The Circulatory System The Structure and Function of the Heart and Blood Vessels

Arteries  Arterioles  Capillaries  Venules  Veins Blood Vessels Arteries: thick-walled blood vessels that ALWAYS carry blood AWAY from the heart. Veins: thin-walled blood vessels that ALWAYS carry blood TOWARD the heart. Arteries  Arterioles  Capillaries  Venules  Veins

Blood Vessels

Arteries Transports blood under pressure. Blood moves in a pulse-like wave throughout the circulatory system. Arterioles have smaller diameter than arteries and are less elastic, BUT the contraction and relaxation of arterioles is the major determinant of the overall blood pressure.

Capillaries Narrowest of all blood vessels. RBCs travel in single file. Branching of the capillaries increases the surface area available for diffusion. Connects the arterial & venous systems.

Veins Thinner walls, larger diameters & less muscle than arteries. Contains 70% of total blood volume. Most veins must work against gravity. Valves allow one-way flow of blood. Contraction of skeletal muscles pushes blood toward heart. If veins are constantly stretched, they will lose their elasticity & varicose veins will form.

The Heart Size of your fist. Hardest-working muscle in the body. Contains four chambers: Left and right atria (receiving chambers). Left and right ventricles (delivery chambers). Left and right sides of the heart are separated by a muscular septum.

The Heart Protected by the ribcage, sternum & spine.

Pathways of Blood Pulmonary Circuit Systemic Circuit Right side of heart. Low-pressure system. Systemic Circuit Left side of heart. High-pressure system.

Pulmonary Circuit The right atrium receives oxygen-poor blood from the: Superior vena cava. Inferior vena cava. Blood enters the right atrium & flows through the tricuspid valve or the right atrioventricular valve (AV-valve).

Pulmonary Circuit The blood passes through the pulmonary semi- lunar valve & enters the pulmonary trunk, which divides into the left & right pulmonary arteries. The pulmonary arteries divide into capillaries at the lungs where external gas exchange occurs. Oxygenated blood travels from the lung capillaries to the left & right pulmonary veins, which return the blood to the left atrium.

Systemic Circuit Blood passes from the left atrium through the left AV-valve or bicuspid (mitral) valve to the left ventricle. Left ventricular walls are two times thicker than in the right ventricle. Blood travels through the aortic semi-lunar valve to the aorta.

Systemic Circuit The aorta branches into smaller systemic arteries, which branch into arterioles and then capillaries. At the capillaries, internal gas exchange occurs with the body cells. Capillaries rejoin as venules & then as veins. Deoxygenated blood returns to the right atrium via the superior & inferior vena cava.

Systemic Circuit

Coronary Circulation Delivers oxygenated blood directly to the heart muscle. Consists of the left & right coronary artery.

Cardiac Contractions Sinoatrial (SA) node or “pacemaker” (in the right atrium) maintains the heart’s intrinsic pumping rhythm. Nerves influence the rate & strength of the heart’s contractions. This signal travels to the atrioventricular (AV) node, where it is delayed for 0.1 seconds. It travels to the ventricles via the Purkinje fibers and the Bundle of His. The delay at the AV node causes the atria to contract simultaneously before the ventricles.

Pacemaker

Specialization of Cardiac Muscle The rhythmic contractions of the heart are due to special characteristics of the cardiac muscle cells.

The Heart Cycle The cardiac cycle is a continuous cycle of contraction and relaxation. Systole Heart contraction. Diastole Heart relaxation.

Systole Blood pressure is increased (120 mm Hg). Atria contract to further fill the ventricles. Tricuspid & bicuspid valves forced closed due to an increase in ventricular pressure. Ventricles contract to force blood from the heart. Right ventricle  (open pulmonary semi-lunar valve)  pulmonary trunk & pulmonary arteries; and Left ventricle  (open aortic semi-lunar valve)  aorta. Blood pressure is increased (120 mm Hg).

Diastole Blood pressure is reduced (80 mm Hg). Blood enters all four chambers: Pulmonary veins  Left atrium; and Vena cava (superior & inferior)  Right atrium. Tricuspid & bicuspid valves (AV valves) open, allowing blood to flow into the left and right ventricles. Pulmonary & aortic semi-lunar valves close due to a decrease in ventricular pressure. Blood pressure is reduced (80 mm Hg).

Systole & Diastole

Heart Valves & Heartbeat Sounds Heart valves open and close at different times to ensure blood flows in the proper direction. “Lub”: tricuspid and bicuspid valves close (beginning of systole). “Dub”: pulmonary and aortic semi-lunar valves close (end of systole).

Blood Pressure