The Heart The heart is a cone-shaped, muscular organ The heart muscle forms the myocardium, with tightly interconnected cells of cardiac muscle tissue. The pericardium is the outer membranous sac with lubricating fluid.

The heart has four chambers: two upper, thin-walled atria, and two lower, thick-walled ventricles. The septum is a wall dividing the right and left sides. Atrioventricular valves occur between the atria and ventricles – the tricuspid valve on the right and the bicuspid valve on the left; both valves are reinforced by chordae tendinae attached to muscular projections within the ventricles. Semilunar valves occur between the ventricles and the attached arteries

External heart anatomy

External heart anatomy The superior vena cava and the pulmonary trunk are attached to the right side of the heart. The aorta and pulmonary veins are attached to the left side of the heart. The right ventricle forms most of the ventral surface of the heart, and the left ventricle forms most of the dorsal surface.

Coronary artery circulation The coronary arteries and cardiac veins pervade cardiac muscle. The coronary arteries bring oxygen and nutrients to cardiac cells, which derive no benefit from blood coursing through the heart.

Path of blood through the heart This diagrammatic representation of the heart allows you to trace the path of the blood through the heart.

Conduction system of the heart The SA node sends out a stimulus, which cause the atria to contract. When this stimulus reaches the AV node, it signals the ventricles to contract. Impulses pass down the two branches of the atrioventricular bundle to the Purkinje fibers, and thereafter the ventricles contract.

Conduction and Contraction SA node SA node in right atrium is pacemaker Electrical signals cause contraction of atria Signal flows to AV node and down septum to ventricles

The Heartbeat Each heartbeat is called a cardiac cycle. When the heart beats, the two atria contract together, then the two ventricles contract; then the whole heart relaxes. Systole is the contraction of heart chambers; diastole is their relaxation. The heart sounds, lub-dup, are due to the closing of the atrioventricular valves, followed by the closing of the semilunar valves.

Electrocardiogram A normal ECG (top) indicates that the heart is functioning properly. The P wave occurs just prior to atrial contraction; the QRS complex occurs just prior to ventricular contraction; and the T wave occurs when the ventricles are recovering from contraction. Ventricular fibrillation (bottom) produces an irregular electrocardiogram due to irregular stimulation of the ventricles. Ventricular fibrillation is of special interest because it can be caused by an injury or drug overdose. It is the most common cause of sudden cardiac death in a seemingly healthy person over age 35. Once the ventricles are fibrillating, they have to be defibrillated by applying a strong electrical current for a short period of time. Then the SA node may be able to reestablish a coordinated beat.

The Vascular Pathways The cardiovascular system includes two circuits: Pulmonary circuit which circulates blood through the lungs, and Systemic circuit which circulates blood to the rest of the body. Both circuits are vital to homeostasis.

Pulmonary Circuit This loop oxygenates blood right pulmonary artery left pulmonary artery This loop oxygenates blood capillary bed of left lung capillary bed of right lung pulmonary trunk (to systemic circuit) (from systemic circuit) pulmonary veins heart lungs

Systemic Circuit Longer loop carries blood to and from body tissues capillary beds of head and upper extremities aorta (to pulmonary circuit) (from pulmonary circuit) Longer loop carries blood to and from body tissues heart capillary beds of other organs in thoracic cavity capillary bed of liver capillary beds of intestines capillary beds of other abdominal organs and lower extremities

Cardiovascular system diagram The blue-colored vessels carry O2-poor blood, and the red-colored vessels carry O2-rich blood; the arrows indicate the flow of blood. Capillaries are present in all parts of the body, so no cell is located far from a capillary.

The Blood Vessels The cardiovascular system has three types of blood vessels: Arteries (and arterioles) – carry blood away from the heart Capillaries – where nutrient and gas exchange occur Veins (and venules) – carry blood toward the heart.

Blood vessels

The Arteries Arteries and arterioles take blood away from the heart. The largest artery is the aorta. The middle layer of an artery wall consists of smooth muscle that can constrict to regulate blood flow and blood pressure. Arterioles can constrict or dilate, changing blood pressure.

The Capillaries Capillaries have walls only one cell thick to allow exchange of gases and nutrients with tissue fluid. Capillary beds are present in all regions of the body but not all capillary beds are open at the same time.

Anatomy of a capillary bed

The Veins Venules drain blood from capillaries, then join to form veins that take blood to the heart. Veins have much less smooth muscle and connective tissue than arteries. Veins often have valves that prevent the backward flow of blood when closed. Veins carry about 70% of the body’s blood and act as a reservoir during hemorrhage.

The Venous System Blood flows from capillaries into venules, then on to veins Veins are large-diameter vessels with some smooth muscle in wall Valves in some veins prevent blood from flowing backward

Changes in thoracic and abdominal pressure that occur with breathing also assist in the return of blood. Varicose veins develop when the valves of veins become weak. Hemorrhoids (piles) are due to varicose veins in the rectum. Phlebitis is inflammation of a vein and can lead to a blood clot and possible death if the clot is dislodged and is carried to a pulmonary vessel.

Major Vessels carotid arteries jugular veins ascending aorta superior vena cava pulmonary arteries pulmonary veins coronary arteries hepatic portal vein brachial artery renal artery renal vein inferior vena cava abdominal aorta iliac arteries iliac veins femoral artery femoral vein

Atherosclerosis Atherosclerosis is due to a build-up of fatty material (plaque), mainly cholesterol, under the inner lining of arteries. The plaque can cause a thrombus (blood clot) to form. The thrombus can dislodge as an embolus and lead to thromboembolism. Thromboembolism is a clot that has moved and is now stationary in a new blood vessel where it can cause damage.

Coronary bypass operation During this operation, the surgeon grafts segments of another vessel, usually a small vein from the leg, between the aorta and the coronary vessels, bypassing areas of blockage. Patients who require surgery often receive two to five bypasses in a single operation.

Angioplasty On the left, a plastic tube is inserted into the coronary artery until it reaches the clogged area. In the middle diagram, a metal tip with a balloon attached is pushed out the end of the plastic tube into the clogged area. On the right, when the balloon is inflated, the vessel opens. Sometimes metal coils or slotted tubes, called stents, are inserted to keep the vessel open.

Atherosclerosis Arteries thicken, lose elasticity, and fill up with cholesterol and lipids High LDL increases risk

Up in Smoke The vast majority of smokers picked up the habit before age 18 Smoking damages the circulatory and respiratory systems Smokers have increased risk of respiratory infections, cancers, high blood pressure, stroke, and heart attack