UNDERSTANDING BASICS
OF THE FULL CARDIOVASCULAR SYSTEM....
The cardiovascular system consists of the heart, blood
vessels, and the approximately 5-7 litres of blood that the blood vessels
transport. Responsible for transporting oxygen, nutrients, hormones, and
cellular waste products throughout the body, the cardiovascular system is
powered by the body’s hardest-working organ — the heart, which is only about
the size of a closed fist. Even at rest, the average heart easily pumps over 5
litres of blood throughout the body every minute.
The Heart
The heart is a muscular pumping organ located medial to the
lungs along the body’s mid-line in the thoracic region. The bottom tip of the
heart, known as its apex, is turned to the left, so that about 2/3 of the heart
is located on the body’s left side with the other 1/3 on right. The top of the
heart, known as the heart’s base, connects to the great blood vessels of the
body: the aorta, vena cava, pulmonary trunk, and pulmonary veins.
Circulatory Loops
There are 2 primary circulatory loops in the human body: the
pulmonary circulation loop and the systemic circulation loop.
1) Pulmonary circulation transports de-oxygenated blood from
the right side of the heart to the lungs, where the blood picks up oxygen and
returns to the left side of the heart. The pumping chambers of the heart that
support the pulmonary circulation loop are the right atrium and right
ventricle.
2) Systemic circulation carries highly oxygenated blood from
the left side of the heart to all of the tissues of the body (with the
exception of the heart and lungs). Systemic circulation removes wastes from
body tissues and returns deoxygenated blood to the right side of the heart. The
left atrium and left ventricle of the heart are the pumping chambers for the
systemic circulation loop.
Blood Vessels
Blood vessels are the body’s highways that allow blood to
flow quickly and efficiently from the heart to every region of the body and
back again. The size of blood vessels corresponds with the amount of blood that
passes through the vessel. All blood vessels contain a hollow area called the
lumen through which blood is able to flow. Around the lumen is the wall of the
vessel, which may be thin in the case of capillaries or very thick in the case
of arteries.
All blood vessels are lined with a thin layer of simple
squamous epithelium known as the endothelium that keeps blood cells inside of
the blood vessels and prevents clots from forming. The endothelium lines the
entire circulatory system, all the way to the interior of the heart, where it
is called the endocardium.
There are three major types of blood vessels:
arteries, capillaries and veins.
Blood vessels are often named after either the region of the
body through which they carry blood or for nearby structures. For example, the
brachiocephalic artery carries blood into the brachial (arm) and cephalic
(head) regions. One of its branches, the subclavian artery, runs under the
clavicle; hence the name subclavian. The subclavian artery runs into the
axillary region where it becomes known as the axillary artery.
Arteries and Arterioles:
Arteries are blood vessels that carry blood away from the
heart. Blood carried by arteries is usually highly oxygenated, having just left
the lungs on its way to the body’s tissues. The pulmonary trunk and arteries of
the pulmonary circulation loop provide an exception to this rule – these
arteries carry deoxygenated blood from the heart to the lungs to be oxygenated.
Arteries face high levels of blood pressure as they carry
blood being pushed from the heart under great force. To withstand this
pressure, the walls of the arteries are thicker, more elastic, and more
muscular than those of other vessels. The largest arteries of the body contain
a high percentage of elastic tissue that allows them to stretch and accommodate
the pressure of the heart.
Smaller arteries are more muscular in the structure of their
walls. The smooth muscles of the arterial walls of these smaller arteries
contract or expand to regulate the flow of blood through their lumen. In this
way, the body controls how much blood flows to different parts of the body
under varying circumstances. The regulation of blood flow also affects blood
pressure, as smaller arteries give blood less area to flow through and
therefore increases the pressure of the blood on arterial walls.
Arterioles are narrower arteries that branch off from the
ends of arteries and carry blood to capillaries. They face much lower blood
pressures than arteries due to their greater number, decreased blood volume,
and distance from the direct pressure of the heart. Thus arteriole walls are
much thinner than those of arteries. Arterioles, like arteries, are able to use
smooth muscle to control their aperture and regulate blood flow and blood
pressure.
Capillaries:
Capillaries are the
smallest and thinnest of the blood vessels in the body and also the most
common. They can be found running throughout almost every tissue of the body
and border the edges of the body’s avascular tissues. Capillaries connect to
arterioles on one end and venuoles on the other.
Capillaries carry blood very close to the cells of the
tissues of the body in order to exchange gases, nutrients, and waste products.
The walls of capillaries consist of only a thin layer of endothelium so that
there is the minimum amount of structure possible between the blood and the
tissues. The endothelium acts as a filter to keep blood cells inside of the
vessels while allowing liquids, dissolved gases, and other chemicals to diffuse
along their concentration gradients into or out of tissues.
Precapillary sphincters are bands of smooth muscle found at
the arteriole ends of capillaries. These sphincters regulate blood flow into
the capillaries. Since there is a limited supply of blood, and not all tissues
have the same energy and oxygen requirements, the precapillary sphincters
reduce blood flow to inactive tissues and allow free flow into active tissues.
Veins and Venuoles:
Veins are the large return vessels of the body and act as
the blood return counterparts of arteries. Because the arteries, arterioles,
and capillaries absorb most of the force of the heart’s contractions, veins and
venuoles are subjected to very low blood pressures. This lack of pressure
allows the walls of veins to be much thinner, less elastic, and less muscular
than the walls of arteries.
Veins rely on gravity, inertia, and the force of skeletal
muscle contractions to help push blood back to the heart. To facilitate the
movement of blood, some veins contain many one-way valves that prevent blood
from flowing away from the heart. As skeletal muscles in the body contract,
they squeeze nearby veins and push blood through valves closer to the heart.
When the muscle relaxes, the valve traps the blood until
another contraction pushes the blood closer to the heart. Venuoles are similar
to arterioles as they are small vessels that connect capillaries, but unlike
arterioles, venuoles connect to veins instead of arteries. Venuoles pick up
blood from many capillaries and deposit it into larger veins for transport back
to the heart.
Coronary Circulation
The heart has its own set of blood vessels that provide the
myocardium with the oxygen and nutrients necessary to pump blood throughout the
body. The left and right coronary arteries branch off from the aorta and
provide blood to the left and right sides of the heart. The coronary sinus is a
vein on the posterior side of the heart that returns deoxygenated blood from
the myocardium to the vena cava.
Hepatic Portal Circulation
The veins of the stomach and intestines perform a unique
function: instead of carrying blood directly back to the heart, they carry
blood to the liver through the hepatic portal vein. Blood leaving the digestive
organs is rich in nutrients and other chemicals absorbed from food. The liver
removes toxins, stores sugars, and processes the products of digestion before
they reach the other body tissues. Blood from the liver then returns to the
heart through the inferior vena cava.
Blood
The average human body contains about 4 to 5 liters of
blood. As a liquid connective tissue, it transports many substances through the
body and helps to maintain homeostasis of nutrients, wastes, and gases. Blood
is made up of red blood cells, white blood cells, platelets, and liquid plasma.
Red Blood Cells: Red blood cells, also known as erythrocytes,
are by far the most common type of blood cell and make up about 45% of blood
volume. Erythrocytes are produced inside of red bone marrow from stem cells at
the astonishing rate of about 2 million cells every second. The shape of
erythrocytes is biconcave—disks with a concave curve on both sides of the disk
so that the center of an erythrocyte is its thinnest part. The unique shape of
erythrocytes gives these cells a high surface area to volume ratio and allows
them to fold to fit into thin capillaries. Immature erythrocytes have a nucleus
that is ejected from the cell when it reaches maturity to provide it with its
unique shape and flexibility. The lack of a nucleus means that red blood cells
contain no DNA and are not able to repair themselves once damaged.
Erythrocytes transport oxygen in the blood through the red
pigment hemoglobin. Hemoglobin contains iron and proteins joined to greatly
increase the oxygen carrying capacity of erythrocytes. The high surface area to
volume ratio of erythrocytes allows oxygen to be easily transferred into the
cell in the lungs and out of the cell in the capillaries of the systemic
tissues.
White Blood Cells:
White blood cells, also known as leukocytes, make up a very
small percentage of the total number of cells in the bloodstream, but have
important functions in the body’s immune system. There are two major classes of
white blood cells: granular leukocytes and agranular leukocytes.
Granular Leukocytes:
The three types of granular leukocytes are neutrophils,
eosinophils, and basophils. Each type of granular leukocyte is classified by
the presence of chemical-filled vesicles in their cytoplasm that give them
their function. Neutrophils contain digestive enzymes that neutralize bacteria
that invade the body. Eosinophils contain digestive enzymes specialized for
digesting viruses that have been bound to by antibodies in the blood. Basophils
release histamine to intensify allergic reactions and help protect the body
from parasites.
Agranular Leukocytes:
The two major classes of agranular leukocytes are
lymphocytes and monocytes. Lymphocytes include T cells and natural killer cells
that fight off viral infections and B cells that produce antibodies against
infections by pathogens. Monocytes develop into cells called macrophages that
engulf and ingest pathogens and the dead cells from wounds or infections.
Platelets :
Also known as thrombocytes, platelets are small cell
fragments responsible for the clotting of blood and the formation of scabs.
Platelets form in the red bone marrow from large megakaryocyte cells that
periodically rupture and release thousands of pieces of membrane that become
the platelets. Platelets do not contain a nucleus and only survive in the body
for up to a week before macrophages capture and digest them.
Plasma:
Plasma is the
non-cellular or liquid portion of the blood that makes up about 55% of the
blood’s volume. Plasma is a mixture of water, proteins, and dissolved
substances. Around 90% of plasma is made of water, although the exact percentage
varies depending upon the hydration levels of the individual. The proteins
within plasma include antibodies and albumins. Antibodies are part of the
immune system and bind to antigens on the surface of pathogens that infect the
body. Albumins help maintain the body’s osmotic balance by providing an
isotonic solution for the cells of the body. Many different substances can be
found dissolved in the plasma, including glucose, oxygen, carbon dioxide,
electrolytes, nutrients, and cellular waste products. The plasma functions as a
transportation medium for these substances as they move throughout the body.
Functions of the Cardiovascular System
The cardiovascular system has three major functions:
transportation of materials, protection from pathogens, and regulation of the
body’s homeostasis.
Transportation:
The cardiovascular system transports blood to almost all of
the body’s tissues. The blood delivers essential nutrients and oxygen and
removes wastes and carbon dioxide to be processed or removed from the body.
Hormones are transported throughout the body via the blood’s liquid plasma.
Protection:
The cardiovascular system protects the body through its
white blood cells. White blood cells clean up cellular debris and fight
pathogens that have entered the body. Platelets and red blood cells form scabs
to seal wounds and prevent pathogens from entering the body and liquids from
leaking out. Blood also carries antibodies that provide specific immunity to
pathogens that the body has previously been exposed to or has been vaccinated
against.
Regulation:
The cardiovascular system is instrumental in the body’s
ability to maintain homeostatic control of several internal conditions. Blood
vessels help maintain a stable body temperature by controlling the blood flow
to the surface of the skin. Blood vessels near the skin’s surface open during
times of overheating to allow hot blood to dump its heat into the body’s
surroundings. In the case of hypothermia, these blood vessels constrict to keep
blood flowing only to vital organs in the body’s core. Blood also helps balance
the body’s pH due to the presence of bicarbonate ions, which act as a buffer
solution. Finally, the albumins in blood plasma help to balance the osmotic
concentration of the body’s cells by maintaining an isotonic environment.
The Circulatory Pump
The heart is a four-chambered “double pump,” where each side
(left and right) operates as a separate pump. The left and right sides of the
heart are separated by a muscular wall of tissue known as the septum of the
heart. The right side of the heart receives deoxygenated blood from the
systemic veins and pumps it to the lungs for oxygenation. The left side of the
heart receives oxygenated blood from the lungs and pumps it through the
systemic arteries to the tissues of the body. Each heartbeat results in the
simultaneous pumping of both sides of the heart, making the heart a very
efficient pump.
Regulation of Blood Pressure
Several functions of the cardiovascular system can control
blood pressure. Certain hormones along with autonomic nerve signals from the
brain affect the rate and strength of heart contractions. Greater contractile
force and heart rate lead to an increase in blood pressure. Blood vessels can
also affect blood pressure. Vasoconstriction decreases the diameter of an
artery by contracting the smooth muscle in the arterial wall. The sympathetic
(fight or flight) division of the autonomic nervous system causes
vasoconstriction, which leads to increases in blood pressure and decreases in
blood flow in the constricted region. Vasodilation is the expansion of an
artery as the smooth muscle in the arterial wall relaxes after the
fight-or-flight response wears off or under the effect of certain hormones or
chemicals in the blood. The volume of blood in the body also affects blood
pressure. A higher volume of blood in the body raises blood pressure by
increasing the amount of blood pumped by each heartbeat. Thicker, more viscous
blood from clotting disorders can also raise blood pressure.
Haemostasis
Haemostasis, or the clotting of blood and formation of
scabs, is managed by the platelets of the blood. Platelets normally remain
inactive in the blood until they reach damaged tissue or leak out of the blood
vessels through a wound. Once active, platelets change into a spiny ball shape
and become very sticky in order to latch on to damaged tissues. Platelets next
release chemical clotting factors and begin to produce the protein fibrin to
act as structure for the blood clot. Platelets also begin sticking together to
form a platelet plug. The platelet plug will serve as a temporary seal to keep
blood in the vessel and foreign material out of the vessel until the cells of
the blood vessel can repair the damage to the vessel wall.
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