Page 141 - 84 human physiolofy part-1

The Cardiovascular System

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 ever very far from a capillary.

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.

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 ever very far from a capillary. 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. Capillaries
come in three types:

• Continuous: Continuous capillaries have a sealed epithelium and only allow small
molecules, water and ions to diffuse.
• Fenestrated: Fenestrated capillaries (as their name implies "fenster") have openings that
allow larger molecules to diffuse.
• Sinusoidal: Sinusoidal capillaries are special forms of fenestrated capillaries that have
larger opening allowing RBCs and serum proteins to enter.

Veins

Veins carry deoxygenated blood to the heart. The only exception to this is in the pulmonary vein
that carries oxygenated blood 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 artery, 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

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