Cardiac Cycle and Basic Haemodynamics

Question 1

What are the pressure and flow changes that occur with systole and diastole?

Answer 1

Diastole

• Ventricular pressure is less than atrial pressure
• Ventricular pressure is much less than pulmonary artery and aortic pressure
• Blood fills the ventricle passively down a small pressure gradient (but high flow because resistance is low). At rest 80-90% of ventricular

Atrial systole - ventricular diastole

• Ventricular pressure is less than atrial pressure.
• Ventricular pressure is much less than pulmonary artery and aortic pressure
• Active filling of ventricles (passive filling slow prior to atrial systole)

Ventricular systole - isovolumic contracion

• Ventricular pressure is greater than atrial pressure
• Ventricular pressure is rising but is still less than pulmonary artery and aortic pressure
• Ventricular volume doesn’t change (ie iso-volumic).

Ventricular systole - ejection

• Ventricular pressure is greater than atrial pressure
• Ventricular pressure is greater than pulmonary artery and aortic pressure
• Blood ejected into pulmonary artery and aorta from R and L ventricles.

Ventricular diastole - isovolumic relaxation

• Ventricular pressure is greater than atrial pressure
• Ventricular pressure has fallen below pulmonry artery and aortic pressure
• Ventricular volume doesn’t change (iso-volumic)

Diastole

• Ventricular pressure is less than atrial pressure
• Ventricular pressure is much less than pulmonary artery and aortic pressure
• Blood fills the ventricle passively down a small pressure gradient (but high flow because resistance is low)

Question 2

Apply physiological principles of the cardiac cycle to explain the normal heart sounds.

Answer 2

There are up to four heart sounds: S1, S2, S3 and S4.

S1 and S2 are each comprised of two sounds, names after the valves that are responsible for them. The two sounds comprising S1 cannot normally be distinguished clinically.

• S1 is the sound of the mitral and tricuspid valves closing, and marks the start of systole.
  
  • M1 = mitral valve closes
  • T1 = tricuspid valve closes
• S2 is the sound of turbulence against the closing of the aortic and pulmonary valves. The aortic valve closure just before the pulmonary valve.
  
  • A2 = aortic valve closes
  • P2 = pulmonary valve closes
• S3 is apparent in youth, exercise and pregnancy. It is the rush of blood during rapid diastolic filling.
• S4 is a presystolic (or atrial) ‘gallop’. Caused by the atrium contracting forcefully to overcome an unusual afterload e.g. in LV hypertrophy.

Question 3

Contrast and describe the different vessels of the circulation.

Answer 3

Elastic aa: large amounts of elastic fibres in the tunica media. Conducting vessels as they conduct blood from the heart to the major regions of the body.

Muscular aa: large amounts of smooth muscle in the tunica media. Also called distributing vessels as they distribute blood to organs.

Arterioles: regulate blood flow from arteries to capillaries. Resistance vessels.

Capillaries: thin, endothelium and BM membrane only vessels between terminal arteriole and post-capillary venule. Called exchange vessels.

Venules: return blood flow from capillaries to venous system. Some exchange in the smallest venules.

Veins: return blood flow to heart. Highly compliant - capacitance vessels.

Question 4

Explain how pressure and resistance affect flow, with reference to Ohm’s law.

Answer 4

Ohm’s law states that flow is directly proportional to change in pressure and inversely proportional to resistance.

Flow = change in pressure / resistance

This principle can be applied to a single vessel, the circulation supplying a whole tissue and the entire systemic circulation of the body (it is the heart that provides the pressure gradient for flow).

If we use the example of the whole circulation , in this case flow is the entire output of the heart (we call this the cardiac output). Therefore:

Cardiac output = (MAP - RAP) / TPR

Question 5

Explain how viscosity and vessel radius affect flow, with reference to Poiseuille’s law.

Answer 5

Poiseuille’s law states that resistance is

• inversely proportional to the 4th power of vessel radius
• directly proportional to vessel length
• directly proportional to blood viscosity

R = 8nl / πr⁴

Therefore, Poiseuille’s law demonstrates that radius is the biggest determinant of resistance and thus flow.

Question 6

Mean arterial blood pressure

Answer 6

MAP = DBP + 1/3 PP

The PP is roughtly proportional to the stroke volume, but also partiall determined by peripheral resistance and compliance of the vessel.

During hypovolemic shock - the heart reduces pump volume, thus pulse pressure decreases.

The MAP is closer to diastolic because the heart spends more time in diastole.

Question 7

Where does the biggest fall in pressure occur?

Answer 7

At the greatest source of resistance. As

Change in pressure = flow x resistance

The bigegr the resistance of a degment, the more the pressure drops across that degment.

The biggest pressure drop is across small arteries and arterioles (ie. the resistance vessels).

Question 8

Compliance and distensibility

Answer 8

Veins are 20 times more compliant than arteries.

This means that veins act as capacitance vessels, with the majority (61%) of blood at any point is held in the systemic veins.