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

Arterial System

• Transports blood under pressure.• Blood moves in a pulse-like wave.• Contraction & 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.

Venous System

• Thinner walls, larger diameters & less muscle than arteries.

• Contains 70% of total blood volume.• Most veins work against gravity.

• Valves allow one-way flow.• Contraction of skeletal muscles pushes blood

toward heart.

Blood Vessels

The Heart

• Size of fist.• Hardest-working muscle.• Contains four chambers:

• Left & right atria (receiving chambers).• Left & right ventricles (delivery chambers).

• Septum separates left & right sides.

The Heart

• Protected by the ribcage, sternum & spine.

Pathways of Blood

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

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

Pulmonary Circuit

• Right atrium receives oxygen-poor blood from:• Superior vena cava.• Inferior vena cava.

• Blood flows through the tricuspid valve or the right atrioventricular (AV) valve to the right ventricle.

Pulmonary Circuit

• Blood flows through the pulmonary semi-lunar valve & enters the pulmonary trunk, which divides into the left & right pulmonary arteries.• Divides into capillaries where external gas exchange occurs.

• Oxygenated blood enters the left & right pulmonary veins, returning blood to the left atrium.

Systemic Circuit

• Blood flows 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

• Aorta branches into smaller systemic arteries, which branch into arterioles & then capillaries.• Internal gas exchange occurs.

• Capillaries rejoin as venules & then veins.• Deoxygenated blood returns to the right

atrium via the superior & inferior vena cava.

Systemic Circuit

Coronary Circulation

• Delivers oxygenated blood 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 Purkinje fibers & Bundle

of His.• The delay at the AV node causes the atria to contract

simultaneously before the ventricles.

Cardiac Contractions

The Heart Cycle

• The cardiac cycle is a continuous cycle of relaxation & contraction.• Diastole

• Heart relaxation

• Systole• Heart contraction.

Diastole (80 mm Hg)

• Blood enters all four chambers.• Pulmonary veins left atrium.• Inferior & superior vena cava right

atrium.• Left & right AV valves are open.• Pulmonary & aortic semi-lunar valves are

closed.

Systole (120 mm Hg)

• Aria 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.• Pulmonary & aortic semi-lunar valves are

open.

Diastole & Systole

Blood Pressure

• Pressure exerted on the arterial walls.• The elastic property of vessels allows

for continuous blood flow throughout the circulatory system.

• The difference between your systolic pressure & your diastolic pressure is called your pulse pressure.

Blood Pressure

• Highest in the aorta.• Blood pressure & velocity both decline as

blood enters the arterioles.• The drop in pressure results from the

resistance (friction) to blood flow.• Low blood pressure reduces your capacity to

transport blood.• High blood pressure weakens & ruptures

arterial walls.

Blood Pressure

Heart Valves & Heart Sounds

• “Lub”: Tricuspid & bicuspid valves close (beginning of systole).

• “Dub”: Pulmonary and aortic semi-lunar valves close (end of systole).

Heart Valves & Heart Sounds