8.08 The Heart

Heart Structure:

• Size and Weight: ~300 g, about the size of a fist.
• Cardiac Muscle:
  • Type: Special muscle type; cells are tightly joined, allowing electrical excitation to pass easily, enabling synchronized contractions.
  • Coronary Arteries: Branch from the aorta, supplying oxygen and nutrients to heart muscle.

Heart Chambers:

• Four Chambers: Two atria (upper chambers) and two ventricles (lower chambers).
  • Septum: Muscular wall that separates the left and right sides, preventing blood flow between sides.
  • Right Side: Receives deoxygenated blood (via vena cava) → pumps to lungs (pulmonary artery).
  • Left Side: Receives oxygenated blood (via pulmonary veins) → pumps to body (aorta).

Heart Valves:

• Atrioventricular Valves (AV Valves):
  • Left (Bicuspid/Mitral Valve): Between left atrium and left ventricle.
  • Right (Tricuspid Valve): Between right atrium and right ventricle.
  • Function: Prevent backflow from ventricles to atria during ventricular contraction.
• Semilunar Valves:
  • Located at the exits of the aorta and pulmonary artery.
  • Function: Prevent blood from flowing back into ventricles after contraction.

The Cardiac Cycle

Phases of Cardiac Cycle:

1. Atrial Systole (~0.1 s):

• Atria contract, pushing blood into ventricles.
• AV valves open; semilunar valves in veins prevent backflow.

1. Ventricular Systole (~0.3 s):

• Ventricles contract, increasing pressure.
• AV valves close, preventing backflow; semilunar valves open, allowing blood into aorta and pulmonary artery.

1. Ventricular Diastole (~0.4 s):

• Ventricles relax, pressure decreases.
• Semilunar valves close to prevent backflow; AV valves open, allowing blood flow from atria into ventricles.

Total Duration: ~0.8 seconds per cycle, enabling a resting heart rate of about 70 beats per minute.

Function of Heart Valves

• AV Valves: Open when atrial pressure > ventricular pressure, allowing blood flow into ventricles; close when ventricular pressure > atrial pressure, preventing backflow.
• Semilunar Valves: Open when ventricular pressure > arterial pressure; close during diastole to prevent arterial blood from re-entering the ventricles.

Pressure Changes in the Heart

• Left Ventricular Pressure:
  • Higher than atrial pressure; required to pump blood throughout the body.
  • Significantly greater than right ventricle pressure (needed only to send blood to lungs).
• Aortic Pressure:
  • Receives high-pressure blood from left ventricle; pressure maintained to ensure steady blood flow.

Differences in Ventricular Wall Thickness

• Right Ventricle: Thinner wall; generates lower pressure to pump blood to nearby lungs.
• Left Ventricle: Thicker wall; needs to create high pressure to pump blood throughout the body, including against systemic resistance in arterioles.

Key Terms

• Atrial Systole: Contraction of atrial muscles.
• Ventricular Systole: Contraction of ventricular muscles.
• Diastole: Relaxation of all heart chambers.
• Cardiac Cycle: Sequence of events in one heartbeat.

Clinical Insight

• Faulty Heart Valves:
• Ineffective closure can lead to backflow of blood, reducing efficiency and possibly leading to heart failure or poor circulation.

This cycle and structure enable efficient blood flow to meet oxygen and nutrient needs throughout the body.