The circulatory system is divided into the heart and blood vessels. The heart pumps blood into the pulmonary and systemic circulations. The right side pumps blood to the lungs and the left side pumps oxygenated blood to the rest of the body. The heart has four chambers, valves to ensure one-way blood flow, and a conducting system to coordinate contractions. Arteries carry blood away from the heart while veins return blood to the heart.

1. UNIT 4
THE CIRCULATORY
SYSTEM

2. • The cardiovascular (cardio – heart, vascular –
blood vessels) system is divided for
descriptive purposes into two main parts:
the heart, whose pumping action ensures
constant circulation of the blood the blood
vessels, which form a lengthy network
through which the blood flows.
• The heart pumps blood into two
anatomically separate systems of blood
vessels : the pulmonary circulation and the
systemic circulation.

4. •The right side of the heart pumps blood to the
lungs (the pulmonary circulation) where gas
exchange occurs, i.e. the blood collects oxygen
from the airsacs and excess carbon dioxide
diffuses into the airsacs for exhalation.
•The left side of the heart pumps blood into the
systemic circulation, which supplies the rest of
the body. Here, tissue wastes are passed into
the blood for excretion, and body cells extract
nutrients and O2.

5. THE HEART
• The heart is a roughly cone-shaped hollow muscular
organ.
• It is about 10 cm long and is about the size of the
owner’s fist.
• It weighs about 225 g in women and is heavier in
men (about 310 g).
• Position The heart lies in the thoracic cavity in the
mediastinum (the space between the lungs).
• It lies obliquely, a little more to the left than the
right, and presents a base above, and an apex below.

6. •The apex is about 9 cm to the left of the
midline at the level of the 5th
intercostal space, i.e. a little below the
nipple and slightly nearer the midline.
•The base extends to the level of the 2nd
rib.

8. Organs associated with the heart
Inferiorly – the apex rests on the diaphragm.
Superiorly – the great blood vessels, i.e. the
aorta, superior vena cava, pulmonary artery and
pulmonary veins.
Posteriorly – the oesophagus, trachea, left and
right bronchus, descending aorta, inferior vena
cava and thoracic vertebrae.
Laterally – the lungs – the left lung overlaps the
left side of the heart.
Anteriorly – the sternum, ribs and intercostal
muscles.

10. Structure
•The heart wall is composed of three
layers of tissue : pericardium,
myocardium and endocardium.

11. Endocardium
•The innermost layer of the cardiac wall is
known as the endocardium. It lines the cavities
and valves of the heart.
•Structurally, the endocardium is comprised of
loose connective tissue and simple squamous
epithelial tissue – it is similar in its composition
to the endothelium which lines the inside of
blood vessels.
•In addition to lining the inside of the heart, the
endocardium also regulates contractions.

12. Myocardium
•The myocardium is composed of
cardiac muscle and is an involuntary
striated muscle. The myocardium is
responsible for contractions of the
heart.

13. Pericardium
•The epicardium is the outermost layer of the
heart, formed by the visceral layer of the
pericardium. It is composed of connective
tissue and fat. The connective tissue secretes a
small amount of lubricating fluid into the
pericardial cavity.
•In addition to the connective tissue and fat,
the epicardium is lined by on its outer surface
by simple squamous epithelial cells.

14. Interior of the heart
Atria : These are the thin-walled receiving
chambers of heart.
They are separated from each other by inter-
auricular septum.
Interauricular septum has an oval depression
called fossa ovalis.
It is a remnant of the embryonic aperture called
foramen ovalis.

16. Superior vena cava, inferior vena cava and
coronary sinus open into the right atrium.
Opening of the postcaval is guarded by a
Eustachian valve while the Thebesian valve
guards the opening of coronary sinus into
right atrium.
Four pulmonary veins open into the left
atrium.
These openings are without valves.

17. Both the atria open into the ventricles of their
respective sides by atrioventricular apertures.
These openings are guarded by cuspid valves.
The tricuspid valve is present in the right AV
aperture and bicuspid valve (mitral valve) is
present in the left AV aperture.
All these heart valves help in maintaining a
unidirectional flow of blood.
They also avoid back flow of blood.

18. Ventricles : These are inferior, thick-walled
pumping chambers of the heart.
The right and left ventricles are separated by
an interventricular septum.
Wall of the left ventricle is more muscular and
about 3-times thicker than the right ventricle.
Inner surface of the ventricles shows several
ridges called columnae carnae or trabeculae
carnae which divide the lumen of ventricle
into small pockets or fissures.

19. •The lumen of ventricles also shows inelastic
fibers called chordae tendinae. These attach
the bicuspid and tricuspid valves to the
ventricular wall (papillary muscles) and
regulate their opening and closing.
•The right ventricle opens into the pulmonary
aorta and left ventricle opens into the aorta.
These openings are guarded by three
semilunar valves each. These valves prevent
the backward flow of blood into the ventricles.

20. Flow of blood through heart

21. •The two largest veins of the body, the
superior and inferior venae cavae, empty
their contents into the right atrium.
•This blood passes via the right
atrioventricular valve into the right
ventricle, and from there is pumped into
the pulmonary artery or trunk (the only
artery in the body which carries
deoxygenated blood).
•The opening of the pulmonary artery is
guarded by the pulmonary valve, formed
by three semilunar cusps.

22. •This valve prevents the backflow of blood
into the right ventricle when the
ventricular muscle relaxes. After leaving
the heart the pulmonary artery divides
into left and right pulmonary arteries,
which carry the venous blood to the lungs
where exchange of gases takes place:
carbon dioxide is excreted and oxygen is
absorbed.

23. •Two pulmonary veins from each lung
carry oxygenated blood back to the left
atrium.
•Blood then passes through the left
atrioventricular valve into the left
ventricle, and from there it is pumped
into the aorta, the first artery of the
general circulation.
•The opening of the aorta is guarded by
the aortic valve, formed by three
semilunar cusps.

25. •From this sequence of events it can be
seen that the blood passes from the
right to the left side of the heart via
the lungs, or pulmonary circulation.
•However, it should be noted that both
atria contract at the same time and
this is followed by the simultaneous
contraction of both ventricles.

26. Blood supply to the heart
(the coronary circulation)
•Arterial supply The heart is supplied with
arterial blood by the right and left coronary
arteries, which branch from the aorta
immediately distal to the aortic valve.
•The coronary arteries receive about 5% of the
blood pumped from the heart, although the
heart comprises a small proportion of body
weight.
•This large blood supply, especially to the left
ventricle.

28. •The coronary arteries traverse the
heart, eventually forming a vast
network of capillaries.
•Venous drainage Most of the venous
blood is collected into a number of
cardiac veins that join to form the
coronary sinus, which opens into the
right atrium. The remainder passes
directly into the heart chambers
through little venous channels.

29. Conducting system of the
heart
•The heart possesses the property of
autorhythmicity, which means it
generates its own electrical impulses and
beats independently of nervous or
hormonal control, i.e. it is not reliant on
external mechanisms to initiate each
heartbeat.

30. •However, it is supplied with both
sympathetic and parasympathetic
autonomic nerve fibres, which increase
and decrease respectively the intrinsic
heart rate.
•In addition, the heart responds to a
number of circulating hormones, including
adrenaline (epinephrine) and thyroxine.

31. •Small groups of specialised neuromuscular
cells in the myocardium initiate and conduct
impulses, causing coordinated and
synchronised contraction of the heart muscle
•This pathway is made up of 5 elements:
•The sino-atrial (SA) node
•The atrio-ventricular (AV) node
•The bundle of His
•The left and right bundle branches
•The Purkinje fibres

33. •Sinoatrial node (SA node)
• This small mass of specialised cells lies in
the wall of the right atrium near the
opening of the superior vena cava. The
sinoatrial cells generate these regular
impulses because they are electrically
unstable.
•This instability leads them to discharge
(depolarise) regularly, usually between 60
and 80 times a minute.

34. •This depolarisation is followed by
recovery (repolarisation), but almost
immediately their instability leads them
to discharge again, setting the heart rate.
•Because the SA node discharges faster
than any other part of the heart, it
normally sets the heart rate and is called
the pacemaker of the heart.
•Firing of the SA node triggers atrial
contraction.

35. •Atrioventricular node (AV node) This
small mass of neuromuscular tissue is
situated in the wall of the atrial septum
near the atrioventricular valves.
•Normally, the AV node merely transmits
the electrical signals from the atria into
the ventricles.
•There is a delay here; the electrical signal
takes 0.1 of a second to pass through into
the ventricles.

36. •This allows the atria to finish contracting
before the ventricles start.
•The AV node also has a secondary
pacemaker function and takes over this
role if there is a problem with the SA
node itself, or with the transmission of
impulses from the atria.
•Its intrinsic firing rate, however, is slower
than that set by the SA node (40–60
bpm).

37. •Atrioventricular bundle (AV bundle or
bundle of His) This is a mass of
specialised fibres that originate from the
AV node.
•The AV bundle crosses the fibrous ring
that separates atria and ventricles then, at
the upper end of the ventricular septum,
it divides into right and left bundle
branches.
•Within the ventricular myocardium the
branches break up into fine fibres, called

38. •The AV bundle, bundle branches
and Purkinje fibres transmit
electrical impulses from the AV
node to the apex of the
myocardium where the wave of
ventricular contraction begins, then
sweeps upwards and outwards,
pumping blood into the pulmonary
artery and the aorta.

39. Nerve supply to the heart
•The heart is influenced by autonomic
(sympathetic and parasympathetic) nerves
originating in the cardiovascular centre in the
medulla oblongata.
•The vagus nerves (parasympathetic) supply
mainly the SA and AV nodes and atrial muscle.
•The sympathetic nerves supply the SA and AV
nodes and the myocardium of atria and
ventricles.

40. The cardiac cycle
• At rest, the healthy adult heart is
likely to beat at a rate of 60–80 bpm.
During each heartbeat, or cardiac
cycle, the heart contracts and then
relaxes. The period of contraction is
called systole and that of relaxation,
diastole.

42. •Stages of the cardiac cycle
•Taking 74 bpm as an example, each cycle
lasts about 0.8 of a second and consists of
•1) atrial systole – contraction of the atria
• 2)ventricular systole – contraction of the
ventricles
•3)complete cardiac diastole – relaxation of
the atria and ventricles.

43. •The superior vena cava and the inferior vena
cava transport deoxygenated blood into the
right atrium at the same time as the four
pulmonary veins bring oxygenated blood into
the left atrium. The atrioventricular valves are
open and blood flows passively through to the
ventricles. The SA node triggers a wave of
contraction that spreads over the myocardium
of both atria, emptying the atria and
completing ventricular filling (atrial systole 0.1
s).

44. • After this brief delay, the AV node
triggers its own electrical impulse, which
quickly spreads to the ventricular muscle
via the AV bundle, the bundle branches
and Purkinje fibres. This results in a wave
of contraction which sweeps upwards
from the apex of the heart and across the
walls of both ventricles pumping the
blood into the pulmonary artery and the
aorta (ventricular systole 0.3 s).

45. •The high pressure generated during ventricular
contraction is greater than that in the aorta
and forces the atrioventricular valves to close,
preventing backflow of blood into the atria.
•After contraction of the ventricles there is
complete cardiac diastole, a period of 0.4
seconds, when atria and ventricles are relaxed.
During this time the myocardium recovers in
preparation for the next heartbeat, and the
atria refill in preparation for the next cycle.

46. Heart sounds
• There are two heart sounds, each corresponding to a
particular event in the cardiac cycle. The sound
through the stethoscope like “lub dup”. The first
sound, ‘lub’, is fairly loud and is due to the closure of
the atrioventricular valves. This corresponds with
the start of ventricular systole. The second sound,
‘dup’, is softer and is due to the closure of the aortic
and pulmonary valves. This corresponds with
ventricular diastole.

48. BLOOD PRESSURE
•Blood pressure is the force or pressure
that the blood exerts on the walls of the
blood vessels.
•Systemic arterial blood pressure
maintains the essential flow of blood into
and out of the organs of the body.

49. •The arterial blood pressure, is the result of the
discharge of blood from the left ventricle into
the already full aorta.
•Blood pressure varies according to the time of
day, the posture, gender and age of the
individual. Blood pressure falls at rest and
during sleep.

50. sphygmomanometer

51. •Systolic and diastolic pressure When the left
ventricle contracts and pushes blood into the
aorta, the pressure produced within the
arterial system is called the systolic blood
pressure.
• In adults it is about 120 mmHg.
•When complete cardiac diastole occurs and
the heart is resting following the ejection of
blood, the pressure within the arteries is much
lower and is called diastolic blood pressure.
• In an adult this is about 80 mmHg.

52. •The difference between systolic and
diastolic blood pressures is the pulse
pressure.
•Arterial blood pressure is measured with a
sphygmomanometer and is usually
expressed with the systolic pressure
written above the diastolic pressure:
•The normal blood pressure- 120/80
mmHg.

53. PULSE
• Pulse is felt by slightly compressing an artery
against an underlying bone or flat muscle by the
three middle fingers.
• It is felt in the radial, brachial, carotids, femoral
and dorsalis pedis arteries.
• The subject should be seated comfortably or
lying supine.
• The radial pulse is best felt with the subject's
forearm in mid prone position and wrist slightly
flexed.
• The following observations are made:

54. • Rate: The rate is expressed as beats per minute. The
counting is started when the subjects pulse has
settled down after an initial quickened due to
nervousness. The beats should be counted for at
least one minute. The normal resting heart rate is 72
± 10 beats/min.
• Causes of increase in pulse rate: Exercise,
anxiety,excitement, fever, thyrotoxicosis, sleep.
• Causes of decrease in pulse rate: Regular athletic
training, myxedema and sleep.
• Rhythm: Normally the pulse beats at regular
intervals and therefore is regular. An irregular pulse
is seen in disease

55. Factors affecting pulse rate are as
follows:
• Age: ↓with age
• Fever: ⬆ with fever
• Gender: ⬆ after puberty male slightly <than female
• Medications: E.g. ⬆ with epinephrine and with digitalis
• Exercise: ⬆ with exercise
• Hypovolemia: ⬆ with blood loss
• Position changes: Sitting or standing→ pooling of blood in
dependent vessels →↓ BP and ⬆ HR
• Stress: ⬆ with stress

56. COMMON
PULSE
SITES

57. BLOOD VESSELS
•Blood vessels vary in structure, size and
function, and there are several types: arteries,
arterioles, capillaries, venules and veins.

58. Arteries and arterioles
•These are the blood vessels that transport
blood away from the heart. They vary
considerably in size and their walls consist of
three layers of tissue
•1)tunica adventitia or outer layer of fibrous
tissue
•2) tunica media or middle layer of smooth
muscle and elastic tissue
•3)tunica intima or inner lining of squamous
epithelium called endothelium.

59. •The amount of muscular and elastic
tissue varies in the arteries depending
upon their size and function.
• In the large arteries, sometimes called
elastic arteries, the tunica media consists
of more elastic tissue and less smooth
muscle.
•This allows the vessel wall to stretch,
absorbing the pressure wave generated
by the heart as it beats.

60. •These proportions gradually change
as the arteries branch many times
and become smaller until in the
arterioles (the smallest arteries) the
tunica media consists almost
entirely of smooth muscle.

62. Capillaries
•The smallest arterioles break up into a
number of minute vessels called
capillaries.
•Capillary walls consist of a single layer of
endothelial cells sitting on a very thin
basement membrane, through which
water and other small molecules can pass.

63. •Blood cells and large molecules
such as plasma proteins do not
normally pass through capillary
walls.
•The capillaries form a vast network
of tiny vessels that link the smallest
arterioles to the smallest venules.

64. Veins and venules
•Veins are blood vessels that return blood at
low pressure to the heart.
•The walls of the veins are thinner than
those of arteries but have the same three
layers of tissue.
• They are thinner because there is less
muscle and elastic tissue in the tunica
media, because veins carry blood at a
lower pressure than arteries.

65. •veins possess valves, which prevent
backflow of blood, ensuring that it
flows towards the heart.
•They are formed by a fold of tunica
intima and strengthened by
connective tissue.
• The cusps are semilunar in shape
with the concavity towards the heart.

66. •Valves are abundant in the veins
of the limbs, especially the lower
limbs where blood must travel a
considerable distance against
gravity when the individual is
standing.
•The smallest veins are called
venules.

68. •Thank you....