WEEK 5 - THE HEART

Question 1

What is the function of the cardiovascular system, also called circulatory system?

Answer 1

circulates blood around the body to deliver nutrients and remove wastes from tissues.

Question 2

How many components is the cardiovascular system made up of?

Answer 2

Three.
1. fluid (blood) - travels through
2. set of tubes (vessels) - connected to
3. a pump (heart)

RIGHT SIDE: PUMPS TO LUNGS (PULMONARY CIRCULATION)
LEFT SIDE: PUMPS TO BODY (SYSTEMIC CIRCULATION)

Question 3

Structures of heart and movement of blood flow

Answer 3

ARTERY = carries blood AWAY from heart
VEIN = carries blood TOWARDS heart

Question 4

Blood vessel function [ARTERIES]

Answer 4

Arteries have thick muscular, elastic walls to withstand pulsing of blood and prevents them from bursting. Elastic stretch and recoil keeps blood flowing at an even pressure

Question 5

Blood vessel function cont. [VEINS]

Answer 5

Veins have thin walls, wide lumen and little muscle, as most of pressure has been lost. They also contain valves to prevent backflow. Flow of blood back to heart aided by ‘skeletal muscle pumps’.

Question 6

Blood vessel function cont. [CAPILLARIES]

Answer 6

Capillaries are smallest blood vessels in body (only 1 cell thick!), so are in close contact with all body tissues to allow for diffusion of nutrients and wastes. There are ~100,000 km of capillaries!

Question 7

Blood Flow TO the heart

Answer 7

The coronary circulation (part of the systemic circulation) supplies blood to and drains blood from the heart tissue

coronary arteriesarise from the aorta, while the coronary veins drain directly into the right atrium.

Question 8

Blood flow TO the heart [Heart conditions]

Answer 8

Conditions affecting cardiac muscle are one of the leading cause of death in developed countries. The most common problem is ischaemic heart disease, where coronary arteries become narrowed or blocked (from atherosclerosis). If blood supply is severely reduced, a myocardial infarction (‘heart attack’) can occur.

Question 9

Cardiac Muscle cells (‘cardiomyocytes’)

Answer 9

• Regular arrangement of contractile proteins (actin and myosin)
• Many mitochondria provide endless energy, making them fatigue resistant
• Individual cells are connected by special intercalated discs that contain:
  Desmosomes - cell to cell adhesion molecules, that ‘weld’ the cells together, so heart can function ‘as one’, and also prevents separation of cells during strong contractions
  Gap junctions - allow depolarising ions to flow easily from cell to cell
• Cardiac cells are ‘auto-rhythmic’, they can generate their own action potentials without input from brain

Question 10

Intrinsic conducting system of heart

Answer 10

This electrical system of the heart sets the rhythmic heart beat
It contains auto-rhythmic cells that initiate and disperse depolarising current over the heart. Depolarisation leads to muscle contraction
The autonomic nervous system (sympathetic and parasympathetic fibres) also innervates the SA and AV node to change heart rate

Question 11

What do intrinsic components include?

Answer 11

Sinoatrial node (SA) –> Atrioventricular node (AV) –> Bundle of His –> Left & Right bundle branches –> Purkinje fibres

Question 12

Intrinsic conducting system of heart
(sequence of electrical excitation during one heart beat)

Answer 12

1. SA node (‘pacemaker cells’)
   generate APs, which spreads across both atria, causing them to contract together
2. Impulse is delayed briefly (0.1sec) at AV node, so atria have enough time to pump blood into ventricles
3. Impulse travels down bundle of His (or ‘AV bundle’), which connects atria to ventricles
4. Impulse continues down left & right bundle branches in wall that separates the ventricles
5. Impulse travels to the Purkinje fibres, depolarising all ventricular cells together, causing them to contract in synch

Question 13

Electrocardiogram (ECG) waves

Answer 13

The electrical activity of the heart can be detected on the surface of the body.
Shape of ECG recording has ‘waves’ that represents depolarisation and repolarisation of different chambers.
This wave is NOT a single action potential, but the summation of millions of cardiac cells undergoing action potentials.

Question 14

PQRST WAVE

Answer 14

P wave - atrial depolarisation which initiates atrial contraction

QRS complex - ventricular depolarisation which initiates ventricular contraction, and atrial repolarisation (hidden by large QRS complex!)

T wave - ventricular repolarisation

Question 15

What is SYSTOLE and DIASTOLE?

Answer 15

SYSTOLE = CONTRACTION (think ‘squeeze’, heart ejecting blood)

DIASTOLE = RELAXATION (heart filling with blood)

Question 16

Movement of blood through heart controlled entirely by pressure changes

Answer 16

ELECTRICAL EVENTS (impulses from SA and AV nodes) lead to MECHANICAL EVENTS (contraction of atria/ventricles) which lead to PRESSURE EVENTS (increasing pressure in contracting chambers cause valves to open and close, keeping blood flowing in the forward direction

blood always moves DOWN its pressure gradient (high to low).

BOYLE’S LAW: AS VOLUME INCREASES, PRESSURE DECREASES
AS VOLUME DECREASES, PRESSURE INCREASES

Question 17

Name and describe 5 stages of blood flow through the heart during one complete heartbeat…

Answer 17

1. VENTRICULAR DIASTOLE: Blood flows passively through open AV valves from atria into relaxed ventricles where the pressure is lower.
2. ATRIAL SYSTOLE: Atria contract (decreasing chamber volume and increasing pressure) forcing remaining blood (20%) into ventricles, where pressure is lower.
3. ISOVOLUMETRIC SYSTOLE: Ventricles contract and intraventricular pressure rises, closing AV valves. Pressure is not high enough to open semilunar valves. Briefly ventricles are completely closed chambers. LUB = 1st heart sound (caused by turbulent blood flow in ventricles)
4. VENTRICULAR SYSTOLE: Ventricles continue to contract, rising ventricular pressure forcing semilunar valves open, ejecting blood from heart (either to lungs via pulmonary arteries or to body via aorta).
5. ISOVOLUMETRIC DIASTOLE: Ventricles relax, dropping ventricular pressure, blood backflows closing semilunar values.
   DUP = 2nd heart sound (caused by turbulent blood flow in pulmonary artery & aorta)

Question 18

Volume Changes in Ventricles

Answer 18

End Diastolic Volume (EDV) - volume of blood in the ventricle at end of diastole (filling)
—>  volume is highest
End Systolic Volume (ESV) - volume of blood remaining in the ventricle after contraction
—>  volume is lowest
Stroke Volume (SV) - volume ejected from each ventricle as it contracts (per heart beat)

SV = EDV – ESV
SV = 130ml - 70ml = 60ml

NOTE: the two sides of the heart eject the same blood volume per contraction, the only difference is that the pressure in the pulmonary circuit is lower

Question 19

Atrial systole (contraction)

Answer 19

atria depolarise then contract,
atrial pressure ↑
> ventricular pressure,
pushing remaining blood
into ventricles (↑ ventricular volume)

Question 20

Isovolumetric systole

Answer 20

ventricles depolarise then contract, ventricular pressure ↑ > atrial pressure, AV valves close,
< pulmonary artery and aorta, semilunar valves stay shut, blood becomes turbulent in closed ventricle chambers (‘same volume’ of blood in ventricles)
= 1st heart sound ‘LUB’

Question 21

Ventricular systole

Answer 21

contracting ventricles continue to ↑ ventricular pressure
> pulmonary artery and aortic pressure, forcing semilunar valves open (↓ ventricular volume)

Question 22

Isovolumetric diastole

Answer 22

ventricles repolarise, ventricular pressure ↓
< pulmonary artery and aortic pressure, semilunar valves close, backflow of blood
= 2nd heart sound ‘DUP’

NOTE: there is a brief rise in the aortic pressure due to the backflow of blood onto the semilunar aortic valve = dicrotic notch

Question 23

Ventricular diastole

Answer 23

Relaxed ventricles have↓ pressure, < atrial pressure
causes AV values to open and blood starts filling the ventricles again, marking the beginning of the next cardiac cycle….

Question 24

Cardiac Output (CO)

Answer 24

= Volume of blood delivered to tissues to supply them with adequate nutrients/oxygen and remove waste.
2 major factors involved are heart rate (HR) and stroke volume (SV).
Working out CO can determine heart failure or inadequate circulation

Question 25

Cardiac Output (CO) cont.

Answer 25

CO = HR x SV
CO: Amount of blood pumped out by each ventricle in 1 min.
CO can increase markedly to meet demands of body, i.e. during exercise or during sympathetic activation

HR: Number of times heart beats per min bpm.
~75bpm in adults at rest
Controlled by sympathetic and parasympathetic input to SA and AV nodes

SV: Amount of blood pumped by each ventricle with each heart beat = 70ml/beat, determined by contractile strength (from sympathetic input), end diastolic volume (EDV) and arterial afterload. [when exercising: ↑ EDV (from ↑ venous return)
↑ ventricular stretch ↑ force of contraction ↑ stroke volume]

CO = 75 beats per min x 70ml/beat
CO = 5250ml/min
CO = ≈5L/min

Question 26

Cardiac Output (CO) cont.

Answer 26

Q. What happens if you suddenly lose a very large amount of blood? (or fluid i.e. from severe diarrhoea/vomiting or burns?)

A: Your body desperately tries to maintain a stable CO. If you go into ‘hypo-volumetric shock’, and your SV decreases, this is compensated for by increased HR, signs are a weak fast pulse..
Treatment: replace fluid volume ASAP