabolic demands of each organ system
what does blood flow to the lungs align with
cardiac output
why does blood flow to the lungs align with the cardiac output
because all blood must pass through the lungs for oxygenation and removal of carbon dioxide
which organ system requires blood flow similar to cardiac output
the lungs
what increases and decreases the blood flow to specific organs
metabolic requirements of the tissues
why does blood flow to skeletal muscle increase during exercise
greater demand for oxygen
why does blood flow to the GI system increase following a meal
there is greater demand for oxygen following ingestion of food
why are there interorgan blood flow differences
as a result of alterations in vascular resistance
how can the mechanisms of blood flow regulation be categorised
local and hormonal
describe local control of blood flow
matches blood flow to metabolic requirements of the tissue systems
direct action on the metabolites on arteriolar resistance
describe the neural/hormonal control of blood flow
action of sympathetic nervous system on vascular smooth muscle
action of vasoactive substances
what are some vasoactive substances involved in hormonal control of blood flow
- histamine
- bradykinin
- prostoglandins
what are the specific needs for blood flow
- delivery of oxygen to tissues and nutrients
- removal of carbon dioxide and hydrogen ions
- maintenance of ion concentrations in tissues
- transport of hormones
what are the nutrients that blood delivers to tissues
glucose
amino acids
fatty acids
what are the mechanisms by which intrinsic control of blood flow can occur
autoregulation
reactive hyperemia
active hyperemia
describe autoregulation
maintenance of constant blood flow while arterial pressure changes
example of autoregulation
if the coronary arterial pressure decreases, there is immediate compensatory vasodilation of coronary arterioles to decrease the coronary vasculature resistance
describe active hyperemia
blood flow to tissues is proportional to its metabolic activity
increased blood flow when metabolic activity increases
increased arteriolar dilation
when would there be an increase in blood flow due to metabolic activity
exercising skeletal muscle tissue which increases oxygen consumption
describe reactive hyperemia
increase in blood flow in response to a prior period of decreased blood flow
example of reactive hyperemia
following a period of arterial occlusion, oxygen debt accumulates. the longer the arterial occlusion, the greater the oxygen debt and the greater the increase of blood flow above the pre occlusion levels. this continues occurs the oxygen debt is reversed
what are the factors that determine resistance to blood flow
vessel diameter
vessel length
viscosity of blood
how does vessel diameter change
with vasoconstriction and vasodilation as a result of contraction and relaxation of vascular smooth muscle
what is the vascular smooth muscle
the tunica media
how does a small change in vessel diameter affect resistance
there are large changes as a result of even just small changes
does viscosity of blood ever really change
no it usually remains within a narrow range
what is the exception to the concept that blood viscosity changes
haemocrit
what can happen to the blood vessels to lead to changes in resistance
change in their diameter
what can cause an increase in blood viscosity
dehydration and immobility
what can reduce the risk of increases blood viscosity
hydration
movement
compression socks
why does movement decreases changes in normal blood viscosity
it encourages venous return
what can a change in blood viscosity lead to
risk of deep vein thrombosis
what is vessel resistance directly proportional to (an increase in one will increase the other)
length of the vessel and viscosity of the blood
what is vessel resistance inversely proportional to
radius to the fourth power of the vessel
describe the equation for the relationship between flow, pressure and resistance
flow = change in pressure over resistance
what is poiseuilles equation
Q = r^4 times change in pressure over viscosity times length of the vessel
what does poiseuilles equation describe
how flow is related to perfusion pressure, radius, length, and viscosity
what does the poiseuilles equation assume
- flow is through a uniform straight pipe
- flow is non pulsatile
- flow is laminar
what does it mean to say that flow is laminar
it is smooth
when can velocity of flow be considered the same across the tube
when the flow has negligible resistance
what happens to the velocity of flow as viscosity increases
it increases to a maximum at the centre of the tube
do veins have a large or small cross sectional area
large
how are venous valves orientated
toward the heart
why are venous valves orientated toward the heart
to maintain blood flow in one direction
what is venous return to the heart aided by
the action of working skeletal muscles intermittently contracting to compress the veins
what happens to intrathoracic pressure during exercise
it becomes more negative because there are more frequent respirations that increase the pressure gradient between abdominal and thoracic veins
what is in control of the sympathetic activation of venous return
noradrenaline
how does noradrenaline play a role in venous return
sympathetic activation which constricts the veins and increases venous return to the heart
what is preload
venous return to the right ventricle
what happens when there is an increase in preload
the heart will have to work harder to pump the blood out and this can be a problem in coronary artery disease and heart failure
what is angina
coronary artery disease
what is coronary circulation
this is circulation that facilitates the perfusion of the myocardium
maintains high basal rate of oxygen supply to cardiac muscle