General description
The circulatory system consists of the heart, which acts as a pump, and
the blood vessels, through which blood circulates. The primary function
of this system is to carry blood containing oxygen, nutrients and chemicals
to cells and to carry waste products away from cells for elimination.
Although the circulatory system is one complete system, three systems
are referred to when considering blood circulation:
Circulatory System
This chapter gives an introduction to the blood and the circulatory system, and
the main organs related to their function.
(i) The Pulmonary Circulation
which is the circulation of blood from the right ventricle of the heart to the lungs (where oxygen is absorbed and carbon dioxide is excreted) and back to the left atrium.
(ii) The Systemic Circulation
which is the circulation of blood from the left ventricle of the heart via the aorta
to all parts of the body and back to the right atrium via the venae cavae.
(iii) The Portal Circulation
(iii) The Portal Circulation, where, instead of returning directly to
the heart, venous blood passes from the capillary bed of the
abdominal part of the digestive system and the spleen, via the
capillary bed of the liver, where the distribution of nutrients to
the body may be controlled.
Blood circulates round the body in arteries and arterioles, veins and
venules, and the smallest blood vessels, capillaries.
The lymphatic system is also part of the circulatory system.
Arteries
Arteries carry blood from the heart. All arteries, with the exception
of the pulmonary artery, carry oxygenated blood. It is brighter than
deoxygenated blood which passes through veins (with the exception
of the pulmonary veins). Most of the major arteries are shown in the
diagram to the right. Arteries divide into smaller and smaller branches.
The smallest of these are called arterioles. The arterial walls comprise
three layers, the middle layer of which is extremely elastic.
Veins
Veins comprise the same three layers as arteries, but are thinner because there is
less muscle and elastic tissue. In certain veins, especially those of the lower limbs,
venous return is dependent on muscle contraction and tissue fluid pressure,
and backflow is prevented by a large number of valves.
Valves prevent backflow of blood in the veins
All veins carry blood to the heart, and all veins, except the pulmonary veins, carry
deoxygenated blood. Deoxygenated blood is darker than oxygenated blood.
Most of the major veins are shown in the following diagram. Venules are
the smallest veins. They join larger veins which empty into the largest
veins, called the venae cavae.
The Great Vessels
The major circulatory vessels connected to the heart are the aorta, venae cavae,
pulmonary artery and pulmonary veins.
All veins carry blood to the heart, and all veins, except the pulmonary veins, carry
deoxygenated blood. Deoxygenated blood is darker than oxygenated blood. Most
of the major veins are shown in the following diagram. Venules are the smallest
veins.They join larger veins which empty into the largest veins, called the venae cavae.
Aorta
The aorta is the main artery leaving the left side of the heart, via the ventricle.
It circulates oxygenated blood to other arteries in the circulatory system.
Venae cavae
The superior and inferior venae cavae are the two largest veins in the body. They feed
deoxygenated blood from the circulatory system into the right atrium.
Pulmonary artery
The pulmonary artery transports deoxygenated blood from the right
ventricle to the lungs.
Pulmonary veins
Pulmonary veins carry oxygenated blood from the lungs to the left atrium. The
pulmonary veins and pulmonary artery are part of the pulmonary circulation.
Circulatory System
Coronary Arteries
Blood Circulation
Heart
The heart is a muscular organ that pumps the blood around the body. The
pumping cycle (heartbeat) normally occurs between 60 and 80 times per
minute and is controlled by the pacemaker (sinoatrial node) heart.
Atrium
There is a left and right atrium (auricle). The atria (right and left) are
two upper chambers which receive blood into the heart.
Ventricles
The ventricles are the left and right lower chambers of the heart.
Septum
The septum, composed of heart muscle, separates the right and left
sides of the heart.
Aortic valve
The aortic valve stops blood flowing back into the heart. It lies at the point
where the aorta leaves the left ventricle.
Mitral valve
The mitral valve (left atrioventricular valve) allows oxygenated blood to pass from the left
atrium to the left ventricle and prevents backflow during systole. It has two cusps.
Tricuspid valve
The right atrioventricular valve is called the tricuspid valve. It allows blood to pass
from the right atrium to the right ventricle.
Pericardium
The heart is composed of three layers of tissue, the pericardium, the myocardium,
and the endocardium. The pericardium is the outermost layer. The two
membranous sacs of the pericardium surround the heart.
Myocardium
The myocardium is the middle layer of tissue. It is composed of cardiac muscle
which is found only in the heart.
Endocardium
The endocardium forms a lining to the myocardium. It is continuous
with the lining of the blood vessels.
Coronary arteries
The heart muscle receives oxygenated blood via the left and right main
coronary arteries.
Interior View of the Heart
Blood and Blood Cells
The blood consists of plasma, a transparent fluid mainly consisting
of water in which many important chemicals are dissolved, and
suspended red and white blood cells and platelets.
White cells (leucocytes)
The main function of leucocytes is to fight infection. There are two main varieties of
leucocytes: granular leucocytes (neutrophils, eosinophils and basophils) and
non-granular leucocytes (lymphocytes and monocytes).
Red cells (erythrocytes)
Erythrocytes carry oxygen from the lungs to cells in the body and carbon dioxide
from cells to the lungs for excretion.
Platelets (thrombocytes)
Platelets initiate the clotting mechanism when injury occurs.
Lymphatic System
General description
The lymphatic system consists of a network of channels throughout the body
(lymph vessels), lymph nodes and other lymphatic tissue, the spleen and the
thymus gland. Its major function is to supplement the action of veins by helping
to return tissue fluids to the blood via the subclavian veins.It also plays a defensive
role against infection by the filtering action of the lymph nodes. The lymphatic
system carries lymph which contains waste and other materials and additional
substances too large to pass through blood capillary walls. Lymph travels in
one direction only – towards the heart. Tissue fluids gathered from all over
the body by the lymphatic system are emptied into the blood stream via
the left and right subclavian veins. The lymph system has no pumping
mechanism, and the process of returning lymph closely resembles venous
return. Virtually all the organs of the body possess a lymphatic system.
Lymph nodes
Lymph nodes are collections of tissue, situated at various points along
the length of lymph vessels where lymph is filtered of microbes and
noxious substances as it passes through. As many as four or five afferent
lymph vessels may enter a lymph node, while only one efferent vessel
carries lymph away from the node.
Spleen
Blood (not lymph) passes through the spleen where blood cells and other cellular materials are broken down. The spleen also provides many of the lymphocytes in the blood.
Thymus gland
Situated in the upper thorax and neck is the thymus gland. It helps to fight infection by assisting in the maturation of lymphocytes.
Lymphatic System
Glossary of Terms—Blood and the Circulatory System
Aorta
(a-or’tah) Largest artery in the body; arises out of the left ventricle of
the heart and courses down the body trunk.
Aortic arch
Curved portion of the aorta which courses upward then turns
downward behind the heart to form an arch.
Aortic bifurcation
(a-or’tick bi-fur-ka’shun) Distal end of the abdominal aorta at which
point the artery divides into the two common iliac arteries.
Aortic valve
Fibrous tissue flaps or leaflets which open and close between the left ven- tricle and
ascending aorta. Valve closure prevents backflow (reflux) of blood into the heart.
Arteries
(ar’ter-es) Elastic, extensible vessels which carry blood in the direction
away from the heart.
Arteriole
(ar-te’re-ol) Small artery.
Atrium
(a’tre-um) One of the two upper chambers of the heart (right and left).
Bicuspid valve
(bi-kus’pid) Also called the mitral valve.
Blood capillary
(kap’i-lar-e) A microscopic vessel through which blood travels from arteriole
to venule. Oxygen/carbon dioxide exchange occurs across capillary walls.
Carotid arteries
(kah-rot’id) Vessels (common, external and internal) which provide the
major blood supply to the head and neck.
Circulatory
Pertaining to movement around a circular route.
Diastole
The resting stage of the cardiac cycle.
Heart
Hollow, muscular organ which pumps blood to all parts of the body.
Iliac artery, common
(il’e-ak) Vessel which carries major blood supply to each leg.
Innominate artery
This is also known as the subclavian artery (see overleaf).
Lymph
(limf) Substance continuously formed by filtration from tissue fluids.
Lymphatics
Lymph vessels.
Lymph nodes
Small bodies of lymphoid tissue arranged in chains to filter lymph and
help prevent the spread of infection.
Lymphocyte
(lim’fo-site) A particular type of white blood cell which is formed in the reticular
(net-like) tissue of lymph glands.
Lymph circulatory
A “one way” drainage system which picks up fluids from tissues and system returns them to the bloodstream.
Mitral valve
(mit’ral) The two-leaflet heart valve between the left atrium and of the heart
(left atrioventricular valve). Closure prevents reflux of blood into
the left atrium. The mitral valve is also called the bicuspid valve.
Oxygenated
(ok’si-je-na-ted) Saturated with oxygen.
Plasma
(plaz’mah) Fluid portion of blood in which cells are suspended.
Portal circulation
Instead of returning directly to the heart, venous blood passes from the capillary bed of the abdominal part of the digestive system and the spleen via the capillary bed of the liver.
Pulmonary artery
(pul’mo-ner-e) Vessel which carries venous blood from right ventricle of
the heart to lung for oxygenation.
Pulmonary circulation
The circulation of blood from the right ventricle of the heart to the
lungs and back to the left atrium.
Pulmonary vein
Vessel which returns oxygenated blood from the lung to the left atrium of the heart.
Red blood cells
Erythrocytes carry oxygen and carbon dioxide between the lungs and all body cells.
Spleen
A large lymphoid organ located in the left upper abdomen behind the stomach. The spleen plays a role in lymphocyte production and in the destruction and
possibly the storage of red blood cells.
Subclavian artery
(sub-kla’ve-an) Large vessel which branches off the aortic arch on the left and the
brachiocephalic trunk on the right to supply blood to areas of the trunk, head and
upper extremities. Also known as the innominate artery.
Systemic circulation
(sis-tem’ik) The circulation of blood from the left ventricle of the heart via the aorta
to all parts of the body and back to the right atrium via the venae cavae.
Systole
The contraction stage of the cardiac cycle.
Thoracic
(tho-ras’ik) Pertaining to the chest.
Thymus gland
(thi’mus) Located in the upper thorax and neck, the thymus helps to fight
infection by assisting in the maturation of lymphocytes.
Tricuspid valve
(tri-kus’pid) Valve with three fibrous tissue leaflets which open and close between the right atrium and ventricle of the heart (right atrioventricular valve). Closure prevents reflux of blood into the right atrium.
Vascular
(vas’ku-lar) Pertaining to vessels.
Vein
Vessel which carries blood in the direction toward the heart.
Venae cavae
(ve’ne ka’ve) Two large veins, inferior and superior, which return
venous blood to the right atrium of the heart.
Venous
(ve’nus) Pertaining to veins.
Ventricle
(ven’trikl) One of the two (right and left) lower chambers of the heart.
Venule
Small vein.
White blood cells
There are two main varieties of white blood cells: granular leucocytes (neu- trophils,
eosinophils and basophils) and non- granular leucocytes (lympho- cytes and monocytes).Their main function is to fight infection.
Exercises on the Circulatory System
A. Write in the table below the medical terms for the numbered items in the diagram.
Then check your answers against the fully labelled diagram in this section.
B. Write in the table below the medical terms for the numbered items in the diagram.
Then check your answers against the fully labelled diagram in this section.
C. Read the definitions of terms below. Each describes an organ, a part
or function of the circulatory system. You may refer to the previous text
to fill in the appropriate word or phrase.
Definition
- The fluid that circulates through the body carrying
nutrients and oxygen to the cells and carrying waste
products away from cells for elimination.
- The transparent fluid in which many important
chemicals are dissolved and in which are suspended
red and white blood cells and platelets.
- Red blood cells which carry oxygen from the lungs to
cells of the body.
- White blood cells which help to prevent infection
and diseases.
- Cells which assist the clotting process when injury occurs.
- Vessels that carry blood from the heart.
- Most arteries carry oxygenated blood,this is the exception.
- Arteries which supply oxygenated blood to the heart muscle.
- Vessels that transport blood to the heart.
- Most veins carry deoxygenated blood,these are the exceptions.
- The three systems referred to when considering blood
circulation.
- The circulation of blood from the right ventricle
of the heart to the lungs and back to the left atrium.
- The circulation of blood from the left ventricle of
the heart via the aorta to all parts of the body.
- Venous blood passes from the capillary bed of the
digestive system, via the capillary bed of the liver in this
circulation system.
- Two membranous sacs which surround the heart.
- The interior lining of the myocardium, which is
continuous with the lining of the blood vessels.
D. Use a medical dictionary to look up the meaning of these terms
which are commonly used in describing conditions related to the
circulatory system.
acrocyanosis
agranulocytosis
aneurysm
angiography
angioplasty
aortic incompetence
aortic stenosis
aplastic anaemia
arrhythmia
arteriosclerosis
atrial flutter
atrial septal defect
bacterial endocarditis
bradycardia
cardiac ischaemia
cardiomyopathy
cerebrovascular accident
claudication
coronary atherosclerosis
cor pulmonale
defibrillation
embolectomy
embolism
endarterectomy
essential hypertension
Fallot’s tetralogy
heart murmur
hypertension
hypotension
ischaemic heart disease
malignant hypertension
mitral stenosis
myocardial infarct
pacemaker
palpitation
paroxysmal tachycardia
pericarditis
phlebitis
portal hypertension
Raynaud’s disease
sclerosis
sphygmomanometer
stethoscope
subacute bacterial endocarditis
tachycardia
thrombocytopenia
thrombophlebitis
thrombosis
varicose veins
ventricular tachycardia
ventricular fibrillation