Cardiac Output Flashcards
What is cardiac output?
volume of blood ejected from heart every minute (ml/min)
What is the equation for cardiac output?
CO = heart rate x
stroke volume
What is cardiac output regulated by?
heart rate
stroke volume
What is heart rate regulated by?
- parasympathetic activity
- sympathetic activity (and epinephrine)
What does increased parasympathetic activity do to heart rate?
decreases HR
What is sympathetic activity controlled by?
norepinephrine and epinephrine
What does increased sympathetic activity do to heart rate?
increases HR
What is stroke volume regulated by?
- sympathetic activity (extrinsic control)
- diastolic volume (intrinsic control)
What does increased sympathetic activity do to stroke volume?
increases SV
What does increased EDV do to stroke volume?
increases SV
What does increased sympathetic activity also control?
moves blood from venous system into heart
- increases EDV
- increases SV
What regulates the sympathetic nervous system?
noradrenaline/adrenaline acting on β1 receptors on nodal cells
- increases slope of phase 4
- increases HR
What regulates the parasympathetic nervous system?
acetylcholine acting on muscarinic receptors on nodal cells
- decreases slope of phase 4
- decreases HR
Extrinsic Control of Stroke Volume
- sympathetic nerves release NA that acts on β1 adrenergic receptors on muscle cells found in ventricle
- increases intracellular cAMP, which activates PKA
- PKA catalytic subunit phosphorylates proteins, which increases intracellular Ca2+
- ↑ entry from outside cell into myocyte from outside cell
- ↑ Ca2+ release from intracellular stores - increased stroke volume – contraction of ventricle
muscles contract more forcefully
What is the Frank Starling Law of the Heart?
SV of left ventricle will increase as left ventricular volume increases due to myocyte stretch, causing more forceful systolic contraction
Starling Law of the Heart
What is the strength of contraction related to?
initial length of cardiac muscle fibers
muscle is more stretched initially = stronger contraction
What is the initial length of heart muscle before contraction equivalent to?
EDV
What is the strength of contraction equivalent to during systole?
SV
What is end diastolic volume (EDV or preload)?
how filled the heart is at end of diastole
How does EDV increase SV?
mobilization of more blood that increases EDV results in increase in SV
What is stroke volume related to in regards to muscle?
muscle tension
What is EDV related to in regards to muscle?
cardiac
muscle fibre length
Starling Curve
What is EDV at rest?
~ 140 ml
Starling Curve
What is SV at rest?
~ 70 ml
What is the relationship between EDV and SV?
increase in EDV = increase in SV
- more blood is pumped out (within reason – heart will pump what it receives)
- if heart receives more but is not pumping out more, will get build-up of blood
What is the equation for blood pressure?
blood pressure = CO x SVR
What determines SVR?
radius of blood vessels
resistance ∝ 1/radius4
- if radius is halved (ie. vasoconstriction), resistance increases by factor of 16 → flow decreases by 16x
What is the equation for arterial pressure?
arterial pressure = CO
x SVR
What are the vasoactive hormone constrictors?
- angiotensin II (AII) – converted from angiotensinogen (liver) in response to renin from kidney
- arginine vasopressin (AVP) – from brain
What do hormones AII and AVP act on?
receptors (each have different ones) linked to Gq proteins (like NA)
What is the vasoactive hormone dilator?
atrial natriuretic peptide (ANP) – from heart
What does the hormone ANP act on?
receptor linked to GI protein
ANP Vasodilation Pathway in Vascular Smooth Muscle Cell
- ANP acts on receptor linked to GI protein
- GI leads to activation and production of cGMP
- cGMP phosphorylates MLCK
- phosphorylated MLCK inhibits contraction
**not same vasodilation as 𝛽2 adrenergic receptor
What is afterload?
pressure that ventricle must generate in order to eject cardiac output
- when heart pushes, it pushes against pressure in aorta
- ie. chronic high arterial pressure – high afterload
What are the 2 Starling forces?
force of filtration: capillary hydrostatic pressure (CHP)
- move fluid out of capillaries
force of reabsorption: capillary oncotic pressure (COP)
- draw fluid into capillaries
What increases filtration?
things that increase CHP
What increases reabsorption?
things that increase COP
Starling Forces Across Entire Length of Capillary
What occurs at arterial end?
CHP > COP
fluid moving out of vessel
- CHP depends on arterial end (high blood pressure)
Starling Forces Across Entire Length of Capillary
What occurs at venous end?
CHP < COP
fluid moving into vessel
blood pressure decreases as blood travels down capillary into venous end
How does blood pressure change as it travels through capillary?
blood pressure decreases as blood travels down capillary into venous end
Starling Forces Across Entire Length of Capillary
What occurs when venous pressure is too high (ie. heart failure)?
- filtration on both ends of vessel
- no reabsorption
- results in edema – accumulation of fluid in tissue
What happens during heart failure?
- blood can’t get pumped efficiently
- blood builds up in right side of heart, and pushes back against pulmonary veins
- increases venous pressure and CHP on venous end
