Basic Sciences - Cardiovascular Physiology Flashcards
Definition of flow
Quantity of fluid being moved passed a point in a given time
Not velocity
Cardiac output equation
CO = SV x HR
Approximate CO in average person
5 L/min
Approximate SV in average person
70 ml
Requirement to generate flow of a fluid
Pressure gradient
Main types of receptor present in the heart
Muscarinic cholinergic receptors - parasympathetic
Beta 1 adrenergic - sympathetic
Main types of receptor present in peripheral blood vessels
Alpha 1 adrenergic - sympathetic
Factors which impact flow of fluid
Most efficient type of flow
Laminar flow
Correlation between pressure, flow and resistance
Pressure = Flow x Resistance
Correlation of pressure, flow and resistance in human circulation
MAP = CO x SVR
Mean arterial pressure
Cardiac output
Systemic vascular resistance
How to calculate flow under laminar flow conditions
Poiseuille’s law
Poiseuille’s law
Which factor has greatest influence on flow
Radius of vessel / pipe (according to Poiseuille’s law)
Calculations for MAP
MAP = CO x SVR
MAP = 2/3 Diastolic BP + 1/3 (Systolic - Diastolic BP)
Reason for MAP representing 2/3 Diastole and 1/3 Systole
Cardiac cycle is 2/3 in diastole
Arterial BP waveform
Dicrotic notch - the change in wave form on the descent
Dicrotic notch on arterial waveform representation
Elastic recoil of aortic wall immediately after aortic valve closure
Normal pressures of right atrium
Systole 5 mmHg
Diastole 2 mmHg
Normal pressures of right ventricle
Systole 25 mmHg
Diastole 0 mmHg
Normal pressures of left atrium
Systole 6 mmHg
Diastole 3 mmHg
Normal pressures of left atrium
Systole 120 mmHg
Diastole 0 mmHg
Reason for large pressure difference in left ventricle between systole and diastole
Diastole pressure must be less than left atrial pressure in order to fill
Systole pressure must overcome aortic resistance
Reason right ventricle has lower systolic pressure than left ventricle
Pulmonary circulation has lower resistance than systemic circulation
Less pressure needed to perfuse it
Pressure changes in different circulation vessels
Why does biggest drop in pressure occur in the arterioles
Small radius produces high resistance
Resistance is inversely proportional to radius^4 according to Poiseuille’s law
What happens to coronary vessels during systole
Pressure generated by LV in systole occludes the coronary vessels running through which provides cardiac blood supply
During which cardiac cycle phase does most coronary blood flow occur
Diastole - LV is relaxed
Two main systems that regulate cardiovascular system
Nervous (autonomic nervous system)
Humoral (essentially Renin-angiotensin-aldosterone system)
Speed of response of autonomic nervous system to hypotension
Seconds - minutes
Sensor of autonomic nervous system to hypotension + location
Baroreceptors
Carotid sinus
Effector of autonomic nervous system response to hypotension
Sympathetic outflow
Response caused by autonomic nervous system from hypotension
Vasoconstriction
Tachycardia
Speed of response of Humoral system to hypotension
Minutes - hours
Sensor of Humoral system to hypotension + location
Juxtaglomerular apparatus
Kidney
Effector of Humoral system response to hypotension
Renin-angiotensin
and subsequently aldosterone
Response caused by Humoral system from hypotension
Vasoconstriction
Na+ / water retention
Duration of action of nervous vs humoral systems
Humoral system is longer lasting but slower onset
Actions of renin
Activates cascade which produces angiotensin II
Stimulates aldosterone release from adrenal cortex
Where is angiotensin I converted to angiotensin II
Lung
Effects of hypervolaemia to reduce circulating volume
Distention of atria
Causes release of ANP (atrial natriuretic peptide)
Results in sodium and water excretion
Stages of hypovolaemic shock
Other useful measurements to assess hypovolaemia
Urine output
Respiratory rate
Central venous pressure
Cardiac myocyte action potential
How is simultaneous cardiac muscle fibre contraction achieved
Specialised conduction system
Syncitial nature of cardiac muscle
Prolongation of action potential
How is cardiac action potential prolonged
Slow Ca2+ inflow through L type channels
Duration of cardiac action potential
300 ms
Duration of nerve cell action potential
1 to 2 ms
How is tetanic / sustained contraction of cardiac muscle prevented
Ion channel inactivation - prolonged refractory period
Natural firing rate of the SA node
100 - 120 bpm
Why is heart rate slower than intrinsic firing rate of SA node
Dominant vagal parasympathetic activity
Ionic sequence of SA node action potentials
1) Continuous slow inward leak of Na+ until the threshold potential of -40 mV is reached
2) Main depolarization brought about by Ca2+ (not Na+) inflow through L-type channels
3) Repolarization from K+ outflow
There is no resting phase or resting membrane potential, and phases 1 and 2 of the action potential are absent. The cycle length determines the heart rate.
How is heart rate changed from an action potential / ionic perspective
Phase 4 slope is altered to make cycle length shorter or longer
Increased Na+ permeability -> Tachycardia
Increased K+ permeability -> Bradycardia
Pacemaker cell action potential graph
Time delay applied to impulse by AV node
~100 ms
