The Heart: Structure and Function

Study Notes

The heart is the central organ of the cardiovascular system, responsible for pumping blood throughout the body.
Heart Size and Location:
- Weighs approximately 250-390 grams in males and 200-275 grams in females.
- Roughly the size of a closed fist.
- Located in the mediastinum, the medial cavity of the thorax, between the two lungs.
- The base of the heart tilts medially and posteriorly, while the apex points inferiorly and laterally.

Pericardium: A double-walled sac that surrounds the heart, keeping it in place and providing protection.
- Outer Fibrous Pericardium: The outermost layer, composed of dense connective tissue.
- Inner Serous Pericardium: Composed of two layers:
  - Parietal Layer: The outer layer of the serous pericardium.
  - Visceral Layer (Epicardium): The inner layer of the serous pericardium, directly attached to the heart's outer surface.
- Pericardial Cavity: A space between the parietal and visceral layers, filled with pericardial fluid, which reduces friction during heartbeats.
Pericarditis: Inflammation of the pericardium, causing chest pain and discomfort.

Epicardium (Outer Layer): The visceral layer of the serous pericardium.
Myocardium (Middle Layer): Responsible for the heart's pumping action.
- Composed of cardiac muscle, arranged in a circular and spiral pattern.
- Fibrous Skeleton of the Heart: A crisscrossing network of connective tissue within the myocardium.
  - Provides structural support for valves and prevents overstretching.
  - Acts as an electrical insulator.
Endocardium (Inner Layer): A thin endothelial lining that covers the inner chambers of the heart.

Two Atria (Receiving Chambers):
- Right Atrium: Receives deoxygenated blood from the body via the superior and inferior vena cava and the coronary sinus.
- Left Atrium: Receives oxygenated blood from the lungs via the four pulmonary veins.
- Interatrial Septum: Separates the two atria.
- Auricles: Pouched extensions of the atria, increasing the volume.
- Pectinate Muscles: Ridges within the atrial walls, particularly prominent in the right atrium.
Two Ventricles (Discharging Chambers):
- Right Ventricle: Pumps deoxygenated blood to the lungs through the pulmonary trunk.
- Left Ventricle: Pumps oxygenated blood to the rest of the body through the aorta.
- Interventricular Septum: Separates the two ventricles.
- Trabeculae Carneae: Irregular muscular columns that project into the ventricular chambers, contributing to the heart's efficient pumping action.

Valves: Composed of dense connective tissue covered by endothelium.
- Atrioventricular Valves (AV):
  - Tricuspid Valve: Located between the right atrium and right ventricle, with three cusps.
  - Bicuspid (Mitral) Valve: Located between the left atrium and left ventricle, with two cusps.
- Semilunar Valves (SV):
  - Pulmonary Semilunar Valve: Located between the right ventricle and the pulmonary trunk, with three cusps.
  - Aortic Semilunar Valve: Located between the left ventricle and the aorta, with three cusps.
Chordae Tendineae: Tiny tendons that anchor the cusps of the AV valves to papillary muscles.
Papillary Muscles: Muscle projections from the ventricular walls that, when contracted, pull on the chordae tendineae to prevent the AV valves from prolapsing back into the atria during ventricular contraction.

Valve Opening and Closing:
- Valves open and close based on pressure differences between the chambers.
  - During ventricular contraction (systole), the AV valves close (preventing backflow into the atria), while the semilunar valves open, allowing blood to be ejected into the pulmonary trunk and aorta.
  - During ventricular relaxation (diastole), the semilunar valves close (preventing backflow from the pulmonary trunk and aorta), while the AV valves open to allow blood to fill the ventricles.

The left ventricle is significantly thicker than the right ventricle because it must generate much higher pressure to pump blood throughout the entire systemic circulation (the entire body), which has a much wider network of vessels and a greater distance compared to the pulmonary circulation (only the lungs).