BS2014: Cardiovascular and Respiratory Physiology

Stroke volume

volume of blood ejected from the heart during each cycle

If a ventricle contains 100ml of blood at the end of filling, and 40 ml at the end of contraction, what is the stroke volume?

60 ml

Cardiac output

total volume of blood pumped by the ventricle per minute

Cardiac output equation

cardiac output = stroke volume x heart rate

What does ECG stand for

electrocardiogram

What does the P wave represent in an ECG

atrial systole

What does the QRS wave represent in an ECG

ventricular systole

What does the T wave represent in an ECG

ventricular diastole

What does the R-R interval of an ECG represent

the heart beat duration

What does the Q-T interval represent in an ECG

the time taken for ventricles to depolarise and then repolarise

Significance of LONG QT intervals

• they are arrhythmogenic

• these long QT intervals can cause sudden death in athletes unaware of their condition

What does the P-R interval represent in an ECG

• shows the onset of atrial systole to the onset of ventricular systole

• length of time for electrical conduction to pass from atria to ventricles

Where is the P-R interval measured from (despite its name)

from the beginning of the upslope of the P wave to the beginning of the QRS wave

Why is atrial repolarisation not visible on an ECG

because it is masked by ventricular systole (QRS wave)

What changes to the ECG occur during exercise

• shortening of R-R interval

• slight increase in P wave amplitude

• shortening of P-R interval

• shortening of Q-T interval

• S-T segment depression

• T-wave amplitude may flatten

Why does the P-R interval shorten during exercise

shorter conduction time between atria and ventricles

Why does the Q-T interval shorten during exercise

faster depolarisation of ventricles

What are the 2 main centres of the control of heart rate

• central command in the motor cortex of the brain

• cardiovascular control centre in the medulla in the brainstem

What are 2 nerves associated with controlling the heart rate + their effect on HR

• Accelerator nerve (sympathetic NS) — when stimulated increases HR

• Vagus nerve (parasympathetic NS) — when stimulated decreases HR

How do sympathetic nerves increase heart rate at the SAN

• accelerator nerves release noradrenaline at the SAN

• noradrenaline binds to β₁ adrenergic receptors on the SAN

• this activates G-proteins, stimulating adenyl cyclase to produce cAMP

• increased cAMP causes faster depolarisation, increasing the heart rate

How do parasympathetic nerves decrease heart rate at the SAN

• The vagus nerves release acetylcholine at the SAN

• acetylcholine binds to M₂ muscarinic receptors on the SAN

• opening K⁺ channels leading to hyperpolarisation of the cell membrane

• inhibiting adenylyl cyclase, reducing cAMP, and decreasing intracellular concentrations of Ca²⁺ and Na⁺

• decreasing heart rate

What is the intrinsic heart rate

100 BPM

What is the normal resting heart rate

60-70 BPM

What is the maximum heart rate?

220 - age

Chronotropic effect

changes in heart rate

Inotropic effect

the force of muscle contraction

Which catecholamines have a positive chronotropic and inotropic effect

adrenaline and noradrenaline

Which catecholamine has a negative chronotropic and inotropic effect

acetylcholine

What does stroke volume depend on

• end diastolic volume

• aortic or pulmonary arterial pressure

• circulating adrenaline and noradrenaline

Frank-Stirling Mechanism

the stroke volume increases in response to an increase in the volume of blood filling the heart (the end diastolic volume) when all other factors remain constant

How does a heart transplant affect heart rate

• takes longer for heart rate to increase during exercise (8-10 minutes) and decrease following exercise

• a higher resting heart rate — typically about 90-110 bpm

Why does a heart transplant cause changes in heart rate

the nerves to the heart are cut therefore the heart is denervated removing vagal tone and allowing the heart to beat at its intrinsic, faster rate

How do baroreceptors respond to high blood pressure

• increased baroreceptor firing

• increased parasympathetic (vagal) output

• decreased sympathetic output

• decrease in heart rate and vessels relax

• blood pressure returns to normal

How do baroreceptors respond to low blood pressure

• decreased baroreceptor firing

• decreased parasympathetic (vagal) output

• increased sympathetic output

• increase in heart rate and vessels constrict

• blood pressure returns to normal

VO₂ Max

the maximum amount of oxygen that can be taken in, transported and utilised in 1 minute, measured in L/min or mL/kg/min

VO₂ max of UK general public

• 30-40 mL/kg/min for men

• 25-35 mL/kg/min for women

Highest recorded VO₂ max

97.5mL/kg/min (Oskar Svendsen)

Atrial fibrillation

an irregular heart rhythm, characterized by rapid, irregular electrical signals causing upper chambers of the heart to quiver instead of beating regularly

Blood distribution in the CVS

• 64% in veins

• 13% in arteries

• 9% pulmonary

• 7% arterioles, capillaries

• 7% heart