Human Heart Anatomy

Heart Structure and Function

• The heart is a hollow muscular organ located between the lungs in the mediastinum.
• It consists of 4 chambers: 2 atria and 2 ventricles, separated by a continuous partition (interatrial septum and interventricular septum).
• The musculature is thicker in the ventricles than in the atria.
• The force of contraction of the heart depends on the muscle.

Right Side of the Heart

• The right side of the heart has two chambers: right atrium and right ventricle.
• The right atrium is a thin-walled, low-pressure chamber that receives venous (deoxygenated) blood from the head, neck, and upper limbs through the superior vena cava, and from the lower parts of the body through the inferior vena cava.
• The right atrium has the pacemaker (sinoatrial node) and the atrioventricular node that conducts impulses to the ventricles.

Left Side of the Heart

• The left side of the heart has two chambers: left atrium and left ventricle.
• The left atrium is a thin-walled, low-pressure chamber that receives oxygenated blood from the lungs through the pulmonary veins.
• The left ventricle pumps arterial blood to different parts of the body through the systemic aorta.

Valves of the Heart

• There are no admixtures between arterial and venous blood.
• The myocardium forms the bulk of the heart and is responsible for the pumping action.
• Myocardium has three types of muscle fibers: contractile, pacemaker, and conductive system.

Endocardium

• Endocardium is the innermost layer of the heart wall, a thin, smooth, and glistening membrane.
• It is formed by a single layer of endothelial cells lining the inner surface of the heart.

Properties of Cardiac Muscle

• The cardiac muscle has two types of muscle fibers: cardiac muscle proper and conductive system.
• It has special properties that characterize it from other types of muscles, including:
  • Histology of the cardiac muscle: unlike skeletal muscle, with anatomical connections between myocardial fibers (intercalated discs and gap junctions).
  • Functional syncytium: stimulation of one cardiac muscle cell results in stimulation of all cells, allowing the heart to contract as one unit.
  • Main source of energy: the cardiac muscle consumes fat as the main source of energy.
  • Blood flow (supply): the cardiac muscle receives its blood supply mainly during diastole.
  • Oxygen extraction: the cardiac muscle extracts a higher amount of oxygen than skeletal muscle.

Blood Vessels

• The circulatory system consists of:
  • Aorta
  • Arteries
  • Arterioles
  • Capillaries
  • Venules
  • Veins
  • Venae cavae

Arterial System

• The arterial system comprises the aorta, arteries, and arterioles.
• Walls of the aorta and arteries are formed by three layers: tunica adventitia, tunica media, and tunica intima.
• Aorta and arteries have two laminae of elastic tissues.
• Arterial branches become narrower and their walls become thinner while reaching the periphery.

Venous System

• The venous system starts from the capillaries and includes venules, veins, and venae cavae.
• Capillaries end in venules, which are smaller vessels with thin muscular walls.
• Venules are continued as veins, which have a larger diameter.
• Veins form superior and inferior venae cavae, which have a larger diameter.

Capillaries

• Capillaries make a connective link between the arterioles and venules.
• They are linked by a single layer of flat endothelial cells.
• The capillary endothelium does not directly touch the elements of other tissues and is always separated from a supporting bed of connective tissue by an intervening layer (basal lamina).

Divisions of Circulation

• Blood flows through two divisions of the circulatory system:
  • Systemic circulation (greater circulation with high resistance circuit)
  • Pulmonary circulation (lesser circulation with low resistance circuit)
• The two systems meet in the heart.