The Circulatory System

The Structure and Function of the Heart and Blood Vessels

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.

• Protected by the ribcage, sternum & spine.

The Heart

Pathways of Blood

• Pulmonary Circuit– Right side of heart.– Low-pressure system.

• Systemic Circuit– Left side of heart.– High-pressure system.

• 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.

Pulmonary Circuit

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

1. Atria contract to further fill the ventricles.

2. Tricuspid & bicuspid valves forced closed due to an increase in ventricular pressure.

3. Ventricles contract to force blood from the heart.

4. Right ventricle (open pulmonary semi-lunar valve) pulmonary trunk & pulmonary arteries; and

5. Left ventricle (open aortic semi-lunar valve) aorta.

Blood pressure is increased (120 mm Hg).

Diastole

Blood enters all four chambers:

1. Pulmonary veins Left atrium; and

2. Vena cava (superior & inferior) Right atrium.

3. Tricuspid & bicuspid valves (AV valves) open, allowing blood to flow into the left and right ventricles.

4. 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