2. Two systems in One
Most of the cells in the human body are not in
direct contact with the external environment, so
they rely on the circulatory system to act as a
transport service for them. Two fluids move
through the circulatory system: blood and
lymph. The blood, heart, and blood vessels form
the Cardiovascular System. The lymph, lymph
nodes and lymph vessels form the Lymphatic
System. The Cardiovascular System and the
Lymphatic System collectively make up the
Circulatory System.
4. The Cardiovascular System
The cardiovascular system serves a number of
important functions in the body. Most of these
support other physiological systems. The major
cardiovascular functions fall into five categories:
1) Delivery
2) Removal
3) Transport
4) Maintenance
5) Prevention
6. -
1) The cardiovascular system delivers oxygen and
nutrients to, 2) and removes carbon dioxide and
metabolic waste products from, every cell in the
body. 3) It transports hormones from endocrine
glands to their target receptors. 4) The system
maintains body temperature, and the blood’s
buffering capabilities help control the body’s pH.
The cardiovascular system maintains appropriate
fluid levels to prevent dehydration and helps 5)
prevent infection by invading organisms.
8. Structure and Function of the
Cardiovascular System
The cardiovascular system responds immediately to
the body’s many and everchanging needs. All
bodily functions and virtually every cell in our
body depend in some way on this system.
Any system of circulation requires three
components:
1) A pump (the heart)
2) A system of channels (the blood vessels)
3) A fluid medium (the blood)
9. The Heart
The heart is a reddish-colored hollow organ that
lies in the thoracic cavity between the lungs and
behind the sternum, supported by the muscles of
the diaphragm. It is conical in shape, with the tip
pointing downwards and to the left. Inside the
heart there are four chambers: two atria in the
upper part, separated by the interatrial wall, and
the two ventricles in the lower part, separated by
the interventricular wall. The right chambers, the
right atrium and the right ventricle communicate
with one another through the tricuspid valve, and
the left chambers through the mitral valve.
11. -
The walls of the heart have three layers: the
endocardium, a very thin internal membrane; the
myocardium, an intermediate layer of striated
muscular tissue, which is thick in the ventricular wall
areas and thin in the atrial walls; and the
pericardium, a membrane that envelops the heart.
The heart has two atria acting as receiving chambers
and two ventricles acting as sending units. The heart is
the primary pump that circulates blood through the
entire vascular system.
13. Blood Flow Through the Heart
Blood that has coursed its way between the cells of the
body, delivering oxygen and nutrients and picking up waste
products, returns through the great veins – the superior
vena cava and inferior vena cava – to the right atrium. This
chamber receives all the body’s deoxygenated blood.
From the right atrium, blood passes through the tricuspid
valve into the right ventricle. This chamber pumps the
blood through the pulmonary semilunar valve into the
pulmonary artery, which carries the blood to the right and
left lungs. Thus the right side of the heart is known as the
pulmonary side, sending the blood that has circulated
throughout the body into the lungs for reoxygenation.
15. -
After receiving a fresh supply of oxygen, the blood exits
the lungs through the pulmonary veins, which carry it
back to the heart and into the left atrium. All freshly
oxygenated blood is received by this chamber. From
the left atrium, the blood passes through the bicuspid
(mitral) valve into the left ventricle. Blood leaves the
left ventricle by passing through the aortic semilunar
valve into the aorta, which ultimately sends it out to all
body parts and systems. The left side of the heart is
known as the systemic side. It receives the
reoxygenated blood from the lungs then sends it out to
supply all body tissues.
17. The Myocardium
Cardiac muscle is collectively called the
myocardium. Myocardial thickness varies directly
with the stress placed on the heart chambers’
walls. The left ventricle is the most powerful of
the four heart chambers. Through its
contractions, the chamber must pump blood
through the entire system route.
The left ventricle’s tremendous power is reflected
by the greater size of its muscular wall compared
to the other heart chambers
18. -
Although striated in appearance, the myocardium
differs from skeletal muscle in one important way.
Cardiac muscle fibers are anatomically
interconnected end-to-end by dark staining
regions called intercalated disks. These disks have
desmosomes, which are structures that anchor
the individual cells together so they don’t pull
apart during contraction, and gap
junctions, which allow rapid transmission of the
impulse signaling contraction. These features
allow the myocardium in all four chambers to act
as one large muscle fiber: All fibers contract
together.
20. The Cardiac Conduction System
Cardiac muscle has the unique ability to generate its
own electrical signal, called autoconduction, that
allows it to contract rhythmically without neural
stimulation. With neither neural nor hormonal
stimulation, the intrinsic heart rate averages 70 to 80
beats (contractions) per minute.
There are four components of the cardiac conduction
system:
1) Sinoatrial (SA) node
2) Atrioventricular (AV) node
3) Atrioventricular (AV) bundle (bundle of His)
4) Purkinje fibers
22. -
The impulse for heart contraction is initiated in the
sinoatrial (SA) node, a group of specialized cardiac
muscle fibers located in the posterior wall of the right
atrium. Because this tissue generates the
impulse, typically at the frequency of about 60 to 80
beats per minute, the SA node is known as the heart’s
pacemaker, and the beating rate it establishes is called
the sinus rhythm. The electrical impulse generated by
the SA node spreads through both atria and reaches
the atrioventriular (AV) node. Located in the right atrial
wall near the center of the heart. As the impulse
spreads through the atria, they are signaled to
contract, which they do almost immediately.
24. -
The AV node conducts the impulse from the
atria into the ventricles. The impulse is
delayed by about 0.13 s as it passes through
the AV node, then it enters the AV bundle.
This delay allows the atria to fully contract
before the ventricles do, maximizing
ventricular filling. The AV bundle travels along
the ventricular septum and then sends right
and left bundle branches into both ventricles.
25. -
These branches send the impulse toward the apex
of the heart, then outward. Each bundle branch
subdivides into many smaller ones that spread
throughout the entire ventricular wall. These
terminal branches of the AV bundle are the
Purkinje fibers. They transmit the impulse
through the ventricles approximately six times
faster than through the rest of the cardiac
conduction system. This rapid conduction allows
all parts of the ventricles to contract at about the
same time.
26. Extrinsic Control of Heart Activity
Although the heart initiates its own electrical
impulses (intrinsic control), their timing and
effects can be altered. Under normal
conditions, this is accomplished primarily
through three extrinsic systems:
1) The parasympathetic nervous system
2) The sympathetic nervous system
3) The endocrine system (hormones)
27.
28. -
The parasympathetic system, a branch of the
autonomic nervous system, acts on the heart
through the vagus nerve (cranial nerve X). At
rest, parasympathetic system activity
predominates in a state referred to as vagal tone.
The vagus nerve has a depressant effect on the
heart – it slows impulse conduction and thus
decreases the heart rate. It also decreases the
force of cardiac contraction.
29. -
The sympathetic nervous system, the other
branch of the autonomic nervous system, has
opposite effects. Sympathetic stimulation
increases impulse conduction speed and thus
heart rate (up to 250 beats per minute).
Sympathetic input also increases the
contraction force. The sympathetic nervous
system predominates during times of physical
and emotional stress, when the body’s
demands are higher.
30. -
The endocrine system exerts its effects
through the hormones released by the
adrenal medulla: norepinephrine and
epinephrine. Like the sympathetic nervous
system, these hormones stimulate the
heart, increasing its rate. In fact, release of
these hormones is triggered by sympathetic
stimulation during times of stress and their
actions prolong the sympathetic response.
31. The Cardiac Cycle
The cardiac cycle consists of all heart
chambers undergoing a relaxation phase
(diastole) and a contraction phase (systole).
During diastole, the chambers fill with blood.
During systole, the chambers contract and
expel their contents. The diastolic phase is
longer than the systolic phase. The pressure
that blood exerts on the arterial walls during
the two phases is called diastolic and systolic
blood pressure.
35. The Vascular System
The vascular system is composed of a series of
vessels that transport blood from the heart to
the tissues and back:
• Arteries
• Arterioles
• Capillaries
• Venules
• Veins
37. The Arterial System
The arterial system consists of a network of
blood vessels called arteries, which start from
the heart and extend throughout the
body, carrying by means of the arterial blood
the oxygen that is essential for the cells to
function. The further these vessels are from
the heart, the narrower they become. Arteries
are the largest, most muscular, and most
elastic vessels, and they always carry blood
away from the heart to the arterioles.
42. -
The largest and most important artery in the
body is the aorta, which leaves the left
ventricle upwards, and describes a curve
known as the aortic arch, after which it starts
to descend. From the aortic arch, arteries
branch off .
44. Pulmonary Arterial System
The pulmonary artery leaves the right
ventricle and then divides into two
branches, the right and left pulmonary
arteries, which enter the lungs and spread out
to form a similar structure to the bronchial
tree, ending as alveolar capillaries
46. The Venous System
The venous system consists of a network of blood
vessels that approximately parallels the structure
of the arterial network but runs in the opposite
direction. Veins start as venules, getting larger on
their way back to the heart. They collect the
deoxygenated blood, loaded with waste
substances (venous blood), to the right-hand
chambers of the heart, where it is passed on to
the pulmonary vessel to be oxygenated and
converted into arterial blood.
49. -
The walls of the veins are less elastic and
muscular than those of the arteries, because
the blood circulating through them is being
drawn in by the suction effect of the heart.
Inside the veins, any backflow of blood is
prevented by a system of valves.
50. -
The two major veins of the body are the
superior and inferior venae cavae, two large
intrathoracic veins; the former receives the
venous circulation from the upper
extremities, the head and the neck, and the
latter receives the blood from the lower
extremities and the abdominal and thoracic
cavities
52. Factors that Help Venous Blood return
to the Heart
• By the time blood has passed from the capillaries into
the venous system the pressure has dropped
significantly. The average blood pressure in the venous
system is only 2 mmHg (millimeters of mercury) as
compared to an average of 100 mmHg in the arterial
system. The low venous pressure is barely adequate to
drive blood back to the heart, particularly from the
legs. Other mechanisms are needed to aid in the return
of blood to the heart. The flow of venous blood back to
the heart is increased by (1) the sympathetic nervous
system, (2) the skeletal muscle pump, and (3) the
respiratory pump.
53. -
• Veins are enervated by sympathetic motor
neurons. Sympathetic input causes
vasoconstriction, which increases
pressure, which drives blood back to the
heart. When the body needs to mobilize more
blood for physical activity, the sympathetic
nervous system induces vasoconstriction of
veins.
54. -
• The action of the skeletal muscle pump. Veins
pass between skeletal muscles. The contraction
of skeletal muscle squeezes the vein, thus
increasing blood pressure in that section of the
vein. Pressure causes the upstream valve
(furthest from the heart) to close and the
downstream valve (the one closest to the heart)
to open. Repeated cycles of contraction and
relaxation, as occurs in the leg muscles while
walking, effectively pumps blood back to the
heart.
55. -
• While the contraction of skeletal muscle in the
legs drives venous blood out of the lower
limbs, the act of breathing helps to drive venous
blood out of the abdominal cavity. As air is
inspired, the diaphragm descends and abdominal
pressure increases. The increasing pressure
squeezes veins and moves blood back toward the
heart. The rhythmic movement of venous blood
caused by the act of breathing is called the
respiratory pump. Gravity helps the backflow of
blood from the areas above the heart
(head, neck, shoulders…)
56. Capillaries
• Capillaries are the smallest and most numerous of
blood vessels. Capillaries function as the site of
exchange of nutrients and wastes between blood and
tissues. The anatomy of capillaries is well suited to the
task of efficient exchange. Capillary walls are
composed of a single layer of epithelial cells
surrounded by a basement layer of connective tissue.
The thin nature of the walls facilitates efficient
diffusion of oxygen and carbon dioxide. Most
capillaries also have pores between cells that allow for
bulk transport of fluid and dissolved substances from
the blood into the tissues and vice versa.
58. -
• Although capillaries are extremely numerous (40
billion in the body), collectively they hold only
about 5% of the total blood volume at any one
time. This is because most capillaries are closed
most of the time. Precapillary sphincters, which
are bands of smooth muscle that wrap around
arterioles, control the amount of blood flowing in
a particular capillary bed. Contraction of the
sphincter shuts off blood flow to a capillary
bed, while relaxation of the sphincter allows
blood to flow.
59. The Blood
• The average adult has about five liters of blood coursing through
blood vessels, delivering essential elements, and removing harmful
wastes. Without blood, the human body would stop working.
• Blood is the fluid of life, transporting oxygen from the lungs to body
tissue and carbon dioxide from body tissue to the lungs. Blood is
the fluid of growth, transporting nourishment from digestion and
hormones from glands throughout the body. Blood is the fluid of
health, transporting disease fighting substances to the tissue and
waste to the kidneys.
• Blood contains red blood cells (erythrocytes) and white blood cells
(leukocytes), which are responsible for nourishing and cleansing the
body. Since the cells are alive, they too need nourishment.
62. -
• Vitamins and Minerals keep the blood healthy. The blood cells have
a definite life cycle, just as all living organisms do. Approximately 55
percent of blood is plasma, a straw-colored clear liquid. The liquid
plasma carries the solid cells and the platelets
( thrombocytes), which help blood clot. Without blood
platelets, we would bleed to death.
• When the human body loses a little bit of blood through a minor
wound, the platelets cause the blood to clot so that the bleeding
stops. Because new blood is always being made inside of our
bones, the body can replace the lost blood. When the human body
loses a lot of blood through a major wound, that blood has to be
replaced through a blood transfusion from other people.
• But everybody's blood is not the same. There are four different
blood types.
63. The Lymphatic System
• The lymphatic system consists of
organs, ducts, and nodes. It transports a watery
clear fluid called lymph.
• This fluid distributes immune cells and other
factors throughout the body. It also interacts with
the blood circulatory system to drain fluid from
cells and tissues.
• The lymphatic system contains immune cells
called lymphocytes, which protect the body
against antigens (viruses, bacteria, etc.) that
invade the body.
66. Functions
Main functions of the lymphatic system:
• "to collect and return interstitial
fluid, including plasma protein to the blood,
and thus help maintain fluid balance,
• to defend the body against disease by
producing lymphocytes,
• to absorb lipids from the intestine and
transport them to the blood."
