Circulation
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Your circulatory system is composed of 96 000 km of blood vessels that supply your
body with nutrients. Every cell is no further than 2 cells away from a blood vessel.
Every minute, 5 L of blood cycle from the heart to the lungs, where oxygen is
received, and then back to the heart. The heart pumps oxygen-rich blood to the
tissues of the body. Here, oxygen and nutrients are given up, and wastes return to
the heart.
Your circulatory system carries nutrients to cells, wastes away from cells, and
chemical messengers from cells in one part of the body to distant target tissues.
Defense against invading organisms is also associated with the circulatory system.
Arteries:
- Arteries are the blood vessels that carry blood away from the heart. Every time the
heart contracts, blood surges from the heart and enters the arteries. The arteries
stretch to accommodate the inrush of blood. The pulse you can feel near your wrist
and on either side of your neck is created by the changes in the diameter of the
artery near the surface of your body following heart contractions. Heart contraction
is followed by a relaxation phase. During this phase, pressure drops and elastic fibers
in the walls of the artery recoil.
- A birth defect or injury can cause the inner wall of the artery to bulge – this is an
aneurysm
- Blood from the arteries passes into smaller arteries called arterioles
- As fat droplets “meet” each other in the blood, they combine to form even larger fat
globules. Calcium and other minerals deposit on top of the lipid, and a fibrous net of
plaque is formed. This is known as arteriosclerosis and can narrow the artery to ¼ its
original diameter and lead to high blood pressure. As fat droplets accumulate,
adequate amounts of blood and oxygen cannot be delivered to the heart muscle,
resulting in chest pains.
Capillaries
- The capillary is the site of fluid and gas exchange between blood and blood cells. The
diameter is so small that red blood cells travel through capillaries in single file
- Oxygen diffuses from the blood into the surrounding tissues through the thin walls of
the capillaries. Oxygenated blood which appears red in colour, becomes a purple-blue
colour as it leaves the capillary. The deoxygenated blood collects in small veins called
venules and is carried back to the heart.
Veins
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Capillaries merge and become larger vessels, called venules. Venules merge into veins,
which have a greater diameter. Gradually, the diameter of the veins increases as
blood is returned to the heart.
The Heart – VIDEO - http://www.pbs.org/wgbh/nova/heart/heartmap.html
- The heart is surrounded by a fluid-filled membrane called the pericardium. The heart
beats about 70 times each minute from the beginning of life until death. The heart
has 2 parallel pumps separated by a wall of muscle, the septum. The pump on the right
receives deoxygenated blood from the body tissues and pumps it to the lungs. Vessels
that carry blood to and from the lungs comprise the pulmonary circulatory system.
The pump on the left receives oxygenated blood from the lungs and pumps it to the
cells of the body. Vessels that carry blood to and from the body comprise the
systemic circulatory system.
- Blood is carried to the heart by veins. The superior vena cava carries deoxygenated
blood from the head to the right atrium. The inferior vena cava carries blood from
the tissues of the body to the same atrium. Oxygenated blood flowing from the lungs
enters the left atrium by way of the pulmonary veins. Blood on both sides of the
heart fills the atria and is eventually pumped into the larger ventricles.
Atrioventricular valves (AV valves) separate the atria from the ventricles. The AV
valves prevent the blood from flowing from the ventricles back to the atria. Blood is
carried away from the heart by arteries. The pulmonary artery carries deoxygenated
blood from the heart to the lungs. Once in the lungs, the blood receives oxygen by
diffusion and returns to the left side of the heart. Oxygenated blood is carried away
from the heart by the aorta, the largest artery in your body.
- In addition to blood’s transport functions, blood helps maintain the water balance of
organ systems, body temperature, and pH balance.
- Blood is also an important part of the immune system
- Blood is composed of 55% fluid (plasma), and 45% blood cells
- There are 3 groups of proteins in the plasma:
o Albumins – establishes an osmotic pressure that draws water back into
capillaries and helps maintain body fluid levels
o Gamma Globulins – produces antibodies that provide protection against invading
microbes
o Fibrinogens – important in blood clotting
Red Blood Cells
- The main function of red blood cells is the transport of oxygen
- Red blood cells are also known as erythrocytes
- Hemoglobin is a pigment in the blood that increases the capacity of blood to carry
oxygen
- Without hemoglobin, your red blood cells would supply only enough O2 to live for 4.5
seconds
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There are 280 million hemoglobin molecules in 1 red blood cell
Red blood cells do not contain a nucleus so they have room for the hemoglobin
A decrease in hemoglobin decreases O2 delivery. This is know as anemia (due to low
iron)
Two Types of White Blood Cells:
1. Leukocytes
- much less numerous than red blood cells
- have a nucleus
- some destroy invading microbes by phagocytosis (engulf or “swallow” the cells)
- once the microbe has been engulfed, the leukocyte releases enzymes that digest the
microbe and the leukocyte itself
2. Lymphocytes
- another group of white blood cells
- produce antibodies (recognize and attack invader species)
- 2 types
o T Cell- produced in bone marrow
 Seeks out and recognizes intruder and signals attack
 Passes information onto B cells
o B Cell – produce antibodies to attack invader species
Platelets
- do not contain a nucleus
- produced in the bone marrow
- initiate blood-clotting reactions
Blood Clots – VIDEO - http://library.thinkquest.org/C0115080/?c=clotting#How
- When blood vessels are damaged, the platelets breaks apart, and release the enzyme,
“thrombokinase”. This enzyme converts the protein “prothrombrin” into “thrombin”.
Thrombin turns fibrinogen (protein in blood) into a network of insoluble “fibrin” that
wraps around the damaged area and seals the cut in the skin with a clot.
- Thrombus – blood clot that seals a blood vessel
o O2 cannot get to the tissue
o can cause a stroke
- Embolus – a dislodged blood clot
o May lodge in a vital organ
HIV (Human Immunodeficiency Virus)
- Attaches to the receptor sites of the T-Cell lymphocytes
- Once attached, the T-cell engulfs the virus, creating another problem for the immune
system
- Antibody production requires a blueprint of the invader. The protein coat of the virus
hides inside the very cells assigned as guards for invading antigens