Anatomy of the Heart

Cardiovascular System

• Consists of the heart, arteries, capillaries, and veins.

The Heart

• Has four chambers: two atria (right and left) and two ventricles (right and left).
• Has three layers: epicardium, myocardium, and endocardium.

Endocardium

• Innermost layer of the heart.
• Consists of a thin layer of squamous endothelium (epithelium) with connective tissue and smooth muscle fibers.
• Has a subendocardial layer that contains the sinoatrial (SA) node or pacemaker and the atrioventricular (AV) node.

Myocardium

• Thick middle layer of the heart.
• Consists mainly of cardiac muscle cells (myocytes).
• Thicker in the ventricular wall than in the atrial wall due to high pressure needed to pump out blood.

Epicardium

• Outermost layer of the heart.
• Consists of simple squamous mesothelium with connective tissue, nerves, vessels, and adipocytes.
• Mesothelium secretes pericardial fluid, which acts as a lubricant to prevent friction and covers and protects the heart.

Cardiac Skeleton

• Made up of irregular dense connective tissue located in the endocardium.
• Forms part of the interventricular and interatrial septa, anchors the valves, and surrounds the two atrioventricular canals.
• Extends into the valve cusps and the chordae tendineae.

Cardiac Valves

• The heart has four valves: two atrioventricular (mitral and tricuspid) and two semilunar (aortic and pulmonary).
• Valves possess connective tissue layers covered by endothelium on each side and are arranged into three layers: spongiosa, fibrosa, and ventricularis.

Vascular System

• Components include arteries, capillaries, and veins.
• Arteries transport blood away from the heart by branching into smaller vessels.
• Capillaries are thin-walled vessels with the smallest diameter, forming capillary beds where gases and substances are exchanged.
• Veins drain capillary beds and return blood to the heart.

Tunics or Layers of the Blood Vessels

• Wall of the blood vessels has three layers: tunica intima, tunica media, and tunica adventitia.

Tunica Intima

• Innermost layer of the blood vessel.
• Composed of simple squamous endothelium (epithelium), subendothelial connective tissue, and internal elastic lamina.
• Internal elastic lamina separates the tunica intima from the tunica media and contains elastin that allows for easy diffusion of substances.

Tunica Media

• Middle and thickest layer of the blood vessel.
• Forms the middle and thickest layer of the blood vessel.

Purkinje Fibers

• Modified cardiac muscle cells with 1-2 nuclei.
• Specialized for impulse conduction rather than contraction.
• Found at the periphery of the heart and stain more pale than contractile cardiac muscle fibers due to low myofibril content.