The Heart & Circulation (Lec 2)

Question 1

Peripheral Input Controlling HR

Answer 1

Modification of HR as a result of input received from:
− Muscle chemoreceptors (e.g. ions)
− Arterial chemoreceptors (e.g. changes in O2, CO2, pH)
− Specific mechanoreceptors (baroreceptors in aortic arch and carotid sinus)

Question 2

Stroke Volume

Answer 2

Volume ejected per beat, known as SV

Question 3

Cardiac Output

Answer 3

Q = Heart Rate (HR) x Stroke Volume (SV), usually measured in ml/min or L/min

Question 4

Fick Equation

Answer 4

VO2 = Q(CaO2 – CvO2)

The volume of oxygen consumed by the cells is equal to the cardiac output multiplied by the amount of oxygen extracted from the blood

Question 5

VO2

Answer 5

Volume of oxygen consumed by the cells
• Units are usually l/min or ml/kg/min
• Resting VO2 is 1 met = 3.5ml/kg/min

Question 6

Cardiac Output and Exercise

Answer 6

↑BF can result from redistribution from non-working muscles and organs and/ or ↑Q

SV = EDV – ESV
– EDV = End Diastolic Volume
– ESV= End Systolic Volume

Question 7

Extrinsic Control of SV

Answer 7

↑ SNS activity and norepinephrine release
– ↑ contractility
– ↑HR

↑ Contractility
– Important because diastolic filling time is significantly reduced at very high heart rates

Question 8

Changes in Cardiac Cycle during Exercise

Answer 8

% Time spent in Diastole:
– At rest 0.5/0.8 = 62.5%
– During Exercise 0.13/0.33 = 39%

Question 9

Factors Assisting the Rate of Venous Return

Answer 9

1. Venoconstriction
2. Muscle Pump
3. Respiratory Pump

Question 10

Venoconstriction

Answer 10

Increases venous return by reducing the volume capacity of veins to store blood. Occurs via a reflex sympathetic constriction of smooth muscle in veins draining muscle.

Question 11

Muscle Pump

Answer 11

The result of the mechanical action of rhythmic skeletal muscle contractions. As muscles contract they compress veins and push blood back towards the heart.

Question 12

Respiratory Pump

Answer 12

The rhythmic pattern of breathing also provides a mechanical pump by which venous return is promoted.

Question 13

Major Factors determining Cardiac Output

Answer 13

↑ SV, ↑ HR

Question 14

Major Factors influencing SV

Answer 14

↑ EDV, ↑ Sympathetic nerve activity to Heart, ↑ Plasma Epinephrine

Question 15

VO2 max

Answer 15

Maximal amount of O2 that the body can utilise

– ‘Gold’ standard for aerobic fitness

Question 16

Prolonged Steady State Exercise

Answer 16

Steady State = stable/unchanging VO2
Remember:
•	Q is maintained
•	SV decreases
•	Therefore HR increases to maintain Q

Question 17

Blood Pressure Response to Exercise

Answer 17

• ↑ Q̇

* ↑ Blood viscosity (↑ sweating = ↓PV)

Question 18

Frank-Starling Law

Answer 18

Intrinsic control of SV

• The SV ↑’s in response to an ↑ in EDV (the volume of blood prior to contraction)

Question 19

EDV

Answer 19

End Diastolic Volume: the volume of blood in ventricles prior to contraction

Question 20

ESV

Answer 20

End Systolic Volume: the volume of blood in ventricles after contraction