The vascular system in the human body consists of arteries, veins, and capillaries, which transport blood throughout the body. Arteries carry blood away from the heart, while veins return it, with capillaries serving as the exchange sites for nutrients and gases. Blood pressure regulation involves multiple mechanisms, including the heart's pumping action and vascular resistance, to maintain optimal circulation.

1. THE VASCULAR SYSTEM
By: Umair Khan

2. Introduction:
• The Vascular System of the human body comprises of vessels of the
body which includes Arteries, Capillaries and Veins.
• The Vessels originates from the heart and have spreaded throughout
the body till the cellular level.
• Arteries are those blood vessels which carries blood away from the
heart.
• Veins are the blood vessels which carries blood towards the heart.
• Capillaries are those blood vessels which connects Arteries and Veins
and are the site of nutrient and gases exchange at the cellular level.
• Small Arteries are Called Arterioles.
• Small Veins are Called Venules

3. Arteries:
Arteries Carry blood away from heart to capillaries. Smaller arteries are
called as Arterioles.
• If we see an Artery in Cross Section we will find 03 layers of it, In
medical Terminology these are known as Tunics of Artery.
• Those three Tunics(layers) are following :
1: Tunica Intema
2: Tunica Media
3: Tunica Externa

4. 1: Tunica Intema:(Internal layer)
 It is the innermost layer of an artery which comes in contact with blood.
 It is made up of simple squamous epithelium called Endothelium.
 This lining is same type of tissue as present in Endocardium.
 Its extreme smoothness prevents abnormal blood clotting.
 It also produce NO which is a vasodilator.
2: Tunica Media(middle layer)
 It is made up of smooth Muscles and elastic connective tissue.
 Both of these tissue are involved in maintaining normal blood pressure
 specially diastolic pressure.
 Smooth muscle is the tissue affected by the vasodilator “NO” produced by
 Tunica Intema.
 Relaxation of these tissue brings about the dilation of vessel, smooth
 muscles has also the nerve supply the sympathetic nerve impulses which
 brings about he vasoconstriction


5. 3: Tunica Externa (External Layer)
It consists of Fibrous connective tissue. This tissue is very strong which is
very important to prevent the rapture or bursting of the artery which carry blood
at very high pressure.

6. Veins:
• Veins carry blood towards the heart from the capillaries .Smaller veins
are known as “Venulues”.
• The same 03 tissue layers are present in Vein as in an artery, but
there are some differences when compared to arterial layers.
• The inner layer of veins is smooth endothelium but at intervals this
lining is folded to form “Valves” which prevents the back flow of blood
and these are present numerously in the veins of legs from where
blood must return to the heart against the force of gravity.
• The middle layer of veins is thin layer of smooth muscles as compared
to arteries where it is thick.
• The outer layer is also thin here not much fibrous connective tissue is
required as in case of arterial layer because B.P in veins is low.

7. CAPILLARIES
• Capillaries are the blood vessels which are smaller and thinner than
Arteries and Veins.
• The walls are only one cell in thickness, it has the same simple
squamous epithelium as in Arteries and Veins.
• Some tissue don’t have capillaries these are the Epidermis,Cartilage
, Lens and cornea of eye.
• Most tissue have extensive networks of capillaries these are Liver,
kidney, Intestines, brain etc.
• The quantity of capillary network reflects the metabolic activity of that
organ.
• The functioning of kidney for example depends upon the blood supply
to it.
• In contrast tendon such as Achilles tendon at the heel would have far
fewer vessels.

8. Summary:
• The pathway in which blood circulates throughout the body is:
Arteries Arterioles Capillaries Veneules
Veins

9. Exchanges in Capillaries:
• Capillaries are the site of exchanging the material between the blood
and extracellular fluids.Some of them move from blood to Extracelluar
fluid and some moves from extracellular fluid to blood.
• Gases move by Diffusion that is, from their area of greater
concentration to the area of lesser concentration, it happens in the
alveoli of lungs where oxygen and carbon dioxide moves as a result of
diffusion.
• Lets us look at blood pressure when blood enters into capillaries its
pressure is about 30-35mmHg and pressure in the surrounding is
2mmHg that’s how process of filtration ocours. In this way nutrients
i.e. glucose,amino acids and vitamins etc brought to cells.

10. • Blood pressure decresses as blood enters the venous end of the
capillaries but notice that protiens like Albumin remains in the blood,
Albumin contribute to the “COLLIODAL OSMOTIC PRESSURE” of the
blood which is also known as “ONCOTIC PRESSURE”
• This is an Attracting pressure rather then pushing which attracts
the plasma fluid back into the blood vessel along with the waste
cellular material which needed to put out of the body.This is how blood
volume remained in optimal condition. One thing which helps in
maintaining the normal blood voulume is the Lymphatic system of the
body which put the plasma which remained in the tissue fluid back into
the circulation.

11. Pathways of Circulation:
1:Pulmonary Pathway
The Right Ventricle Pumps blood towards the lungs the two arteries
known as Pulmonary arteries carry the blood to the lungs.
“Pulmonary Artery is the only Artery in the body which carry
de-oxygenated blood”
From the lungs blood gets Oxygen and comes back to Left Atrium of the
heart by the veins known as pulmonary veins.
“Pulmonay vein is the only vein of the body which carry Oxygenated
blood”

13. Systemic Circulation
The left ventricle pumps blood into the aorta, the largest artery of the
body. The branches of aorta takes the blood to the arterioles and
capillaries network throughout the body. Capillaries merge to form
veneules and veins.
The veins from the lower body takes blood to the inferior vena cava,
Veins from the upper body takes blood to the superior vena cava,
These two caval veins return blood to the right atrium.
Aorta is an continious artery but for the sake of better understanding it
is named anatomically from where it passed i.e. Thorasic and
abdominal aorta.
From the arch of aorta 03 braches protrudes out which supply the blood
to the head and arms of the body.
Some of the arteries of the head make an important anastomosis
known as circle of willis which is an effective backup mechanism for
blood supply of the brain.

14. The Systemic Circulation

15. Braches from arch of aorta

16. • Hepatic portal Circulation
• Hepatic portal circulation is the subdivision of systemic circulation in
which blood from the abdominal digestive organs and the spleen
circulates the liver before returning to the heart.
• Blood from capillaries of stomach, small and large intestine, spleen
and pancreas flows into two large veins: The superior mesentric
vein and splenic vein which unite to form the portal vein.
• The portal vein takes blood into the liver where it spread into the
sinusoids the capillaries of the liver from where blood flows into the
hepatic vein to the inferior vena cava and to the right atrium of the
heart.
• The purpose of Hepatic portal circulation is to detoxify the material
which gets absorbed from the digestive organ e.g. alcohol gets
absorbed from the stomach gets into the liver where it detoxified by
liver if not it could damage the brain and other organs of the bdoy.

17. Hepatic portal circulation

18. •Fetal Circulation
• The fetus depends upon mother for oxygen and nutrients and also for the
removal of carbon dioxide and other waste products.
• The site of exchange between fetus and the mother is the placenta which
contains fetal and mother blood vessels which are very close to one
another. The blood of the mother does not mix with the blood of the fetus.
• The substances are exchanged through the process of difficion and through
the Active transport system.
• The fetus is connected to the placenta through the umblical cord which
contains two umblical arteries and one umblical vein and the function of
arteris and veins is same as in adults.

19. Fetal Circulation

20. Blood Pressure
• Blood pressure is the pressure on the walls of blood vessels exerted
by the blood itself.
• Filtration in capillaries depends upon the blood pressure, filtration
brings nutritions to the tissues.
• The pumping of ventricles creates the blood pressure which is
measured in mmHg. When the systemic blood pressure is obtained,
two no are usually considered: Systolic and Diastolic
• Systolic pressure is always the higher of the two and represents the
blood pressure when the left ventricle is contracting
• Diastolic pressure is the lower no when the left ventricle is relaxed
and does not exert force.
• Systemic blood pressure is highest in the aorta, which receives all of
the blood pumped by the left ventricle, as blood travels further away
from the heart blood pressure decreases.

21. • Normal Blood pressure: 120/80 mmHg
• 120 is Systolic blood pressure
• 80 is Diastolic blood pressure
• The brachial artery is often used for obtaining the reading of blood
pressure.
• The abnormal conditions related to the blood pressure are
Hypertension and Hypotension
• which are blood pressure above normal and blood pressure below
normal.

22. Maintenance of Systemic blood pressure
• As blood pressure is so important so many physiological factors and
processes interects to keep the blood pressure within normal range.
1: Venous Return
The amount of blood that returns to the heart by means of veins is
called venous return which is very important heart will only pump the
blood it receives if venous return decreases then the cardiac muscle
fiber will not be stretched then the force of ventricle systole get
decreased (starling law) the blood pressure will decrease. So proper
venous return is very important for maintaning the noraml level of the
blood pressure.

23. • 2.Heart rate and force
• Generally if heart rate and force increases the blood prssure is also
increases this is what happens during exercise. However if the heart
rate is beating extremely rapidly the ventricles may not fill completely
between the beats then the cardiac output and blood pressure
decreases.
• 3.Peripheral Resistance
The term refers to the resistence offered by the vessels to the flow of
the blood. The arteries and veins are usually constricted which
maintains normal diastolic blood pressure.
4.Elasticity of large Arteries
when the left ventricle contracts it stretches the wall of the arteries if
the arteries don’t have elasticity the will repture and the consequencies
of that repture will be quite devastating involving the decrease of blood
pressure as well.

24. Regulation of Blood pressure
• The mechanism that regulates systemic blood pressure may be
divided into two types:
• 1: The Interinsic Mechanism
• 2: The Nervous Mechanism
• The nervous mechanism involves the nervous system
• The interinsic mechanism does not require nerve impulses

25. • INTRINSIC MECHANISMS
• The term intrinsic means “within.” Intrinsic mechanisms work because
of the internal characteristics of certain organs. The first such organ is
the heart. When venous return increases, cardiac muscle fibers are
stretched, and the ventricles pump more forcefully (Starling’s law).
Thus, cardiac output and blood pressure increase. This is what
happens during exercise, when a higher blood pressure is needed.
When exercise ends and venous return decreases, the heart pumps
less forcefully, which helps return blood pressure to a normal resting
level.
• The second intrinsic mechanism involves the kidneys. When blood
flow through the kidneys decreases, the process of filtration
decreases and less urine is formed. This decrease in urinary output
preserves blood volume so that it does not decrease further

26. • The kidneys are also involved in the reninangiotensin mechanism.
When blood pressure decreases, the kidneys secrete the enzyme
renin, which initiates a series of reactions that result in the formation of
angiotensin II.

27. The Nervous Mechanism
• The medulla and the autonomic nervous system are directly involved
in the regulation of blood pressure
• nervous mechanism involves peripheral resistance, that is, the degree
of constriction of the arteries and arterioles and, to a lesser extent, the
veins.
• The medulla contains the vasomotor center, which consists of a
vasoconstrictor area and a vasodilator area. The vasodilator area may
depress the vasoconstrictor area to bring about vasodilation, which
will decrease blood pressure. The vasoconstrictor area may bring
about more vasoconstriction by way of the sympathetic division of the
autonomic nervous system.

28. • Sympathetic vasoconstrictor fibers innervate the smooth muscle of all
arteries and veins, and several impulses per second along these
fibers maintain normal vasoconstriction. More impulses per second
bring about greater vasoconstriction, and fewer impulses per second
cause vasodilation. The medulla receives the information to make
such changes from the pressoreceptors in the carotid sinuses and the
aortic sinus. The inability to maintain normal blood pressure is one
aspect of circulatory shock.