Download Introduction to the Circulatory System

Introduction to the Circulatory System
The Circulatory System:
The circulatory system mediates the continuous movement of all body fluids, its principal
functions being the transport of oxygen and nutrients to the tissues and transport of carbon
dioxide and other metabolic waste products from the tissues.The circulatory system is also
involved in tempreture regulation and the distribution of molecules such as hormones and cells
such as those of the immune system. The circulatory system has two functional components:
the blood vascular system and the lymph vascular system.
The blood vascular system comprises a circuit of vessels through which flow of blood is
maintained by the continuous pumping of the heart.These blood vessels are not rigid tubes but
dynamic structures that pulsate, constrict and relax, and even proliferate( under the influence of
many hormones and factors), as demanded by the changing needs of the human body. The
arterial system
provides a distribution network to the capillaries which are the main sites of interchange of
gases and metabolites between the tissues and blood. The
venous syste
m returns blood from the capillaries to the heart. In contrast,
the lymph vascular system
is merely a passive drainage system for returning extravascular fluid, the
lymph
, to the blood vascular system. The lymph vascular system has no intrinsic pumping
mechanism.
Components of the blood vascular system
Three major types of blood vessels found in the body are, the arteries, veins and capillaries.
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Introduction to the Circulatory System
According to the anatomical definition, the arteries are the vessels that carry blood away from
the heart. While, the
veins are the vessels
that bring back blood towards the heart.
Capillaries
are the tiny microscopic vessels connecting the arteries to the veins.
The arteries are said to "branch", "diverge", or "fork" as they carry blood away from the heart.
Veins, by contrast, are said to" join", "merge", "converge", or "serve as tributaries" as they carry
blood towards the heart.
General microscopic structure of blood vessel walls:
The walls of all blood vessels, except the very small ones( i.e. the capillaries and small
venules), are composed of three distinct layers, or tunics (= cloaks). These tunics surround the
central blood filled space, the vessel lumen.
1. The Inner most layer is called the tunica intima. This tunic is in intimate contact with the
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blood in the lumen. This tunic contains the
e
ndothelium
, the simple squamous epithelium that lines the lumens of all vessels. The endothelial cells are
resting upon a
basement membrane
. The endothelium is the continuation of the endocardial lining of the heart, and its flat cells fit
closely together, forming a slick surface that minimizes friction as blood moves through the
lumen.
2. The middle layer, or tunica media, is mostly composed of circularly arranged smooth
muscle cells
and
sheets of elastin
(in the form of
internal
and
external elastic laminae
). Generally, the tunica media is the bulkiest layer in arteries, which bear the chief responsibilty
for maintaining blood pressure and continuous blood circulation.
3. The outermost layer, or tunica adventitia, is composed largely of loosely woven collagen
fibers that protect and reinforce the vessel, and anchor it to surrounding structures. The tunica
adventitia is infiltrated with nerve fibers, lymphatic vessels, and, in larger veins, a network of
elastin fibers. In larger vessels, the tunica adventitia contains a system of tiny blood vessels, the
vasa vasorum - literally, "vessels of the vessels"
that nourish the more external tissues of the blood vessel wall. The innermost or luminal portion
of the vessel wall obtains its nutrients directly from blood in the lumen.
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Introduction to the Circulatory System
proceed to Circulation 2. Classification of Vessels
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