The circulatory system II The Heart Chapter 20 The circulatory system II The Heart

Circulatory systems Pulmonary circuit: Blood flows to the lungs to exchange CO2 for O2 in the alveoli. Systemic circuit: Provides oxygenated blood to all organs and tissues in the body, including the heart and lungs.

The Heart Located: in the thoracic cavity in the mediastinum between the lungs and deep to the sternum. Function: to pump blood to all tissues and cells in the body Structure: the base of the heart is superior, and is where the great vessels are attached. The heart tapers down to the apex (tip) situated immediately above the diaphragm (left upper quadrant). Size/Orientation – weighs ~300 gm, ~9 cm diameter at base,~13 cm from base to apex and 6 cm diameter at the apex.

Location and Orientation

The pericardium Two coverings that surround the heart, enclosing it in a sac with a tough outer layer, the fibrous pericardium and a thin serous parietal pericardium, with a serous visceral pericardium (epicardium) attached to the myocardium. The pericardial space between these layers is filled with pericardial fluid.

Tissues of the Wall of the Heart The outer layer is epicardium and is a serous membrane (the visceral pericardium). The myocardium is cardiac muscle and consists of layers circularly and spirally wrapped around each other. The endocardium is a layer of specialized epithelial cells called endothelium. Endothelial cells cover the heart valves and line the entire cardiovascular and lymphatic system.

Pattern of cardiac muscle wrap

Heart Chambers The heart contains four chambers: 2 upper atria and 2 lower ventricles The right side of the heart (right atrium and right ventricle) receives oxygen poor blood (high in CO2) from the body and pumps it to the lungs where exchange of CO2 for O2 takes place. The left side of the heart (left atrium and left ventricle) receives oxygenated blood (high in O2) from the lungs and pumps it to every cell in the body.

Heart chambers

Heart Structure The atria (right and left) are separated from each other by the thin interatrial septum. The ventricles (left and right) are separated from each other by a thick muscular interventricular septum. The atria and ventricles are separated from each other by valves that regulate blood flow in a one way direction and by dense fibrous connective tissue. The ventricles are separated from their outflow trunks, the pulmonary trunk and the aorta (great vessels), by semilunar valves.

Right Atrium Oxygen poor blood from the entire body and the heart flows into the right atrium via the superior and inferior vena cavae and the coronary sinus. Occupies most of upper right side of heart Right auricle is conspicuous, looks like a serrated dogs ear. Externally right atrium is separated from right ventricle by right coronary sulcus containing the right coronary artery. Internally, the right atrioventricular (tricuspid) valve separates the right atrium and right ventricle.

Right Atrium

Right ventricle Receives blood from RA via tricuspid valve. Externally it occupies most of anterior right side of heart. Internally the RV exhibits muscular ridges (trabeculae carneae) and papillary muscles with chordae tendonae that anchor the tricuspid valve. The chordae tendonae prevent the valve from flopping (prolapsing) back into the right atrium when RV pumps and assures a one way flow of the blood. Pumps blood through pulmonary semilunar valve into pulmonary trunk for delivery to the lungs.

Right ventricle

Left atrium Receives oxygen rich blood from the lungs via 2 to 5 pulmonary veins (right and left). Separated from right atrium by the interatrial septum, with the fossa ovale, a remnant of fetal foramen ovale. Left auricle is on the upper left side of heart and is the only part of LA visible from front of heart. Separated externally from the left ventricle by the left coronary sulcus and internally by the left atrioventricular (bicuspid or mitral) valve. Pumps oxygen rich blood into left ventricle.

Left auricle of left atrium

Left ventricle Large thick walled pumping chamber of the heart. Receives oxygen rich blood from LA via bicuspid valve (left atrioventricular valve). Contains trabeculae carneae, papillary muscles and chordae tendonae similar to right ventricle. Occupies anterior inferior wall of heart down to apex. Pumps oxygen rich blood through Aortic valve (left semiliunar valve) into the aorta. Externally anterior interventricular sulcus separates RV from LV and contains left anterior descendens (LAD) coronary (anterior interventricular) artery.

Left ventricle

Ventricular wall thicknesses The left ventricular wall is much thicker than the right, as it must pump blood to the entire body and back again, whereas the right ventricle pumps blood at lower pressure through the lungs and back to the left atrium. The right side is a lower pressure side as compared to the left side of the heart (~20mm Hg vs 120 mm Hg).

Comparison of wall thickness

Heart Valves The four heart valves assure one way flow of blood through the heart. Atrioventricular valves are tricuspid (right side) and bicuspid or mitral (left side). Attached to ventricular walls by chordae tendonae to prevent prolapse. The pulmonary (right side) and aortic (left side) semilunar valves contain three half moon-shaped cusps each. All valves are made of flaps of endocardium and are reinforced by dense connective tissue. The heart valves open and close depending on the pressure exerted on them within each chamber.

Papillary muscle connected to chordae tendonae

Heart Valves

Heart Valves/Open-Closed

Papillary muscle connected to chordae tendonae

Heart Valves/Open-Closed

Blood flow in heart

Blood flow to the heart Left and right coronary arteries come off of aorta behind cusps. Venous return from heart drains into great cardiac vein and into coronary sinus and into right atrium

Coronary arteries

Coronary veins

Coronary stenosis

Cardiac conduction system For the heart to pump it must be activated by an electrical impulse generated from its own pacemaker tissue. This site is the sinoatrial (SA) node located in the right atrium. The impulse spreads through the atrial muscle to the atrioventricular (AV) node → AV bundle → the right and left bundle branches → Purkinje fibers → ventricular depolarization and contraction.

Cardiac tissue The myocardium is a syncitium of muscle fibers that interdigitate at intercalated discs

Cardiac tissue

Cardiac conduction system

Conduction pathways Sinoatrial Node (SA Node)-“sinus rhythm” – rate generated by the sinoatrial node in right atrium Atrioventricular Node (AV Node)- located near tricuspid valve; AV node acts as electrical gateway to ventricles Atrioventricular Bundle/”Bundle of His”- pathway by which impulses leave AV node. Forks into right and left bundles branches which enter interventricular septum and descend to the apex. Right/Left Bundle Branches- continuation of Bundle of His which descend to apex and gives rise to the purkinje fibers. Purkinje Fibers- conducting fibers which turn upward and extend into ventricular myocardium. They form a more elaborate network in LV than RV.

Cardiac conduction system

Cardiac innervation Although the heart can beat without neural control, it receives both sympathetic and parasympathetic input from the autonomic nervous system. The parasympathetic input is via CN - X (Vagus) Sympathetic input is from the medullary cardioacclerator center → sympathetic chain (paravertebral) ganglia → the sympathetic nerve. The heart can also be regulated by neuro-humors circulating in the blood.

Sympathetic and parasympathetic input to the heart.