A Presentation

Cardiovascular System: Blood Vessels and Circulation

Structure and Function• The vital functions of the cardiovascular

system occur at the capillary level

• Chemical and gaseous exchange between the blood and interstitial fluid occurs across capillary walls.– Tissues relay on capillary diffusion for

nutrients and oxygen and to remove metabolic waste.

Structure of vessel walls• Three layers

– Tunica intima or innermost layer. Includes a lining of endothelium and a connective tissue layer.

– Tunica media or middle layer. Contains smooth muscle tissue and collagen and elastic fibers.

– Tunica externa forms sheath of connective tissue around the vessel.

Arteries• Elastic: large, able to absorb the pressure

changes of the cardiac cycle, contain many elastic fibers that stretch and return to original dimension. Examples: aorta, pulmonary trunk

• Muscular arteries: medium sized distribute blood to skeletal muscles and internal organs. Not as elastic. Example: external carotid arteries.

• Arterioles: small (30u), diameter of muscle layer very thin.

Capillaries• Only vessels that permit exchange

between blood and interstitial fluid.• Walls are thin(no tunica externa or tunica

media)• Small diameter slows blood flow• Permits water, small solutes and lipid-

soluble materials to pass.• Interconnect to form capillary bed.

Veins collect blood and return it to the heart

• Venules: smallest• Veins have relatively thin walls.• In medium veins contain valves that

prevent the backflow of blood due to low pressure and gravity

• Stretching and distortion of these valves cause varicose veins.

Pressure and resistance determine blood flow and affect

rates of capillary exchange• Highest pressure at the base of the aorta• Resistance opposes movement of blood.

– Sources include• Vascular resistance• Viscosity• turbulence

Pressures within the systemic circuit

• Highest in the aorta and lowest at the vena cava

• Arterial pressure rises during ventricular systole and falls during ventricular diastole.

• 120/80 reflects the separate systolic and diastolic pressures.

• Read the Clinical Note page 435.

Forces Acting Across Capillary Walls

Venous pressure• Pressure is only 1/10th of the arterial

system at the beginning.• When standing two factors help venous

return– Muscular compression of skeletal muscles– Respiratory pump: inhalation causes both the

vena cava and rt. atria to expand and fill.

Cardiovascular Regulation• Involves

– Autoregulation– Neural mechanisms– Endocrine mechanisms

Pulmonary Circuit• Deoxgenated blood enters the lungs in

arteries• Oxygenated blood leaves the lungs in

veins.

Systemic Circuit• Oxygenated blood from the left ventricle

goes to tissues other than the lungs’ exchange surfaces.

• Deoxygenated blood returns to the right atrium.

Fetal Circulation• Embryonic lungs are collapsed and

nonfunctional• All nutritional and respiratory needs are

provided by diffusion across the placenta.• Umbilical arteries carry deoxygenated

blood from fetus to placenta• Umbilical veins returns oxygenated blood

from placenta to fetus.

• Veins bypass developing liver through ductus venosus.

• Foramen ovale in fetal heart and ductus arteriosus between pulmonary and aortic trunks by pass collasped lungs.

• At birth lungs expand and smooth muscles in ductus arteriosus contract closing connection. Increased pressure in L atrium closes foramen ovale.

Aging• Decreased hematocrit• Blockage of peripheral veins• Pooling of blood in the veins• Reduction in max. cardiac output• Changes in heart conduction• Reduction in elasticity of cardiac skeleton• Progressive atheroschlerosis• Replacement of damaged heart muscle by

scar tissue.

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