Animal Physiology for Msc And BS Zoology students.

1. THE PERIPHERAL CIRCULATION
Arterial System
Abuzar Tabassum

2. Peripheral Circulatory System
• Systemic vessels
• Transport blood through most all body parts from left ventricle and back to
right atrium.
• Pulmonary vessels
• Transport blood from right ventricle through lungs and back to left atrium.

3. THE PERIPHERAL CIRCULATION
• Blood pumped from the left ventricle of the mammalian heart carries
oxygenated blood via the arterial system to capillary beds in the
tissues, where the oxygen is exchanged for carbon dioxide.
• The venous system returns the deoxygenated blood to the right atrium.
• Although all blood vessels share some structural features, the vessels
in various parts of the peripheral circulation are adapted for the
functions they serve.

4. Structure of Blood Vessels
• A layer of endothelial cells, called the endothelium, lines the lumen of
all blood vessels.
• In larger vessels, the endothelium is surrounded by a layer of elastic
and collagenous fibers.
• the walls of capillaries consist of a single layer of endothelial cells.
• Circular and longitudinal smooth muscle fibers may intermingle with
or surround the elastic and collagenous fibers.

5. structure of Blood Vessels
• The walls of larger blood vessels comprise three layers:
1) Tunica adventitia: the limiting fibrous outer coat
2) Tunica media: middle layer consisting of circular and
longitudinal muscle.
3) Tunica intima: inner layer, closest to the lumen, composed of endothelial
cells and elastic fibers.

7. structure of Blood Vessels
• The boundary between the tunica intima and the tunica media is not
well defined; the tissues blend into one another.
• Owing to increased muscularization, arteries have a thickened tunica
media
• The larger arteries close to the heart are more elastic, with a wide
tunica intima.
• The thick walls of larger blood vessels require their own capillary
circulation, termed the vasa vasorum.
• In general, arteries have thicker walls and much more smooth muscle
than veins of similar outside diameter.
• In some veins, muscular tissue is absent.

8. Arterial System
• The arterial system consists of a series of branching vessels with walls
that are thick, elastic, and muscular-well suited to deliver blood from
the heart to the fine capillaries that carry blood through the tissues.

9. Functions of Arteries
• Arteries serve four main functions:
1) Act as a channel for blood between the heart and capillaries
2) Act as a pressure reservoir for forcing blood into the small-diameter
arterioles
3) Dampen oscillations in pressure and flow generated by the heart and
produce a more even flow of blood into the capillaries
4) Control distribution of blood to different capillary networks via selective
constriction of the terminal branches of the arterial tree

10. • Arterial blood pressure is determined by:
the volume of blood in the arterial system and
the properties of its walls.
• If either is altered, the pressure will change.
• The volume of blood in the arteries is determined by the rate of filling via
cardiac contractions and of emptying via arterioles into capillaries.
• If cardiac output increases, arterial blood pressure will rise.
• If capillary flow increases, arterial blood pressure will fall.
• Normally, however, arterial blood pressure varies little, because the rates of
filling and emptying are evenly matched (i.e., cardiac output and capillary
flow are evenly matched).

11. • Blood flow through the capillaries is proportional to the pressure
difference between the arterial and venous systems.
• Because venous pressure is low and changes little, arterial pressure
exerts primary control over the rate of capillary blood flow and is
responsible for maintaining adequate perfusion of the tissues.
• Arterial pressure varies among species, generally ranging from 50 to
150 mm Hg.
• Pressure differences are small along large arteries (less than 1mm Hg).
• But pressure drops considerably along small arteries and arterioles
because of increasing resistance to flow with decreasing vessel
diameters.

12. • The oscillations in blood pressure and flow generated by contractions
of the heart are dampened in the arterial system, because of the
elasticity of arterial walls.
• As blood is ejected into the arterial system, pressure rises and the
vessels expand.
• Although elastic, arteries become progressively stiffer with increasing
distension.
• As a result, they are easily distended at low pressures, but then resist
further expansion at high pressures.

13. • Elastic vessels are unstable and tend to balloon; that is, since they
cannot develop high wall tension as pressure increases, they tend to
bulge out.
• In blood vessels, this instability is prevented by a collagen sheath that
limits their expansion.
• Ballooning of a blood vessel (aneurism) can occur, however, if the
collagen sheath breaks down.

14. Blood pressure
• Blood pressures reported for the arterial system are usually
transmural pressures (i.e., the difference in pressure between the
inside and outside, across the wall of the blood vessel).
• The pressure outside vessels is usually close to ambient, but changes
in the extracellular pressure of tissues can have a marked effect on
transmural pressure and therefore on vessel diameter and consequently
blood flow.
• For example, contractions of the heart raise pressure around coronary
vessels and result in a marked reduction in coronary flow during
systole.

15. • During a heartbeat cycle, the maximum arterial pressure is referred to as
systolic pressure and the minimum as diastolic pressure; the difference is
the pressure pulse.
• The pressure pulse travels at a velocity of 3-5 m.s-l.
• The velocity of the pressure pulse increases with decrease in artery diameter
and increasing stiffness of the arterial wall.
• In the mammalian aorta, the pressure pulse travels at 3-5 m.s-1 and reaches
15-35 m.s-1 in small arteries.
• Transmural pressures are typically given in millimeters of mercury
• Both the systolic and diastolic pressures generally are indicated with a slash
between them (e.g., 120/80 mmHg).

16. Effect of gravity and body position on pressure
and flow
• When a person is lying down, the heart is at the same level as the feet
and head, and pressures will be similar in arteries in the head, chest,
and limbs.
• Once a person moves to a sitting or standing position, the relationship
between the head, heart, and limbs changes with respect to gravity, and
the heart is now a meter above the lower limbs.
• The result is an increase in arterial pressure in the lower limbs and a
decrease in arterial pressure in the head.
• The height of the column of blood simply results in a higher blood
pressure due to gravity.

17. • Gravity has little effect on capillary flow, which is determined by
the arterial-venous pressure difference.
• That is, gravity raises arterial and venous pressure by the same
amount and therefore does not greatly affect the pressure
gradient across a capillary bed.

18. Velocity of arterial blood flow
• Blood flow and the oscillations in flow with each heartbeat are
greatest at the exit to the ventricle, decreasing with increasing
distance from the heart.
• At the base of the aorta flow is turbulent and reverses during diastole
as closure of the aortic valves creates whirlpools in the blood
ejected into the aorta during systole.
• In most other parts of the circulation, flow is laminar and
oscillations in velocity are damped by the compliance of the
aorta and proximal arteries.

20. • Mean velocity in the aorta, the point of maximal blood velocity, is
calculated as about 33 cm/sec in humans (based on a cross-sectional
area of about 2.5 cm2 and cardiac output of about 5 L/min).
• If we assume that maximal velocity in a vessel is twice the mean
velocity then the maximal velocity of blood flow in the human aorta
would be 66 cm/s.
• If cardiac output is increased by a factor of 6 during heavy exercise,
maximal velocity is raised to 3.96 m/s.
• In contrast, the pressure pulse associated with each heartbeat travels
through the circulation at 3-35 m/s; thus, the pressure pulse travels
faster than the flow pulse.