L24 Systemic Circulation Flashcards
Systemic circulation
Most vascular beds in parallel
Function to maintain sufficient pressure to push blood through all vascular components or mean arterial pressure (MAP)
Arterial side v venous side
Arterial:
Lower compliance
Higher pressure
Lower volume
Venous:
Higher compliance
Lower pressure
Higher volume
TPR - total peripheral resistance (resistance heart has to overcome to get blood from arterial to venous side)
It’s variable , we can control TPR and CO
Arteriole
Major resistance vessel
Resistance causes a dramatic pressure drop
Convert pulsatile pressure swings of cardiac cycle into non-fluctuating pressure
Radii of arterioles supplying organs can be adjusting to distribute CO and reg BP
Pulse pressure
Difference btw systolic pressure and diastolic pressure
Compliance
Pressure in blood vessels is determined by the volume of blood in vessel and compliance of vessel
Compliance- ability of vessel to expand/give way to stress
Influenced by: vessel geometry, elastic properties of wall, vascular smooth muscle tone
Decreases with age
Arterial pressure
Depends on arterial blood volume and arterial compliance
Rises during ventricular systole as arterial inflow is transiently greater than outflow - volume transiently increases
Pressure falls during diastole as blood leave the arterial side to enter venous system
Transfer of blood to venous circulation during diastole is called peripheral run off
Arterial pressure
MAP
Peak pressure = systolic pressure (SP)
Pressure just prior to ventricular ejection= diastolic pressure (DP) - determined by arterial compliance and residual arterial volume immediately prior to next cardiac ejection
Pulse pressure PP= SP-DP
MAP ~ DP + PP/3
Capillary flow continues during
Diastole
Elastic recoil of the arterial walls converts the stored energy into capillary flow during diastole (maintains blood flow)
Pulse pressure and systolic pressure depend on
Aortic compliance and SV
PP= SP -DP = deltaP =deltaV/C
At constant compliance, aortic pulse pressure and systolic pressure
Increase as SV increases
At a constant SV, aortic pulse pressure and systolic pressure
Increase as aortic compliance decreases
Diastolic pressure mainly influence by
HR (determines time interval for blood transfer from arterial system to venous system (peripheral run off))
And
TPR(determines rate of volume transfer to periphery)
Both affect volume of blood remaining in arterial system at end of diastolic period
Reduced TPR
More run off
Decreased DP
Elevated TPR
Less run off
Increased DP
Increase HR
Less time for run off
Increase DP
Decrease HR
More time for run off
Lower DP
Increasing SV
Directly increases PP
MAP increases
Decreasing HR
Increases run off time
DP and SP decrease
MAP decrease
Increase SV while decreasing HR
Increase PP
DP will decrease
SP May increase
Venous system
Low resistance
Low pressure
High compliance
75% blood volume may be in venous system at any one time
Venous return
Blood flow from the peripheral veins to the right atrium
Equal to cardiac output at steady-state
Pressure gradient for venous return is difference btw peripheral venous pressure (pressure of blood entering peripheral veins from capillaries) and right atrial pressure
Anything that increases pressure of vena cava (central venous pressure), increases venous return
Effects of posture on CO
On standing there is a rapid translocation of blood from thorax to lower extremities
Blood pools in peripheral veins and capillary hydrostatic pressure increase
Venous return decreases; CO and MAP falls transiently
Neurally-mediated reflexes are activated to return MAP towards the supine level
Venous return may be augmented by muscle pump mechanism
Muscle pump
When standing , skeletal muscles of lower extremities begin rhythmic cycles of contraction and relaxation
Reflex initiated by stimulation of plantar surface of feet
Muscle contractions squeeze veins in muscles and drive blood towards the heart (augments venous return)
Venous valves prevent retrograde blood flow
Veins refill during relaxation of skeletal muscle
Skeletal muscle pump activity is intensified with walking and running
