3.5.2 The Human Heart

Structure of the Heart:

1. Two Pumps:

• The heart is divided into two pumps:
• Right side: Pumps deoxygenated blood to the lungs.
• Left side: Pumps oxygenated blood to the rest of the body.

2. Chambers:

• Each pump has two chambers:
• Atrium:
• Thin-walled and elastic.
• Designed to receive blood from veins and stretch as it fills.
• Ventricle:
• Thick muscular walls.
• Pumps blood out of the heart with enough force to reach the lungs or body.

3. Valves:

• Prevent backflow of blood and maintain one-way flow.
• Atrioventricular (AV) valves:
• Left AV valve: Bicuspid valve (two cusps).
• Right AV valve: Tricuspid valve (three cusps).
• Semilunar valves:
• Found at the entrances of the pulmonary artery and aorta.

4. Septum:

• A muscular wall that separates the left and right sides of the heart, preventing mixing of oxygenated and deoxygenated blood.

Why Two Pumps are Needed:

5. Pulmonary Circulation:

• Blood pressure decreases as blood flows through the lungs.
• A second pump is required to maintain pressure for systemic circulation.

6. Systemic Circulation:

• The left ventricle pumps oxygenated blood at high pressure to supply the entire body.

The Cardiac Cycle:

The cardiac cycle involves three main phases:

7. Atrial Systole:

• The atria contract, pushing blood into the ventricles.
• AV valves are open, and semilunar valves are closed.

8. Ventricular Systole:

• The ventricles contract, pumping blood out of the heart:
• Right ventricle pumps to the lungs via the pulmonary artery.
• Left ventricle pumps to the body via the aorta.
• AV valves close to prevent backflow, and semilunar valves open.

9. Diastole:

• The heart relaxes, allowing blood to flow into the atria from the veins.
• Both AV and semilunar valves are closed initially, but AV valves reopen as atrial pressure increases.

Pathway of Blood Through the Heart:

10. Deoxygenated Blood:

• Enters the right atrium from the vena cava.
• Passes through the tricuspid valve into the right ventricle.
• Is pumped to the lungs via the pulmonary artery during ventricular systole.

11. Oxygenated Blood:

• Returns from the lungs to the left atrium via the pulmonary vein.
• Passes through the bicuspid valve into the left ventricle.
• Is pumped to the body via the aorta during ventricular systole.

Adaptations of the Heart:

1. Thick Left Ventricle Wall:

• Generates high pressure to pump blood throughout the body.

2. Valves:

• Prevent backflow, ensuring efficient circulation.

3. Elasticity of the Atria:

• Allows them to stretch and accommodate incoming blood.

4. Septum:

• Prevents mixing of oxygenated and deoxygenated blood, ensuring efficient gas exchange.

Key Definitions:

• Atrium: Thin-walled upper chamber of the heart that receives blood.
• Ventricle: Thick-walled lower chamber of the heart that pumps blood.
• Atrioventricular Valves: Valves between atria and ventricles (bicuspid and tricuspid).
• Semilunar Valves: Valves at the base of the pulmonary artery and aorta.
• Cardiac Cycle: Sequence of events in one heartbeat.