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THE CIRCULATORY SYSTEM

Definition/Description: This is the composite body system which is responsible for the continuous

movement of all body fluids and their contents.

It is subdivided into:

Blood Vascular (Cardiovascular) system and Lymphatic Vascular system.

THE CARDIOVASCULAR SYSTEM

Functions: The functions of the cardiovascular systems include:

1. Continuous movement of body fluids

2. Oxygen transport to body tissues

3. CO2 transport from body tissues

4. Transport of metabolic wastes

5. Transport of Chemical Messengers (Hormones)

6. Transport of Drugs and other Biologically active substances/agents

7. Transport of Blood cells & Plasma Proteins

8. Thermoregulation

9. Electrolyte regulation

10. Acid-Base regulation

11. Body defense

12. Biosynthesis of biologically active substances (Endothelins (vasoconstrictor) and Nitric oxide

(vasodilator).

13. Conversion of Angiotensin I to Angiotensin II in the lung’s capillaries.

14. Inactivation of biologically active substances (Serotonin, nor-epinephrine, Thrombin,

Bradykinin, Prostaglandins).

13. Lipolysis of lipoproteins by enzymes on endothelial cell surface to triglycerides and cholesterol

Component Parts of the CVS (See Diagram 1)

The CVS is composed of a circuit of a hollow muscular organ (The heart) and vascular channels through

which blood flows to and from the body tissues. Its component parts are:

1. The heart

2. The arterial vascular system

3. The microcirculation

4. The capillary vascular system

5. The venous vascular system

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Diagram 1

Microstructure of the CVS (Diagram 2)

All the component parts of the CVS share a common basic microstructure which is found in their walls.

The wall is composed of three main layers, viz.

1. Tunica intima (Tunica internus) – Epithelial tissue

2. Tunica media (Tunica intermedus) – Muscle tissue

3. Tunica adventitia (Tunica externus) – Connective tissue

Structural as well as functional variations occur in the 3 layers from one part of the body to another on

the basis of functional requirements of various tissues of the body. The variation is further influenced by

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mechanical (Blood pressure) and metabolic (Local tissue needs) factors. The most variable layer is the

tunica media.

Diagram 2A

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Diagram 2B

THE HEART (Diagram 2b)

1. Tunica intima (Endocardium).

a. Endothelium of squamous epithelial cells. (Inner surface of the vessel on which blood flows)

b. Basement membrane of the endothelial cells.

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c. Delicate collagen tissue.

d. Robust Fibroelastic tissue.

e. Adipose and smooth muscle cells.

f. Vessels, nerves fibres, conducting tissue and ganglion cells of the autonomic nervous

system.

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NB: c, d, e and f are found in the sub-endothelial region, which is located between the basement

membrane of the endothelium and the tunica media

2. Tunica Media (Myocardium).

This is composed of several layers of cardiac muscle fibres which are attached to the

central fibrous cardiac skeleton.

3. Tunica Adventitia (Epicardium).

a. Serous mesothelium of squamous epithelial cells. (The outermost surface of the heart) –

[Visceral mesothelium]

b. Loose connective tissue (Beneath the mesothelium)

c. Thin fibroelastic tissue.

d. Nerves, Vessels, Adipose cells and ganglion cells of the autonomic nervous system. (Note the

absence of conducting tissue)

4. Fibrous Pericardium:

This is composed of:

a. Inner serous mesothelial layer of squamous epithelial cells – Parietal mesothelium

b. Outer thick and dense fibrous tissue.

5.Other Microscopic Structures in cardiac wall:

a. Heart valves: Heart valves are communication channels between heart chambers and are

guarded by cusps (Leaflets) composed of the following structures:

Endothelium on the atrial surface of the cusp

Lamina fibrosa of collagen and Elastic tissue in the middle layer of the cusp

Endothelium on the ventricular surface of the cusp

Fibrous annular ring into which the bases of the cusps are attached

b. Excitatory and conducting cells of the sinoarterial (SA) and atrioventricular node and

bundle of His

These cells are modified Cardiac muscle cells (They are not nerve cells/fibres) and exist in

three types:

1. Nodal cells

2. Internodal tract cells

3. Purkinje cells

The Arterial Vascular System (Diagram 3)

Functions:

a. Blood distribution to tissue capillaries

b. Control of blood pressure and rate of blood flow to the tissues.

Organization: There are three types of arteries viz.

1. Elastic artery.

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2. Muscular artery.

3. Arteriole.

Elastic Artery.

Elastic arteries are responsible for stabilization of blood flow in the vascular system. Examples of elastic

arteries are the aorta and its major branches. They appear yellowish in color due to large deposit of

elastic tissue in their tunica media.

Microstructure:

1. Tunica intima: This is composed of:

a. Endothelium of squamous cells

b. Basement membrane of endothelial cells.

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c. Mixed collagen and Elastic fibres.

d. Myointimal cells & Fibroblasts.

Internal Elastic Lamina.__________________________________

2. Tunica Media: This is composed of;

a. Fenestrated sheets of elastic tissue alternating with,

b. Smooth muscle sheets.

c. Collagen fibres (Type III)

d. Ground substance of proteoglycan and glycoprotein

3. Tunica Adventitia: This is not well developed but consists of:

a. Collagen (Type I) and elastic fibres.

b. Vasa vasorum

c. Nerve fibres.

d. Lymphatics.

Muscular Artery

Examples of muscular arteries are all the named arteries supplying areas/organs: (Axillary artery,

femoral artery, renal artery, Uterine artery, Superior mesenteric artery, maxillary artery) Muscular

arteries regulate blood flow to the tissues and organs.

Microstructure:

1. Tunica intima: This is composed of;

a. Endothelium of squamous cells

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b. Basement membrane of endothelial cells.

c. Collagen fibres.

d. Few smooth muscle cells

Internal Elastic Lamina.___________________________________________________________-

2. Tunica Media: This is composed of;

a. Several layers of smooth muscle cells (up to 40 layers) alternating with,

b. Elastic lamina, reticular fibres (Type III collagen) & proteoglycans.

External Elastic Lamina____________________________________________________________

3. Tunica Adventitia: This is composed of:

a. Collagen (Type I) & Elastic fibres.

b. Fibroblasts & Adipose cells.

c. Vasa Vasorum.

d. Nerve Fibres.

e. Lymphatics.

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Diagram 3

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LARGE ARTERIOLES (Diagram 3) Large arterioles are involved in local regulation of blood flow

Microstructure of Large Arterioles:

1. Tunica Intima: This is composed of:

a. Endothelium.

b. Basement membrane.

c. Collagen fibres.

Internal Elastic Lamina____________________________________________________

2. Tunica Media: This is composed of up to 6 layers of smooth muscle.

3. Tunica Adventitia: This is composed of:

a. Thick collagen fibres.

b. Vasa Vasorum.

c. Nerve fibres.

d. Lymphatics.

The Microcirculation (See Diagram 4)

The microcirculation is composed of a network of vessels at which the following events occur:

a. Exchange of substances between the blood vascular system and the tissues of the body

b. Control of tissue blood flow.

c. Implicated in inflammatory response of the body

The microcirculation is composed of the following structures in sequential order:

a. Metarteriole.

b. Precapillary sphincters.

c. Capillaries (Surrounded by pericytes)

d. Post-capillary venule (Surrounded by pericytes and has the loosest endothelial junction in the

system).

e. Collecting venule (Surrounded by pericytes)

f. Small muscular venule

Arteriovenous shunts are part of microcirculation but are not included in the six components

listed above. They stand out on their own.

Portal circulatory system is also a component of microcirculation which stands out on its

own. It exist in two types:

a. Venous portal system as in the GIT and Hypothalamus and

b. Arterial portal system as in the glomeruli of the kidney

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Diagram 4

The Capillary (Exchange Vessels)

The capillary is an endothelial-lined vessel which also has a basement membrane and tunica externus of

Pericytes.

Functions of the capillary:

a. Permits selective passage of molecules.

b. Serves as the exchange site between blood and tissues of the body

c. It metabolizes substrates in its endothelial cells, e.g.

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Conversion of Angiotensin I to angiotensin II in the lungs

Inactivation of substances such as, Serotonin, Thrombin, and Bradykinin nor-epinephrine

Lipolysis of lipoprotein to produce cholesterol and triglycerides

Production of Vasoactive factors/substances such as

1. Nitriic oxide- a vasodilator in skeletal muscles

2. Endothelin- a vasoconstrictor

d. Serves as an Antithrombogenic channel for blood

Classification of capillaries:

Four types of capillaries are recognized on the bases of the structural characteristic of the endothelium

lining the lumen of the capillary. The four types of endothelium are:

1. Continuous (Somatic) endothelium: (Found in: Muscles, Connective tissues, Nervous tissue

Exocrine glands).

2. Fenestrated endothelium with diaphragm (Visceral endothelium): (Found in: the kidney, small

intestine, endocrine glands).

3. Fenestrated endothelium without diaphragm (These are restricted to the kidney glomeruli).

4. Discontinuous endothelium (Sinusoidal Capillary). This is characterized by wide gaps between

endothelial cells which are also fenestrated and have no diaphragms. Sinusoids are found in:

The liver

Bone marrow

Spleen

Endocrine glands such as the anterior pituitary gland

The Venous Vascular System (Capacitance Vessels) Diagram 3

Functions:

a. Collection of blood from the capillary system

b. Return of blood to the heart for recirculation to the Pulmonary and Systemic circulatory systems.

Characteristic features of Veins:

a. Low pressure blood flow.

b. Valve assisted blood flow.

c. Skeletal muscle assisted blood flow.

d. Inspiration assisted blood flow.

e. Larger capacity than the arterial vascular system hence retains 70 of total blood volume.

Classification of Veins:

1. Muscular venule

2. Small, Medium and Large-sized Veins

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Muscular Venule:

The wall of a muscular venule is composed of:

Tunica Intima, consisting of:

Endothelium of squamous cells

Basement membrane

Collagen fibres

Tunica Media consisting of:

Smooth muscle fibres (Up to two layers)

Tunica Adventitia consisting of:

Connective tissue

Collagen fibres

Vasa vasorum

Lymphatics

Nerve fibres

Small, Medium and Large-sized Veins:

The wall of this group of veins is composed of:

Tunica Intima consisting of:

Endothelium of squamous cells

Basement Membrane

Admixture of reticular, collagen and elastic fibres

Internal Elastic lamina: This is often poorly developed

Tunica Media, consisting of:

Few layers of smooth muscle fibres (Arrange in circular fashion)

Reticular fibres

Few elastic fibres

Tunica Adventitia (Broadest Layer of veins) consisting of:

Collagen tissue with longitudinally arranged fibres

Smooth muscle fibres arranged longitudinally in large veins

Vas vasorum

Lymphatics

Nerve fibres

In the Vena Cavae, the smooth muscles fibres in the media are also arranged longitudinally.

Furthermore, the vena cavae and the pulmonary veins acquire cardiac muscle fibres in their tunica

adventitia as they approach the heart. The large veins also have valves in the lumen

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The Lymphatic Vascular System (Diagram 5)

The vessels of the lymphatic vascular system are blind-ended tubes arising from tissue spaces. The

functions of this vascular system are:

1. drainage of excess tissue fluid (Lymph)

2. Passive transport of lymph, lymphocytes and other immunologic factors/agents

3. Release of lymph into the blood vascular system

Characteristic features of the lymphatic system: This system is characterized by:

a. Unidirectional flow of fluid towards the heart

b. Valves and muscle assisted flow of fluid

c. Filtration of fluid through lymph nodes

d. Vessels formed from blind-ended lymph capillaries

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Microstructure of the Lymphatic System: This comprises:

Blind-ended, thin-walled, endothelial-lined capillaries

Non-fenestrated endothelium

Rudimentary basement membrane

No pericytes

Large vessels posses circular and longitudinal layers of smooth muscles

Fine elastic filaments (Anchoring fibres) attach endothelium to surrounding connective tissue

The large vessels contain vasa vasorum and nerve fibres

Lymphatic vessels are not found in the following areas/structures:

1. Central nervous system (CNS)

2. Cornea

3. Thymus gland

4. Teeth

5. Bone

6. Bone marrow

7. Cartilage

8. Placenta.

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Diagram 5A.

APPLIED ANATOMY OF THE CIRCULATORY SYSTEM

1. HAEMOPHILIA:

This is characterized by prolonged bleeding due to deficiency of factor VIII (von Willebrand's factor) in

endothelial cells of all blood vessels larger than the capillary. This protein is present in granules called

Weibel-Palade granules in the cytoplasm of the endothelial cells. Capillary endothelial cells do not contain

these granules.

2. ANEURYSM:

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This occurs in arteries following weakening of the tunica media, resulting in dilation and rupture of the

artery. A common cause of aneurysm is Ehlers-Danlos syndrome (Type IV) in which there is deficiency in

the synthesis of collagen Type III. Aneurysm could lead to spontaneous aortic rupture and sudden death.

3. TUMORS:

The commonly encountered tumors of the vascular system include:

a. Angiosarcoma: Malignant (Cancerous) endothelial cell tumor.

b. Haemangiopericytoma: Benign (Innocent) tumor of pericytes.

The presence of factor VIII is used to distinguish between a. and b. above. Furthermore,

Haemangiopericytoma does not affect large vessels since they lack pericytes.

4. ATHEROSCLEROSIS:

This is an abnormality in the arterial wall. It is characterized by:

a. Thickening of the intima from lipoprotein deposit in the subendothelial layer

b. Increased deposition of extracellular connective tissue elements in the subendothelium

c. Proliferation of connective tissue cells

d. Proliferation of muscle cells

e. Deposit of cholesterol in smooth muscles

f. Deposition of cholesterol in macrophages

g. Deposit of cholesterol in myointimal cells **

h. Ultimate occlusion of vascular lumen.

The coronary arteries are the most vulnerable to this lesion.

5. EDEMA:

This is escape of fluids from the circulatory system into tissue spaces and accumulation therein in

abnormal situations such as inflammation. This process occurs principally at the junctions between

endothelial cells of small venules, particularly the post-capillary venules which have the loosest

endothelial junctions.

6. DIAPEDESIS:

This is the escape of Leucocytes from the blood stream into tissue spaces through the junctions between

endothelial cells of capillaries and post-capillary venules. It is frequently associated with inflammatory

states.

7. PERICYTES IN TISSUE DEFENSE AND REPAIR:

Pericytes are elongated contractile cell found on the outer surface of the endothelium of capillaries and

post-capillary venules. They are involved in tissue defense during which they act as phagocytes. They also

proliferate to form new blood vessels (Angiogenesis) and are supportive of the endothelial cells. They are

also capable of forming mesenchymal and muscle cells.

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8. THROMBOEMBOLISM

Thromboembolism is the detachment of a solid mass of a thrombus within the vascular lumen and its

circulation in the vascular system. The circulating thrombus is referred to as an embolus which might get

impacted in a small vessel and completely occlude the vessel leading to tissue ischaemia, infarction and

ultimately necrosis.

9. AGING AND VASCULAR CONNECTIVE TISSUE

The aging process is characterized by:

Distortion of vascular connective tissue due to excess secretion of

1. Type I & III collagen and

2. Glucosaminoglycan

Molecular distortion of Glycoprotein and Elastin leading to

Excess secretion and deposition of Lipoprotein and Calcium ions.

These events result in calcification and hardening of the vessels due to formation of atherosclerotic

plaques.

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Diagram 5B

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Diagram 5C