Basic form and function of the heart

Question 1

Layers of heart

Answer 1

• Endocardium
• Myocardium
• Epicardium
• Surrounded by parietal pericardium

Question 2

Myocardium

Answer 2

-made up of cardiac muscle tissue filled with cardiomyocytes (contractile cells)

Question 3

Endocardium

Answer 3

• Made up of endothelium and connective tissue
• Inner layer

Question 4

Epicardium

Answer 4

• Made up of mesothelium and connective tissue
• Outer layer
• Visceral pericardium

Question 5

Parietal pericardium

Answer 5

-made up of mesothelium
- fixes heart to cardiac cavity and provides lubricant to the heart

Question 6

Pericardiums impact on cardiac output and stroke volume

Answer 6

When pericardium is removed experimentally, saw that there was an increase in stroke volume and an increase in cardiac output. Therefore the pericardium actually decreases the filling capacity of the heart

**Reverse is also true. Fluid build up or inflammation will restrict stroke volume

Question 7

Cardiomyocytes

Answer 7

• Predominant cell type in the heart
• Bifurcated
• Single, centrally located nucleus
• Highly aerobic (high myoglobin, mitochondria, vascularized)
• Involuntary contraction
• Connect to adjacent cells via intercalated discs, forming a single unit (Syncytium)

Question 8

Intercalated discs

Answer 8

Link cardiomyocytes together allowing them to contract and pump blood together

Question 9

Contraction steps

Answer 9

1. Atrial systole: atrial contraction forces a small amount of additional blood into the relaxed ventricle
2. Atrial systole ends, atrial diastole begins
3. Ventricular systole 1st phase: ventricular contraction pushes the AV valves closes (because pressure in the ventricles is greater than in the atria) but does not create enough pressure to open semilunar valves (because pressure in ventricles is lower than that in the pulmonary artery or aorta)
4. Ventricular systole 2nd phase: ventricular pressure rises and exceeds pressure in the arteries, and the semilunar valves open and blood is ejected
5. Ventricular diastole early stage: as ventricles relax, pressure in ventricles drops and blood flows back against cusps of semilunar valves and forces them closed. Blood flows into the relaxed atria
6. Ventricular diastole late stage: all chambers are relaxed. Ventricles fill passively

Question 10

Isovolumetric contraction

Answer 10

-contraction of the ventricles when the volume of blood is not changing

• occurs during ventricular systole because contraction occurring but no change in blood volume due to pressure distribution

Question 11

Isovolumetric relaxation

Answer 11

-volume of blood not changing, but during relaxation period

-occurs when the ventricles finish contracting, and they begin to relax. Pressure inside the ventricles is still greater than atrial pressure so no blood coming in BUT pressure in ventricles is lower than arterial pressures so the pulmonic and aortic valves are closed

Question 12

Isovolumetric relaxation and filling period length

Answer 12

-accounts for 2/3rds of cardiac cycle (no contraction of heart) therefore heart contracting for much less of cycle than one would assume

Question 13

Cells of the conducting system

Answer 13

Autorhythmic or pacemaker cells = SA and AV nodes

• generate action potentials spontaneously
• fastest rate sets the pace… SA>AV

Question 14

Conduction fibers

Answer 14

-spread impulses throughout the heart

-generate action potentials spontaneously but at a much slower rate

Question 15

Depolarization/impulse wave

Answer 15

1. Action potential initiated in SA node
2. AP travels by conduction fibers across atria
3. AP arrives at AV node. Momentary delay occurs which allows atrial contraction to occur and fill ventricles prior
4. AP leaves AV node and travels in AV bundle/bundle of His (separate quite quickly into L/R bundle fibers) of interventricular septum
5. Purkinje fibers transport AP/impulse to the myocardial cells of the ventricles
6. Recovery (repolarization phase)

Question 16

Refractory period

Answer 16

The cells of the AV node go into refractory period after they are stimulated by impulses from SA node

Question 17

AV node spontaneous?

Answer 17

-It can generate spontaneous contraction although it usually does not have much control as it follows the pace of the SA node
-AV node will take over if SA node fails or is blocked

Question 18

Impulse spread

Answer 18

Spreads from the apex of heart through the septum and walls of the ventricles to the base of the heart allowing for greater contraction efficiency when pumping blood from the apex to the base when blood is ejected

Question 19

Cardiac fiber arrangement

Answer 19

-helical formation results in circumferential, vertical, longitudinal contractions

Question 20

Cardiac output and Age

Answer 20

Stroke volume is greater in younger animals than older animals which contributes to a greater cardiac output

Question 21

Physical activity and remodelling of heart

Answer 21

-exercise training results in remodelling of heart muscle (increase in left ventricle size resulting in better pumping ability)