Module 4: How is cardiac function controlled?

Question 1

List the functions of the heart.

Answer 1

1. Generating blood pressure- allows blood to flow through heart and body
2. Routing blood: separates pulmonary and systemic circulations
3. Ensuring one-way blood flow: happens due to valves
4. Regulating blood supply to match changing metabolic needs

Question 2

What are the two major circulations the heart pumps blood into?
What is the length of their loops and what is their pressure and resistance like?

Answer 2

1. Pulmonary- heart –> lungs–> heart
   -takes the deoxygenated blood from the heart to the lungs where it is oxygenated and then returns back to the heart for distribution throughout the body.
   -because of the close proximity of the heart to the lungs this is a relatively short loop and as a result it is both low pressure and low resistance
   (Re-oxygenates blood)
2. Systemic- heart–> all other organs–> heart
   -because of the size of human beings the systemic circulation is a relatively long loop and as a result of this it is both high pressure and high resistance.
   (provides oxygenated blood to body)

Question 3

Where is the heart located?

Answer 3

• The heart is centrally located in the mediastinum

* Although the heart is located centrally, it is angled to the left

Question 4

What is the function of the mediastinum and what does it contain?

Answer 4

• The Mediastinum divides the thoracic cavity into right & left parts
• The mediastinum contains:
-The heart
-trachea, esophagus, thymus, blood vessels & nerves
• The lungs are situated on either sides of the mediastinum

Question 5

List the coverings of the heart.

Answer 5

Coverings:

1. Fibrous pericardium: for anchorage & protection to the heart
2. Serous pericardium: parietal layer (adheres to fibrous pericardium)
   - visceral layer: outer layer of heart)
   - pericardial cavity (between layers)- filled with pericardial fluid, the purpose of which is to reduce friction when the heart is beating

Question 6

List the wall layers of the heart.

Answer 6

1. Epicardium also the same thing as the visceral pericardium
   - smooth outer surface of heart
2. Myocardium -largest layer, composed of cardiac muscle & involved in contraction of heart
3. Endocardium- inner layer of the heart, composed of simple squamous cells that extend into blood vessels. This layer a single continuous layer that lines the inner chambers of the heart & covers the valves

Question 7

Name and describe the function of the third circulation of the heart.

Answer 7

• Coronary circulation

- supplies blood to the myocardium itself.

Question 8

Why is coronary circulation essential?

Answer 8

This is essential as the thickness of the myocardium makes it difficult to provide nutrients to the heart via simple diffusion, this to get its nutrients the heart requires its own blood supply.

Question 9

Give a brief description of how the heart gets its own blood supply.

Answer 9

• The right & left coronary arteries branch off the ascending aorta- these are also the 1st branches of the ascending aorta.
• Large arteries travel in sulci (bunches of fatty tissue) on the heart surface, whereas smaller branches actually penetrate myocardium
• The blood then returns via cardiac veins which drain into the coronary sinus on the posterior surface of the heart & then into the right atrium to be re-oxygenated and that’s how the heart gets its own blood supply.

Question 10

Why are heart valves important?

Answer 10

The heart valves are important to ensure directional flow of blood.

Question 11

Describe the two different types of valves.

Answer 11

1. Atrioventricular valves:
   - located between the atria & ventricles
   1) Left valve= bicuspid= mitral valve
   1) Right valve= tricuspid valve
2. Semi-lunar valves:
   - between ventricles & great vessels
   1) right ventricle & pulmonary trunk
   2) Left ventricle & aorta

Question 12

Describe the fibrous skeleton of the heart and its function.

Answer 12

• There is a dense connective tissue ring around the 4 valves
• Provides: anchorage, structural stability
-also to acts as electrical insulation between atria & ventricles- important for the cardiac cycle

Question 13

What are the characteristics of cardiac muscle fibres?

Answer 13

• heavily striated
• branched- connects to neighbouring fibres
• intercalated discs between the fibres and numerous nuclei throughout
• desmosomes (intercellular junctions that provide strong adhesion between cells)
• gap junctions

Question 14

The heart has the properties of a functional syncytium.

What does this mean and why is it important?

Answer 14

• it can act as a single cell mass even though its comprised of multiple cells
• it allows very fast electrical conduction from one fibre to another which is important for maintaining normal heart function.

Question 15

a) Are myocardial fibers aerobic or anaerobic? b) what cellular organelle do they have a lot of?
c) What do the cells use for energy? Why is this important?

Answer 15

a) Myocardial fibres are very aerobic with many mitochondria
b) are able to oxidise fatty acids glucose for energy
c) important as the heart is an organ the works continuously and requires a lot of energy

Question 16

Explain why some cardiac tissue are non-contractile.

Answer 16

Some cardiac muscle fibres are non-contractile and from the specialised pacemakers and conduction system which regulates heartbeat.

Question 17

What is the function of the pacemaker?

Answer 17

The pacemaker is responsible for the initiation and maintenance of a heartbeat and it begins in the SA node and then passes onto the AV node

Question 18

List the three specialized cardiac tissue and explain their function.

Answer 18

1. Sino-atrial (SA) node- initiates the electrical signal that makes the heart contract
2. Atrioventricular (AV) node- sends conduction & results in spontaneous polarisation
3. Atrioventricular bundle & purkinjie fibres- rapid electrical impulse conductance which results in contraction

Question 19

Why is a fast conduction system important?

Answer 19

The fast conduction system allows for rapid transmission of action potentials from atria through the ventricles to the myocardium. This allows for spontaneous depolarisation and autorhythmicity of the heart.