Cardiovascular physiology Flashcards
What is the difference between adequate blood flow and adequate blood pressure
Flow and pressure are different entities closely related by the Hagan-Poiseuille equation. It should be considered that all tissues require adequate blood flow for adequate nutrient delivery and waste removal. However, in some situations pressure at which the blood is delivered is also critical:
- The kidney (requires a certain perfusion pressure)
- A disease state that requires a high pressure to overcome an obstruction (carotid stenosis)
Explain the dichrotic notch in the arterial waveform
The interruption in the decline in arterial pressure waveform is called the dichrotic notch and is thought to be caused by elastic recoil of the aortic wall immediately after aortic valve closure.
Why is mean arterial pressure (87) approximately one third of the way between the diastolic (70) and systolic (120) pressures.
At rest the cardiac cycle spends about 2 thirds of the time in diastole and one third of the time in systole
MAP = 1/3 SBP + 2/3 DBP
Why is the systolic RV pressure so much lower than the systolic pressure in the LV
Low pulmonary vascular resistance –> lower pressure needed to perfuse it
Cardiac muscle differs from other excitable tissues by having two specific requirements. What are these and how are these achieved
- Simultaneous contraction of all cells
- Prevention of tetany (sustained contraction)
Achieved through three factors:
- Specialized conduction system in the heart ensures contraction of all fibres.
- The myocardium is a functional syncitium
- Cardiac action potential is prolonged via slow Ca2+ inflow through L-type channels, thus ensuring total ventricular depolarization.
Ion channel inactivation results in a prolonged refractory period, thus preventing repeated tetanic contraction
How much longer is the cardiac action potential than that of a nerve cell?
200 times longer (300ms vs 1-2ms)
What prevents tetanic contraction in the heart
Ion channel inactivation resulting in a prolonged refractory period
What is the natural firing rate of the SA node. Is this different to the resting heart rate? why?
100 - 120
resting rate is 70
Slower because of tonic PSNS activity
What makes pacemaker cells different from ordinary cardiac myocytes
- Automaticity - they fire without external stimulus
2. Rhythmicity - in a set rhythm
How is automaticity in pacemaker cells achieved?
- “Funny” currents - continuous slow leak of Na ions into the cell until the threshold potential of -40mV is reached.
- Main depolarization caused by Ca2+ influx (T-type and then L-type) rather than Na= influx
- Repolarization from K + outflow
How is the heart rate change
The slope of phase 4 is increased (Increased Na+ permeability) or decreased (increase K+ permeability). This causes a tachycardia and bradycardia respectively.
How much does the AV node slow conduction by
100ms
How does depolarization occur through the ventricles
The AP descends therough the AV node into the bundle of His where it then divides into the left and right bundles and finally into the purkinje fibers which branch into the ventricles. the ventricles depolarize from inside to out.
What are the advantages of delayed conduction of the AV node
- Allows time for atrial emptying
2. Protects the ventricles from atrial tachyarryhthmias
What happens when baroreceptors are stretched?
Their firing rate increases –> inhibits output from the vasomotor centre –> relative increase PSNS tone = vasodilation
What is the difference between the carotid body and the carotid sinus?
Carotid body is a chemoreceptor situated at the bifurcation of the common carotid artery sensitive to O2, CO2 and H+
Carotid sinus is the initial dilatation of the internal carotid that contains baroreceptors sensitive to stretch
Where are high pressure baroreceptors found? How are these receptors stimulated and what are the effects of their stimulation?
In the carotid sinus. They are stretch receptors. Distension –> inhibition of SNS stimulation to the heart and circulation and increase in PSNS stimulation to the heart.
Bradycardia
Peripheral vasodilation
Reduced SNS also reduces release of adrenalin and noradrenalin from the adrenal medulla into the circulation
Where are low pressure baroreceptors located and what is the effect of their stimulation?
Location: Right atrium.
Small increases in atrial pressure distend the low pressure baroreceptors which lead to the release of atrial natriuretic peptide –> increase sodium and water excretion by the kidneys
ANP
1. Natriuresis
–> Increase GFR: Afferent VD and efferent VC
–> reduced tubular Na reabsorption
2. Inhibits renin release from kidney
3. Inhibits aldosterone release from adrenal cortex
When are both high and low pressure baroreceptors stimulated and what happens when they are not stimulated
Increase in pressure, distension and stretch –> bringing about their effects. The response to hypovolaemia and hypotension is therefore brought about by the fall in activity of these systems and abolition of the responses that they generate.
What are the major problems with massive blood transfusion
- Immune reaction
- Infection
- Dilutional coaguloathy (FFP + Plt required)
- Hypothermia
- Hyperkalaemia
- Hypocalcaemia
- Acidosis
