The Circulatory System
The circulatory system is extremely important in sustaining life.
It’s proper functioning is responsible for the delivery of oxygen and
nutrients to all cells, as well as the removal of carbon dioxide, waste
products, maintenance of optimum pH, and the mobility of the elements,
proteins and cells, of the immune system. In developed countries, the
two leading causes of death, myocardial infarction and stroke are each
direct results of an arterial system that has been slowly and
progressively compromised by years of deterioration.
Arteries
Arteries are muscular blood vessels that carry blood away from the
heart, oxygenated and deoxygenated blood . The pulmonary arteries will
carry deoxygenated blood to the lungs and the sytemic arteries will
carry oxygenated blood to the rest of the body. Arteries have a thick
wall that consists of three layers. The inside layer is called the
endothelium, the middle layer is mostly smooth muscle and the outside
layer is connective tissue. The artery walls are thick so that when
blood enters under pressure the walls can expand.
Arterioles
An arteriole is a small artery that extends and leads to
capillaries. Arterioles have thick smooth muscular walls. These smooth
muscles are able to contract (causing vessel constriction) and relax
(causing vessel dilation). This contracting and relaxing affects blood
pressure; the higher number of vessels dilated, the lower blood
pressure will be. Arterioles are just visible to the naked eye.
Capillaries
Capillaries are the smallest of a body’s vessels; they connect
arteries and veins, and most closely interact with tissues. They are
very prevalent in the body; total surface area is about 6,300 square
meters. Because of this, no cell is very far from a capillary, no more
than 50 micrometers away. The walls of capillaries are composed of a
single layer of cells, the endothelium. This layer is so thin that
molecules such as oxygen, water and lipids can pass through them by
diffusion and enter the tissues. Waste products such as carbon dioxide
and urea can diffuse back into the blood to be carried away for removal
from the body.
The "capillary bed" is the network of capillaries present throughout
the body. These beds are able to be “opened” and “closed” at any given
time, according to need. This process is called autoregulation and
capillary beds usually carry no more than 25% of the amount of blood it
could hold at any time. The more metabolically active the cells, the
more capillaries it will require to supply nutrients.
Veins
Veins carry blood to the heart. The pulmonary veins will carry
oxygenated blood to the heart awhile the systemic veins will carry
deoxygenated to the heart. Most of the blood volume is found in the
venous system; about 70% at any given time. The veins outer walls have
the same three layers as the arteries, differing only because there is
a lack of smooth muscle in the inner layer and less connective tissue
on the outer layer. Veins have low blood pressure compared to arteries
and need the help of skeletal muscles to bring blood back to the heart.
Most veins have one-way valves called venous valves to prevent backflow
caused by gravity. They also have a thick collagen outer layer, which
helps maintain blood pressure and stop blood pooling. If a person is
standing still for long periods or is bedridden, blood can accumulates
in veins and can cause varicose veins. The hollow internal cavity in
which the blood flows is called the lumen. A muscular layer allows
veins to contract, which puts more blood into circulation. Veins are
used medically as points of access to the blood stream, permitting the
withdrawal of blood specimens (venipuncture) for testing purposes, and
enabling the infusion of fluid, electrolytes, nutrition, and
medications (intravenous delivery).
Venules
A venule is a small vein that allows deoxygenated blood to return
from the capillary beds to the larger blood veins, except in the
pulminary circuit were the blood is oxygenated. Venules have three
layers; they have the same makeup as arteries with less smooth muscle,
making them thinner. | 
