the circulatory system the structure and function of the heart and blood vessels
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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
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.
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.
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.
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).
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).