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Mammalian
transport systems III - circulation
Joanna
George-Johnson, Contributor
In
the last two lessons, we have looked at two of the three components of the mammalian
transport system - the blood and the heart. Today, we finish the series by looking
at the blood vessels. We will also discuss the path of blood through the body.
Blood
vessels 
There
are three types of blood vessels, each with its own structure and function. As
the blood vessels move farther from the heart they become progressively smaller.
Arteries divide repeatedly, until they form very small vessels known as capillaries.
The capillaries eventually reunite to form veins Now
that we have looked at the various types of blood vessels, we can now study the
movement of the blood around the body. The
knowledge of three terms is necessary when speaking about mammalian circulation.
 - Blood:
Solution of plasma, blood cells, and platelets that are circulated by the
heart through the vertebrate vascular system.
- Tissue
fluid: blood without plasma proteins and red blood cells, which is forced
through the pores in capillaries. Lymph fluid provides a constant externals environment
for the body cells, as well as their nutrient requirements.
- Lymph:
Recollected tissue fluid.
Blood
follows a certain path around the body. Please remember that although the term
blood is used, in the interstitial spaces and lymph vessels it is known as tissue
fluid and lymph respectively. The
blood flows through the arteries into the arterioles, then into the capillaries.
There, the plasma leaks into the spaces between the cells. The fluid which remains
in the capillaries continues into the venules then the veins. The tissue fluid
is recollected by the lymph vessels, which join back up with the general circulation
at the subclavian vein. | | Arteries | Veins | Capillaries | | Function | Take
blood away from the heart to the tissues (except pulmonary artery) | Take
blood from the tissues towards the heart (except pulmonary vein) | Allows
exchange of materials between the blood and the tissues | | Walls | Thick
with many elastic fibres. This is so that they may resist the high pressure of
blood coming from the heart. | Thin
with few elastic fibres and muscles, as blook is at low pressure returning to
the heart | Only
one cell thick, with no elastic fibres. Extremely permeable in order to allow
exchange of materials between the blood and tissue. | | Size
of Lumen | Smaller
than that of veins but larger than that of capillaries. The size changes as the
vessels expand and recoil. | Large
lumen. This is necessary so as to ensure little resistance to blood flow, as the
blood is flowing at low pressure. | Very
small lumen, only one blood cell being able to pass through at a time. | | Valves? | No.
Not necessary as the blood is at high pressure | Yes.
Blood at low pressure thus valves are necessary to prevent backflow. | No.
| | Blood
pressure | High | Low | Falling
| Note:
High blood pressure does not necessarily mean high flow rate! |
Exercise:
Look
up the names of the arteries and veins that supply the various parts of the human
body. Joanna
George-Johnson teaches at Ardenne High School.
Email: Masterbio@gmail.com.
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