The circulatory system consists of the heart, arteries, veins, and capillaries. The
circulatory system carries oxygen, nutrients, and hormones to every cell of the
body, and carries away carbon dioxide, waste chemicals, and heat. Blood circulates
through a closed system of tubes that includes the lung and tissue capillaries,
heart, arteries, and veins.
The very large surface areas required for ample diffusion of gases in the
lungs and tissues are provided by the thin walls of the capillaries. Every part of the
body is completely interwoven with intricate networks of extremely small blood
vessels called capillaries. In the lungs, capillaries surround the tiny air sacs
(alveoli) so that the blood they carry can exchange gases with air.
The heart (Figure 3-1) is the muscular pump that propels the blood
throughout the system. It is about the size of a closed fist, hollow, and made up
almost entirely of muscle tissue that forms its walls and provides the pumping
action. The heart is located in the front and center of the chest cavity between the
lungs, directly behind the breastbone (sternum).
The interior of the heart is divided lengthwise into halves, separated by a wall of
tissue called a septum, that have no direct conduit to each other. Each half is
divided into an upper chamber (the atrium), which receives blood from the veins
of its circuit and a lower chamber (the ventricle) which takes blood from the
atrium and pumps it away via the main artery. Because the ventricles do most of
the pumping, they have the thickest, most muscular walls. The arteries carry blood
from the heart to the capillaries; the veins return blood from the capillaries to the
heart. Arteries and veins branch and rebranch many times, very much like a tree.
Trunks near the heart are approximately the diameter of a human thumb, while the
smallest arterial and venous twigs are microscopic. Capillaries provide the
connections that let blood flow from the smallest branch arteries (arterioles) into
the smallest veins (venules).
Figure 3-1. The Heart.
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The circulatory system consists of two
circuits with the same blood flowing through the body. The pulmonary circuit
serves the lung capillaries; the systemic circuit serves the tissue capillaries. Each
circuit has its own arteries and veins and its own half of the heart as a pump.
Figure 3-2 shows how the circulatory system is arranged. In complete circulation,
blood first passes through one circuit and then the other, going through the heart
twice in each complete circuit.
figure3-2
Respiration and Blood Circulation. The lung’s gas exchange system is essentially three pumps. The thorax, a gas pump, moves air through the trachea and
bronchi to the lung’s air sacs. These sacs, the alveoli, are shown with and without their
covering of pulmonary capillaries. The heart’s right ventricle, a fluid pump, moves blood that
is low in oxygen and high in carbon dioxide into the pulmonary capillaries. Oxygen from the
air diffuses into the blood while carbon dioxide diffuses from the blood into the air in the
lungs. The oxygenated blood moves to the left ventricle, another fluid pump, which sends
the blood to the systemic capillaries which deliver oxygen to and collect carbon dioxide from
the body’s cells.
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Blood follows a continuous circuit through the human
body. Blood leaving a muscle or organ capillary has lost most of its oxygen and is
loaded with carbon dioxide. The blood flows through the body’s veins to the main
veins in the upper chest (the superior and inferior vena cava). The superior vena
cava receives blood from the upper half of the body; the inferior vena cava
receives blood from areas of the body below the diaphragm. The blood flows
through the main veins into the right atrium and then through the tricuspid valve
into the right ventricle.
The next heart contraction forces the blood through the pulmonic valve into the
pulmonary artery. The blood then passes through the arterial branchings of the
lungs into the pulmonary capillaries, where gas transfer with air takes place. By
diffusion, the blood exchanges inert gas as well as carbon dioxide and oxygen with
the air in the lungs. The blood then returns to the heart via the pulmonary venous
system and enters the left atrium.
The next relaxation finds it going through the mitral valve into the left ventricle to
be pumped through the aortic valve into the main artery (aorta) of the systemic
circuit. The blood then flows through the arteries branching from the aorta, into
successively smaller vessels until reaching the capillaries, where oxygen is
exchanged for carbon dioxide. The blood is now ready for another trip to the lungs
and back again.
The larger blood vessels are somewhat elastic and have muscular walls. They
stretch and contract as blood is pumped from the heart, maintaining a slow but
adequate flow (perfusion) through the capillaries.
The average human body contains approximately five liters
of blood. Oxygen is carried mainly in the red corpuscles (red blood cells). There
are approximately 300 million red corpuscles in an average-sized drop of blood.
These corpuscles are small, disc-shaped cells that contain hemoglobin to carry
oxygen. Hemoglobin is a complex chemical compound containing iron. It can
form a loose chemical combination with oxygen, soaking it up almost as a sponge
soaks up liquid. Hemoglobin is bright red when it is oxygen-rich; it becomes
increasingly dark as it loses oxygen. Hemoglobin gains or loses oxygen depending
upon the partial pressure of oxygen to which it is exposed. Hemoglobin takes up
about 98 percent of the oxygen it can carry when it is exposed to the normal partial
pressure of oxygen in the lungs. Because the tissue cells are using oxygen, the
partial pressure (tension) in the tissues is much lower and the hemoglobin gives up
much of its oxygen in the tissue capillaries.
Acids form as the carbon dioxide dissolves in the blood. Buffers in the blood
neutralize the acids and permit large amounts of carbon dioxide to be carried away
to prevent excess acidity. Hemoglobin also plays an important part in transporting
carbon dioxide. The uptake or loss of carbon dioxide by blood depends mainly
upon the partial pressure (or tension) of the gas in the area where the blood is
exposed. For example, in the peripheral tissues, carbon dioxide diffuses into the
blood and oxygen diffuses into the tissues.
Blood also contains infection-fighting white blood cells, and platelets, which are
cells essential in blood coagulation. Plasma is the colorless, watery portion of the
blood. It contains a large amount of dissolved material essential to life. The blood
also contains several substances, such as fibrinogen, associated with blood clotting.
Without the clotting ability, even the slightest bodily injury could cause death.
