| THE CIRCULATORY SYSTEM This information will help you to develop your
understanding of the elastic hosiery available to the nurse prescriber.
Blood from veins of the forelimbs, neck
and head enter the right atrium via the superior vena cava, and from the rest of the body
by the inferior vena cava. It then passes into the right ventricle via the right
atrioventricular opening, which is guarded by the tricuspid valve.
The pulmonary artery takes blood away
from the right ventricle. This artery divides and passes to the capillaries of the lung.
The blood then moves into the pulmonary vein and passes into the left atrium and the left
ventricle.
The left atrioventricular opening is
guarded by the mitral valve. Blood leaves the left ventricle by the aorta. Semi lunar
valves guard the openings of the pulmonary artery and the aorta. From the aorta, blood
passes into other arteries. Arteries carry the blood away from the heart. Each artery
branches approximately fifteen to twenty times, becoming smaller and smaller. These small
arteries are called arterioles, which lead into a network of minute capillaries. Oxygen
and nutrients diffuse through the thin walls of these vessels into the tissues of the
body. The capillaries eventually form venules, which lead onto veins. Veins take the blood
back to the right atrium. The blood therefore has two journeys: around the body in the
systemic circulation, and to the lungs and back in the pulmonary circulation. At any one
time, three quarters of the blood is in veins, one fifth in arteries, and one-twentieth in
capillaries.
The structure of blood vessels
Arteries and veins are composed of three layers:
- Tunica externa or tunica adventia.
- Tunica media.
- Tunica intima or tunica interna.
Tunica externa or tunica adventia
This is the outermost layer of the
vessel and forms a connective tissue sheath, which helps to stabilise and anchor the
vessel by the blending of connective tissue fibres into adjacent tissue.
Tunica media
This layer contains smooth muscle,
connective tissue and elastic fibres and is stimulated by sympathetic nerve fibres. It is
responsible for the change in lumen size of the vessel, and is very thick in arteries.
Tunica intima or tunica interna
This layer is comprised of a smooth
lining of endothelial cells, which also form the valves, and a basement membrane.
- Arteries are able to cope with the
high-pressure surges of blood from the heart because they have very thick muscular walls.
- The single layer of cells that make up a
capillary enables the passage of oxygen and nutrients from capillaries into the
surrounding tissue.
- Veins have loose, slack walls as the
blood inside them is under very little pressure.
Figure 4 - The
structure of blood vessels
Venous Return
Three mechanisms are involved in the
return of the venous blood to the heart:
- The respiratory pump. The thoracic
cavity expands as a person inhales and air is pulled into the lungs as a result of this
drop in pressure in the pleural cavities. At the same time, blood is also pulled into the
inferior vena cava and right atrium. During exhalation, the internal pressure in the
thoracic cavity rises. Air is forced out of the lungs, and venous blood pushed into the
right atrium.
- The venous pump. During normal standing
and walking, the venous pump assists venous return. As the calf muscles contract, they
compress the nearby blood vessels propelling blood towards the heart. During muscle
relaxation, the vessel once again fills with blood and the cycle is repeated during the
next contraction. Blood pools in the legs when a person is standing still for a long
period of time. The venous pump does not operate.
- Venous valves. The valves in the leg
veins prevent the blood from flowing back towards the capillaries. In the absence of
valves, gravity would cause pooling of blood in the leg veins. When lying down, venous
valves have little effect as the heart and major vessels are at the same level. The valves
in the perforator connecting veins (Figure 5 - The
venous system of the leg) have the most important role. If these valves fail
to work effectively, the high pressure in the deep veins, is transmitted to the much
weaker, unsupported superficial veins. These veins become distended and tortuous (varicose
veins). Capillary pressure becomes increased, and fluid is forced out into the
extravascular space. This can then progress onto chronic venous insufficiency
characterised by oedema, pigmentation, eczema, and ulceration of the leg.
Multiple Choice Question (MCQs)
1. Which of the following statements are true?
A. The tunical media is very thick in arteries.
B. The tunica media is very thick in veins.
C. The tunica externa in veins is is generally thicker than the tunica
media.
D. Veins have loose, slack walls as the blood inside is under very
little pressure.
2. What are the mechanisms involved in the return of venous
blood to the heart?
A. The venous pump
B. Peripheral resistance
C. Respiratory pump
D. Venous valves
3. With regards to the venous pump
A. Calf muscles contract and compress the nearby blood vessels
propelling blood towards the heart.
B. If this system fails to work effectively, the high pressure in the
deep veins, is transmitted to the much weaker,
unsupported superficial veins.
C. During exhalation, the internal pressure in the thoracic cavity
rises. Air is forced out of the lungs, and venous
blood pushed into the right atrium.
4. Which of the following statements are true?
A. Venous valves point in the opposite direction to blood flow.
B. Venous valves are folds of tissue formed from the endothelial lining
of the tunica intima.
C. In the absence of valves, pooling of blood in the leg veins would
occur.
E. During movement, smooth muscles surrounding veins squeeze the blood
towards the heart.
To see the answers to these questions click here
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