Download 7- Introduction and functional anatomy of vascular physiology

7- Introduction and functional anatomy of
vascular physiology
The cardiovascular system consists of the heart, blood vessels, Blood,
and the lymphatic system. Lymphatic system consists of lymphatic
vessels, lymph nodes, and lymph.
History:
The English physician William Harvey (1578 – 1657) demonstrated
that the heart pumps blood through a closed system of vessels.
Functions of CVS:
1- The blood and lymph carry absorbed products of digestion to the liver.
Then the blood transport of these substances which are essential for
cellular metabolism.
2- The blood carries oxygen from the lungs to all the body cells.
3- Excrete metabolic waste products such as urea, excess water and ions
in the urine. Carbon dioxide produced by cell is carried by blood to the
lungs for elimination in the exhaled air.
4- It contributes in both hormonal and temperature regulation.
5- It protects against blood loss through clotting mechanism and foreign
microbes by immune function.
Vascular system:
The blood vessels are a closed system, which carry blood from the heart
to the tissue, and back to the heart.
The wall of blood vessels is made of:A- Inner layer includes endothelial and sub endothelial connective tissue
(Intima).
B- Middle layer includes smooth muscles (Media).
C- Outer layer includes connective tissue (Adventitia). See figure (34).
.
Figure (34): Structure of normal muscle artery (Ganong's review of
medical physiology 2010).
Types of blood vessels:
1- Arteries: The arteries are thick-walled structures with extensive
development of elastic tissue. They are stretched during systole and recoil
during diastole, such property prevents an excessive rise in blood pressure
(during systole) and excessive fall (during diastole). They carry blood
from the heart mostly oxygenated (except pulmonary artery), under the
highest pressure to tissues. The volume of blood in the arteries is called
the stressed volume. The muscles of arteries are innervated by
noradrenergic nerve fibers, in some instances by cholinergic fibers. The
elasticity of arteries is decreased by aging process due to atherosclerosis.
2- Arterioles: They are the smallest branches of the arteries. Their walls
have less elastic tissue but much more innervated by noradrenergic nerve
fibers and some time by cholinergic nerve fibers, which constrict or dilate
the vessels. They are the site of highest resistance to blood flow. They can
alter blood flow to tissue because of their ability of changing their
diameter.
3- Capillaries: The arterioles divide into smaller muscle-walled vessels
called metarterioles and these in turn feed into capillaries. The openings of
the capillaries are surrounded by minute smooth muscle (precapillary
sphincters). The capillaries are thin-walled structures which are about 1
μm thick, lined with a single layer of endothelial cells. They are the site
where nutrients, gases, water and solutes exchange between the blood and
the tissues. They are about 5m in diameter at the arterial end and 9 m at
venous end. The diameter of capillaries is just sufficient to permit red
blood cells in single file; the total area of all capillary walls in the body
are more than 6300 m² in the adult. See figure 35.
Figure (35): Microcirculation (Fox, 2006).
4- Venules and Veins: The venules collect blood from the capillaries and
coalesce into larger veins. The walls of them are only slightly thicker than
those of the capillaries. The veins have thin wall in comparism to the
arteries, easily distended. The veins have a very large capacitance
(capacity to hold blood). They contain the largest percentage of blood in
the cardiovascular system (65-75% of circulating blood volume), figure
37. The volume of blood in the veins is called the unstressed volume. The
smooth muscle in the walls of the veins is innervated by sympathetic
nerve fibers. The blood in veins is under lower pressure. The veins contain
valves prevent retrograde flow. These valves function was first
demonstrated by William Harvey in 17th century. No valves are present in
the very small veins or the veins from the brain and viscera. The veins
carry blood from tissue to right atrium and collect blood from lung to the
left atrium.
Arteriovenous anastomosis: In the finger tips, palms of hand and ear
lobes, there are short channels that connect arterioles to venules (arteriovenous anastomosis or shunt). These permit the passage of large
molecules such as WBCs. See figure 36.
Figure (36): The blood percentage in CVS (Fox 2006).
Neural innervations of the blood vessels:
Sympathetic nerves innervate the most blood vessels by release
norepinephrines which bind to specific receptors on the smooth muscles
called alpha receptors. Stimulation of the alpha receptors causes the smooth
muscles to contract. Blood vessels supplying skeletal muscles have a
different type of receptors called β2 receptors, when stimulated by
norepinephrine cause the vessels to relax. Sympathetic vasodilator response
plays a significant role in the exercise, serving to prime the skeletal muscles
with oxygen and nutrient. Also skeletal muscles blood vessels do not appear
to
be
innervated
by
parasympathetic
neurons.
Lymphatic system
Formation of lymph: As fluid pressure increase in the interstitial space,
this will open the valve at the terminal lymphatic capillary, so protein and
fluid move to lymphatic vessel. Normally filtration of fluid out the
capillaries is slightly greater than absorption of fluid into the capillaries.
The excess filtered fluid and protein are return to circulation via lymph.
Function of lymph:
1- It transports interstitial fluid to the blood.
2- It transports of absorbed fat from small intestine.
3- Lymphocytes help to provide immunological defenses against
infectious disease.
Lymph comes back to the circulation by following mechanisms:
1- Intrinsic rhythmic contraction at rate 10 – 15 per minutes.
2- Contractile filaments and valves in wall of lymphatic capillaries.
3- Contraction of surrounding skeletal muscles.
4- Movement of organs of the body.
5- Pulsation of arteries adjacent to the lymphatic vessels.
6- Interstitial fluid pressure.
7- One way flap valves which permit interstitial fluid to enter lymph
vessels but not
leave it. Figure (37).
Figure (37): structure of lymphatic capillary (Guyton & Hall 2006).
Structures of lymphatic vessel:
A- Lymphatic capillaries:
Their diameter 10 – 50 µm, very thin wall, highly permeable to plasma
proteins. The junction of Endothelial cells run very obliquely and may
function like flap valves allowing fluid to enter but closing to prevent
back flow.
B- Collecting or afferent lymphatic trunk.
C- Lymph nodes.
D- Efferent lymphatic trunk.
E- Major lymphatic duct.
All the lymph of body flow to the thoracic duct and right lymphatic
duct which empty into venous system. Normal lymph flow is 2 – 3 L /
day. The rate of lymph flow varies in different organs, highest in the GIT
and the liver. Figure 38.
Figure (38): lymphatic pathway(Fox 2006) .