The Circulatory System
I.
Composition of Blood
Erythrocytes (also called red blood cells) are about seven to eight micrometers in diameter and
have a disk shape. RBCs contain the blood's hemoglobin, which is an iron-containing protein that
binds to and helps transport oxygen. Mature red blood cells lack a nucleus and other organelles in
mammals.
Leukocytes (also called white blood cells) are usually larger in size (10-14 micrometers in
diameter) than red blood cells and they lack hemoglobin. They are part of the body's immune
system. They destroy and remove old or aberrant cells and cellular debris, as well as attack
infectious agents (pathogens) and foreign substances.
Platelets are between one to two micrometers in
diameter. They are membrane-bound cell
fragments that lack nuclei and are responsible for
blood clotting (coagulation). Their production is
regulated by a hormone called Thrombopoietin.
The circulating life of a platelet is eight to 10 days.
The sticky surface of the platelets allows them to
accumulate at the site of broken blood vessels to
form a clot.
In addition to the cellular components of blood,
blood also contains plasma. Plasma is a watery solvent that contains dissolved ions, nutrients,
proteins, and other substances that are transported by the blood.
Blood
Component
Erythrocytes
Leukocytes
Platelets
Plasma
Description
Function
II.
Blood Vessel Structure
Blood vessels are intricate networks of hollow tubes
that transport blood throughout the entire body.
Blood vessels are constructed of layers of connective
tissue and muscle. The inner blood vessel layer is a
lining of epithelial cells. In capillaries, the majority of
the blood vessel is epithelial cells. There are three
major types of blood vessels: the arteries, which
carry the blood away from the heart; the capillaries,
which enable the actual exchange of water and
chemicals between the blood and the tissues; and the
veins, which carry blood from the capillaries back
toward the heart.
Arteries carry blood away from the heart. They carry deoxygenated blood to the lungs, and
oxygenated blood out to body cells. They have thick muscular walls because they must be able to
withstand the pressure of the blood as it is pumped out of the heart. Blood is at high pressure
closest to the heart, and loses pressure as it travels out to the rest of the body.
Veins always carry blood towards the heart. They carry oxygenated blood from the lungs back to
the heart, and then deoxygenated blood from body cells back to the heart. Veins have thin walls in
comparison to arteries because they carry blood under low pressure. Veins have muscular valves to
prevent blood from flowing backwards as it loses pressure.
Capillaries are the smallest blood vessels, found in the muscles and tissues of the body and the
alveoli of the lungs. The walls of capillaries are microscopic – one cell thick to allow gas exchange
to take place. Oxygen passes through the capillary wall and into the tissues, carbon dioxide passes
from the tissues into the blood.
Blood Vessel
Artery
Vein
Capillary
Description
Function