The Mammalian Heart: External and Internal Structure

This section details the structure of the mammalian heart, covering both its external features and the intricate organization of its internal chambers and valves. Understanding this structure is fundamental to comprehending the circulatory system's function.

External Structure

The heart is a roughly cone-shaped, muscular organ located within the mediastinum, between the lungs.

• Position: Situated in the mediastinum, slightly left of the midline.
• Size: Approximately the size of a clenched fist.
• Pericardium: A double-walled sac surrounding the heart.
  • Fibrous Pericardium: The outer, tough layer that protects the heart and anchors it within the mediastinum.
  • Serous Pericardium: A thinner, double-layered membrane.
    • Parietal Pericardium: The outer layer, lining the fibrous pericardium.
    • Visceral Pericardium (Epicardium): The inner layer, directly covering the heart's surface.
• Great Vessels: Major blood vessels entering and leaving the heart.

Internal Structure

The heart is divided into four chambers: two atria and two ventricles. These chambers work together to efficiently pump blood throughout the body.

Atria

The atria are the two upper chambers of the heart. They receive blood returning to the heart.

• Right Atrium: Receives deoxygenated blood from the body via the superior vena cava and inferior vena cava.
• Left Atrium: Receives oxygenated blood from the lungs via the pulmonary veins.
• Auricles: Small, ear-like appendages on the atria that increase their volume.
• Fossa Ovalis: A depression in the interatrial septum that remains from the fetal circulation.

Ventricles

The ventricles are the two lower chambers of the heart. They pump blood out of the heart to the lungs and the body.

• Right Ventricle: Pumps deoxygenated blood to the lungs via the pulmonary artery.
• Left Ventricle: Pumps oxygenated blood to the body via the aorta.
• Thickness: The left ventricle is significantly thicker and more muscular than the right ventricle because it has to pump blood to the entire body.

Valves

Heart valves ensure that blood flows in the correct direction through the heart.

Valve	Location	Function
Tricuspid Valve	Between the right atrium and right ventricle	Allows blood to flow from the right atrium to the right ventricle.
Pulmonary Valve	Between the right ventricle and the pulmonary artery	Allows blood to flow from the right ventricle to the pulmonary artery.
Mitral Valve (Bicuspid Valve)	Between the left atrium and left ventricle	Allows blood to flow from the left atrium to the left ventricle.
Aortic Valve	Between the left ventricle and the aorta	Allows blood to flow from the left ventricle to the aorta.

Septa

Septa are muscular walls that divide the heart into four chambers.

• Interatrial Septum: Divides the two atria.
• Interventricular Septum: Divides the two ventricles.

Blood Supply to the Heart (Coronary Arteries)

The heart itself requires a constant supply of oxygenated blood. This is provided by the coronary arteries, which branch off from the aorta.

• Left Coronary Artery: Supplies blood to the left atrium and left ventricle.
• Right Coronary Artery: Supplies blood to the right atrium and right ventricle.

Blood Vessels Associated with the Heart

The heart is connected to major blood vessels:

• Superior Vena Cava: Returns deoxygenated blood from the upper body to the right atrium.
• Inferior Vena Cava: Returns deoxygenated blood from the lower body to the right atrium.
• Pulmonary Artery: Carries deoxygenated blood from the right ventricle to the lungs.
• Pulmonary Veins: Carry oxygenated blood from the lungs to the left atrium.
• Aorta: Carries oxygenated blood from the left ventricle to the rest of the body.