The Cardiovascular System Explained

The
cardiovascular system, also called the circulatory
system
, consists of the heart and a
closed system of vessels – the arteries, veins, and capillaries.
The heart is the muscular device that pumps the blood around the
circuit of vessels.

The
cardiovascular system has some characteristics that make it a unique
and complex hydraulic system, such as the fact that it is a closed
circle, the fact that it is elastic and the fact that it is filled
with liquid at a positive mean pressure ("mean cardiovascular
pressure"), which exists independent of the pumping action of
the heart. The heart fills passively, rather than by actively
sucking, and the flow from it is intermittent, while the flow to it
is continuous.

The
most important functions
of the system
are to maintain
homeostasis and a favorable cellular environment. These functions
depend on the continuous and controlled flow of blood through the
thousands of miles of capillaries that reach every cell in the body.
Blood performs its ultimate transport
function (the purpose of circulation)

with the help of these microscopic capillaries: oxygen and nutrients
pass from capillary blood into fluids surrounding the cells and waste
products are removed in the same manner, being taken into the
capillary blood flow.The Cardiovascular System

The heart
is situated slightly to the left of the middle of the thorax,
underneath the sternum (breastbone), and is surrounded by the lungs.
Its strong muscular walls contract and pump blood to the arteries.
The heart expands and contracts 100,000 times per day, pumping
five or six quarts of blood each minute (about 2000 gallons per day).

Its wall
is made up of three distinct layers: the outer epicardium, composed
of a layer of flattened epithelial cells and connective tissue;
beneath, a much thicker myocardium (cardiac muscle); and the
endocardium, a further layer of flattened epithelial cells and
connective tissue lining the chambers of the heart.

The
heart
(Anatomy
of the Heart)
is a four-chambered
organ, divided into the left and right side by a muscular wall called
septum, which prevents blood from passing between them. The right and
left sides of the heart have each a top chamber (atrium) and a bottom
chamber (ventricle). Atria and ventricles are separated by valves –
the atrioventricular valves
- (Valve positions in the cardiac cycle)
,
which maintain unidirectional blood flow from the atria to the
ventricles. The atria receive blood from the veins, while the
ventricles pump blood into the arteries.

Atria and
ventricles work together, contracting and relaxing to pump blood out
of the heart. In the right side, blood enters the heart through the
inferior and the superior venae cavae; this is blood coming from the
body, which is poor in oxygen and enters into the right atrium. When
the atrium contracts, the blood flows through the tricuspid valve
into the right ventricle. After the ventricle is full, the tricuspid
valve shuts, preventing blood from returning into the atrium. As the
ventricle contracts, blood is forced into the pulmonary artery
through the pulmonic valve. It will flow to the lungs, where it will
be oxygenated.

In
the left side, at the same time, the pulmonary vein brings oxygenated
blood from the lungs to the left atrium. When the atrium contracts,
blood is driven into the left ventricle through the mitral valve,
which then shuts (when the ventricle is full). As the ventricle
contracts, the oxygenated blood flows into the aorta through the
aortic valve and then to the entire body. The pattern repeats over
and over (see also: cardiac
cycle
), with the left and right
sides of the heart working together.

In the
lungs, oxygen travels from the tiny air sacs through the walls of the
capillaries, into the blood and, at the same time, carbon dioxide
passes from blood into the air sacs in the same manner. Carbon
dioxide is exhaled, and the oxygenated blood travels back to the left
atrium of the heart through the pulmonary veins.

Apart from
the arteries that leave the heart, the heart has its own network of
arteries, called the coronary arteries, which supply it with oxygen
and nutrients. The right and left main coronary arteries branch off
from the aorta near the point where the aorta and the left ventricle
meet; the right coronary artery supplies the right atrium and right
ventricle with blood, while the left main coronary artery branches
into the circumflex artery and the left anterior descending artery
(left atrium and ventricle).

The
heartbeat is triggered by electrical impulses that travel down a
special pathway through the heart. The impulse starts in the
sinoatrial node (SA node), located in the right atrium, which
consists of a small bundle of specialized cells. This node is the
heart’s natural pacemaker. The electrical activity spreads through
the walls of the atria and causes them to contract. The
atrioventricular node (AV node), located in the center of the heart,
between the atria and ventricles, has the function of a gate that
slows the electrical signal before it enters the ventricles.

At rest, a
normal heart beats around 50 to 99 times a minute. During exercise,
strong emotions, fever or during treatment with some medications, the
heart may beat faster, sometimes to well over 100 beats per minute.

The heart
has other physiological functions, as well, such as secreting ANF
(atrial natriuretic factor), a powerful peptide hormone that has the
role of regulating blood pressure and volume. This hormone affects
the blood vessels, the adrenal glands, the kidneys and the regulatory
regions of the brain.

The
heart pumps blood through a system of vessels – elastic tubes
that carry blood to all parts of the body. The blood vessel system,
with veins,
arteries and capillaries
, reaches a
total length of 60,000.

The system
of arteries begins with the aorta, the largest artery, which
leaves the heart, carrying oxygen-rich blood. Arteries branch several
times, becoming smaller and smaller as they carry blood further from
the heart. Oxygenated blood will reach, through arteries, all body
tissues.

Capillaries
are the smallest, thinnest blood vessels, which connect the arteries
and the veins. They play the essential role in substance exchange, as
their thin walls allow oxygen, nutrients, carbon dioxide and other
waste products to pass to and from cells.

Veins are
blood vessels that take oxygen-poor blood from the body back to the
heart; this blood is rich in waste products that are to be excreted
or removed from the body. Veins become larger and larger as they get
closer to the heart. The superior vena cava enters the heart bringing
blood from the head and arms and the inferior vena cava brings blood
from the abdomen and legs.

The
histological makeup is basically the same in all types of blood
vessels: an inner endothelium, followed by subendothelial connective
tissue, then by a layer of vascular smooth muscle, very well
developed in arteries. A further layer of connective tissue follows -
the adventitia, which contains nerves as well as nutrient capillaries
in the larger blood vessels. Capillaries consist of little more than
a layer of endothelium and occasional connective tissue.

Although
blood vessels do not actively engage in the transport of the blood
(they do not have peristalsis), arteries (and, to a degree, veins)
can adjust their caliber by contraction of the muscular layer. This
is determined by the autonomic nervous system. Vasodilation and
vasoconstriction are also used antagonistically as a method of
thermoregulation.

The blood
pressure in blood vessels is traditionally expressed in millimetres
of mercury (1 mmHg = 133 Pa). Blood pressure is usually around 120
mmHg systolic (high pressure wave due to contraction of the heart)
and 80 mmHg diastolic (low pressure wave), in the arterial system. In
contrast, pressures in the venous system are constant and rarely
exceed 10 mmHg.

Blood
is an essential component of the system, as the transport function
(the
Purpose of Circulation
) is
indissolubly linked to it. Blood carries fresh oxygen from the lungs
and nutrients to body tissues, and it takes the body’s waste
products, including carbon dioxide, away from the tissues. This is
necessary to sustain life and promote the health of all tissues.

There are
numerous control mechanisms that help regulate the diverse functions
and components of the cardiovascular system, in order to supply blood
to specific areas according to need. These mechanisms ensure a
constant internal environment surrounding each body cell.

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