Cardiovascular Physiology

Question 1

What is the approximate stroke volume and cardiac output?

Answer 1

Stroke volume =70ml
Cardiac output 5L

Question 2

What are the main types of neuro-receptors in the heart and peripheral blood vessels

Answer 2

Hearts:
-cholinergic (muscarinic) - parasympathetic
-adrenergic β1 - sympathetic

Peripheral blood vessels:
-adrenergic α1 - sympathetic

Question 3

What is required for flow of a liquid?

Answer 3

A difference in pressure

Question 4

What equation links pressure, flow and resistance.

Answer 4

Pressure = flow x resistance

Question 5

What is the equation for MAP

Answer 5

MAP = CO x SVR

Question 6

What is Poiseuille’s formula?

Answer 6

Q= ( ΔP* π * r4) / (8* η * L)

ΔP = pressure difference

r = radius

η = viscosity

L = length

Question 7

What are the approximate pressures in the right atria, left atria, right ventricle and left ventricle?

Answer 7

Right atria: 5/2
Left atria: 6/3
Right ventricle: 25/0
Left ventricle: 120/0

Question 8

In which vessels is there the biggest drop in pressure?

Answer 8

Arterioles (resistance vessels).

Their small radius produces a high resistance, causing reduced flow rate and drop in pressure.

Resistance is inversely proportional to the fourth power of the radius.

Question 9

During which stage of the cardiac cycle does most of the coronary blood flow to the LV occur?

Answer 9

During diastole, as during systole due to contraction the coronary blood flow is occluded by the pressure in the LV.

(Helps explain rate related ischaemia, increased demand + reduced coronary perfusion)

Question 10

Describe the nervous control of the cardiovascular system?

Answer 10

Baroreceptors in the carotid sinus respond to stretch.

When stretched there is an inhibitory effect on the cardiovascular system causing a reduction in heart rate and vasodilation.

When there is hypotension and reduced stretch then there is therefore increased heart rate and vasoconstriction due to reduced inhibitory nerve supply.

Question 11

Describe the humoral control of the cardiovascular system?

Answer 11

Reduced blood flow in the renal tubule is detected by the juxtaglomerular apparatus

This causes the conversion of prorenin to renin.

Renin causes the conversion of angiotensinogen to angiotensin I.

Angiotensin I is then converted to angiotensin II by angiotensin converting enzyme, (ACE is found in the vascular endothelium of the lung and kidney).

Angiotensin II is a potent vasoconstrictor.

Angiotensin II also stimulates aldosterone release from the adrenal cortex, leading to Na+ and H2O retention.

Question 12

Describe the stages of hypovolaemic shock, including its effect on HR, cap refill and BP.

Answer 12

Stage 1:
<750ml blood loss
<15% blood loss
HR <100
Cap refill normal
BP Normal

Stage 2:
750-1500ml
15-30%
HR 100-120
Cap refill >2
BP normal

Stage 3
1500-2000ml
30-40%
HR 120-140
Cap refill>5
BP reduced

Stage 4
>2000mls
>40%
HR>140
Cap refill undetectable
BP very low

Question 13

Cardiac muscle differs from other excitable tissues in having two specific requirements. What are these?

Answer 13

1. Need for simultaneous contraction of all the fibres.
2. Prevention of sustained contraction (tetany), as this would prevent cardiac filling.

Question 14

How is simultaneous contraction of all the cardiac fibres achieved?

Answer 14

Simultaneous contraction of all fibres is achieved by:

The specialized conduction system: SA node to AV node, down bundle of HIS and up via the purkinje fibres.

The syncitial nature of cardiac muscle*

By prolonging the action potential via slow Ca2+ inflow through L-type channels**, thus ensuring total ventricular depolarization.

*aka all the cardiac cells are connected meaning they contract as a unit

**type of calcium channel, L stands for long lasting referring to the length of activation

Question 15

How long is a cardiac action potential, and how does this compare to a nerve action potential.

Answer 15

Cardiac 300ms
Nerve 1-2ms

Question 16

What is the natural firing rate of the SA node?

Answer 16

100-120/min. The heart rate is normally slower because of dominant vagal parasympathetic activity.

Question 17

Describe the ionic sequence that brings about an action potential in a cardiac pacemaker cells?

Answer 17

Continuous slow inward leak of Na+ until the threshold potential of -40 mV is reached. There is also a small influx of Ca2+ via the T channels.

Main depolarization brought about by Ca2+ (not Na+) inflow through L-type channels.

Repolarization from K+ outflow.

Question 18

What can influence the rate of firing of the pacemaker cells?

Answer 18

Changes to permeability of Na+ causing a tachycardia, as the threshold of voltage gated Ca2+ channels is more readily reached

Or changes in permeability to K+ causing a bradycardia due to the membrane being more re-polarised (aka more -ve).

Question 19

What is the advantage of delayed conduction at the AV node?

Answer 19

Allows for atrial emptying and helps protect the heart from atrial tachyarrhythmias.

Question 20

Draw a pressure volume loop for the right ventricle?

Answer 20

More triangular shaped
Google

Question 21

What happens to the diastolic/systolic time in a tachycardic patient and what is the significance?

Answer 21

The cardiac cycle is shorter but diastole becomes proportionally more shorter than systole.

This is significant as the majority of coronary blood flow occurs during diastole and the majority of ventricular filling occurs during diastole.

Aka in tachycardias more likely to get cardiac ischaemia (rate related ECG changes)

Will have a reduced stroke volume due to reduced pre-load.