THE CIRCULATORY SYSTEM 

DEFINITION/DESCRIPTION 1. This is the composite body system that is responsible for the continuous movement of all body fluids and their contents. 1. It is subdivided into:

Blood vascular (Cardiovascular) system and Lymphatic vascular system.

The Cardiovascular SystemFunctions:  The functions of the CVS include:1.Continuous movement of body fluids2.Oxygen transport to body tissues3.CO2 transport from body tissues4.Transport of metabolic wastes5.Transport of hormones•Transport of drugs and other biologically active substances7. Transport of blood cells8. Thermoregulation9. Electrolyte regulation10. Body defense11. Biosynthesis of biologically active substances12. Inactivation of biologically active substances

Component Parts of the CVSThe 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.

It is composed of:1.The heart2.The arterial vascular system3.The capillary vascular system4.The venous vascular system

Microscopic Anatomy of the CVSAll 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): This is the mucous membrane which lines the cavities of the heart and the lumen of the vessels.2. Tunica media (Tunica intermedus): This is composed of striated and smooth muscle fibres which contract to propel blood through the heart and blood vessels.3. Tunica adventitia (Tunica externus): This is the fibrous connective tissue layer which supports and protects the heart and vessels.

•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 most variable layer is the tunica media.

Microscopic Anatomy of the HeartThe cardiac wall is composed of:1.Tunica intima (Endocardium): This is lined by squamous cell and contains blood vessels, nerve fibres and conducting tissue in its connective tissue2. Tunica Media (Myocardium): This is composed of several layers of cardiac muscle fibres (Striated muscle).3. Tunica Adventitia (Epicardium): This is lined by serous squamous cells and contains blood vessels, nerve fibres and adipocytes in its connective tissueFurthermore, the heart is enclosed in a coat of fibrous tissue which is lined by serous squamous cells (Fibrous and Serous Pericardium)

The Arterial Vascular SystemFunctions:a. Blood distribution to tissue capillariesb. Control of Pressure and rate of blood flow to the tissues.Organization: There are three types of arteries;•Elastic arteries: These include the aorta and its main branches2. Muscular arteries: These include the named arteries supplying regions and organs of the body3. Arterioles: These are vessels found in the walls and substances of organs of the body

The MicrocirculationThe microcirculation consists of a network of vessels at which:

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

2.Blood flow into tissues is controlled.The microcirculation is composed of:Metarterioles.Precapillary sphincters.CapillariesPost-capillary venuleCollecting venule1.Small muscular venule2.Arteriovenous shunts

The Capillary (Exchange Vessels)Vascular system The capillary is an endothelial-lined vessel which also has a basement membrane.Functions:a. Permits selective passage of molecules.b.Serves as the exchange site between blood and tissues of the body.c. It metabolizes substrates through the endothelial cells.d. Serves as an Antithrombogenic channel for blood

Classification of capillaries:There are four types of capillaries viz.1.Continuous (Somatic) capillaries (Pharynx, Brain and Muscles)2. Fenestrated capillaries with diaphragm (Stomach and Endocrine glands)3. Fenestrated capillaries without diaphragm (Kidney glomeruli)•Discontinuous capillaries (Liver, Spleen and Bone marrow )

The Venous Vascular System (Capacitance Vessels)Functions:1.Collection of blood from the capillary system2.Return of blood to the heart for recirculationCharacteristic 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. Smallest vein

(connecting venous system to the microcirculation)

2. Small veins 3. Medium-sized veins and4. Large-sized Veins

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

The functions of the lymphatic system are:1. Drainage of excess tissue fluid (Lymph)2. Passive transport of lymph, lymphocytes and other immunologic factors3. Release of lymph into the blood vascular system

Characteristic features of the lymphatic system: This system is characterized by: • Unidirectional flow of fluid towards the heart• Valves and muscle assisted flow of fluid• Filtration of fluid through lymph nodes• Vessels formed from blind-ended lymph capillaries

Microstructure of the Lymphatic System: This comprises:1.Blind-ended, thin-walled, endothelial-lined capillaries2. Non-fenestrated endothelium3. Rudimentary basement membrane4. Large vessels posses circular and longitudinal layers of smooth muscles5. Fine elastic filaments (Anchoring fibres) attach endothelium to surrounding connective tissue6. 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. Teeth4. Bone5. Cartilage6. Bone Marrow7. Thymus gland• Placenta.