Anatomy and Function of the Heart

Anatomy of the Heart

• The heart is cone-shaped and about the size of a fist
• It weighs less than a pound
• The heart sits within the mediastinum, located between the two lungs
• The vertebral column is on the back side of the heart, and the sternum is located on the front

Cardiovascular System Functions

• The major purpose of the heart is to transport oxygen, digested food, cell wastes, electrolytes, and hormones
• The heart is a muscular pump made of cardiac muscle
• Contractions of the heart force blood through blood vessels
• It uses a one-way valve system

Coverings of the Heart

• The pericardium is a double-walled fibro-serous sac that surrounds the heart
• The fibrous pericardium is the superficial layer
• The deep two-layer serous pericardium has a parietal layer, which lines the internal surface of the fibrous pericardium
• The visceral layer, also known as the epicardium, lines the surface of the heart
• A fluid-filled pericardial cavity separates the layers
• Pericarditis is an inflammation of the pericardium due to adhesion between the serous pericardial layers, which interferes with heart movement

Heart Membrane Layers

• The heart membrane comprises a double-walled fibro-serous sac called the pericardium, positioned on top of the epicardium
• The epicardium is a thin layer of connective tissue and fat that provides an additional protective layer for the heart, underneath the pericardium
• The myocardium is the middle layer and the muscle tissue of the heart, composed of cardiac muscle cells that contract and conduct electricity to coordinate contraction
• The endocardium is the innermost layer of the heart and is composed of endothelial cells that provide a smooth, non-adherent surface for blood collection and pumping

Chambers of the Heart

• The heart has four chambers: two atria on the superior position and two ventricles on the inferior position
• The interatrial septum separates the atria
• The interventricular septum separates the ventricles
• The right ventricle makes up most of the anterior surface
• The left ventricle forms the apex of the heart and dominates the back surface
• Two grooves mark the boundaries of the four chambers
• The coronary sulcus is located at the junction between the atria and ventricle
• The anterior interventricular septum separates the right and left ventricles

Atria

• Atria are the receiving chambers for blood from circulation
• They are small and thin-walled, pushing blood minimally down to the ventricle
• Three veins enter blood to the right atrium
• The superior vena cava returns blood from body regions superior to the diaphragm
• The inferior vena cava returns blood from body regions inferior to the diaphragm
• The coronary sinus returns blood from the myocardium

Ventricles

• Ventricles make up most of the volume of the heart
• The right ventricle dominates the anterior surface
• The left ventricle dominates the posterior surface
• Ventricles are the actual pump of the heart
• Walls are very thick compared to the atria
• The right ventricle pumps blood into the pulmonary trunk
• The left ventricle pumps blood into the aorta, which is the largest artery in the body

Heart Valves

• A total of four valves ensure unidirectional blood flow through the heart
• Two atrioventricular (AV) valves between the atria and ventricular junctions prevent backflow into the atria when ventricles contract
• Flaps of the valves are made up of collagen cords called chordae tendineae or heart strings
• Chordae tendineae anchor AV valves to papillary muscles located on the inside surface of the heart

Semilunar Valves

• Two semilunar (SL) valves are between the ventricles and big arteries like the aorta and pulmonary artery
• The aortic semilunar valve lies between the left ventricle and the aorta
• The pulmonary semilunar valve lies between the right ventricle and pulmonary trunk
• Semilunar valves prevent backflow of blood into the ventricles and open and close in reaction to pressure in the ventricles
• Faulty valves can be replaced

Blood Flow Through the Heart

• There are two circuits: pulmonary and systemic
• Equal volumes of blood pump through both circuits
• Pulmonary circuits originate from the right ventricle and does not need much pressure because it travels to the lungs for a short trip
• The systemic circuit originates from the left ventricle requiring travelling all over the body needing more pressure
• The wall of the left ventricle is 3x thicker than the right

Pulmonary and Systemic Circuits

• Th right side of the heart receives O2 poor blood from throughout the body and pumps to the lungs to pick up O2 and drop CO2
• The blood vessels that carry blood to and from the lungs are called the “pulmonary circuit.”
• The left side of the heart receives O2-rich blood from the lungs and pumps it throughout the body to supply O2 and nutrients
• The blood vessels that carry blood to and from the body tissues are called the “systemic circuit.”

Coronary Circulation

• The heart receives nourishment from coronary arteries and its branches
• The right (R) and left (L) coronary arteries are the first branches of the ascending aorta
• The R coronary artery supplies R atrium and the back of both ventricles, branching into the marginal artery and others
• The L coronary artery supplies the L atrium and the L ventricle, branching into the circumflex artery and others

Cardiac Muscle Cells

• Cardiac muscle cells are short, fat, branched, and interconnected
• Like skeletal muscle, cardiac muscle is striated (less than skeletal) and contracts by the "sliding filament theory"
• Have thick and thin filament and Z, A, and I bands in sarcomere (like skeletal muscle)
• Each cardiac cell has one or two nuclei
• Mitochondria makes up 25-35% of cardiac cell volume
• Cardiac muscle cells (muscle fibers) are connected with next cell through “intercalated discs”
• Each disc has desmosomes for an adherence structure between two cells
• Each disc has gap junctions for plasma membrane channels for communication
• Desmosomes prevent adjacent cells from separating during contraction
• Gap junctions allow ions to pass from cell to cell, transmitting current across the entire heart called functional syncytium