The document summarizes key aspects of the venous system and peripheral circulation. It describes how veins return blood to the heart at low pressure, acting as a reservoir. It discusses venous blood flow regulation via skeletal muscle contraction, breathing, and venous smooth muscle. It also describes capillaries and their role in material exchange, noting their structure varies to control permeability. In summary, it provides an overview of how blood is returned to the heart and exchanged with tissues through the venous and capillary systems.

1. Peripheral Circulation
Venous System
Abuzar Tabassum

2. Venous System
• The venous system acts as a conduit for the return of blood from the
capillaries to the heart.
• It is a large-volume, low pressure system consisting of vessels with a
larger inside diameter than the corresponding arteries.
• In mammals, about 50% of the total blood volume is contained in veins.
• Venous pressures seldom exceed 11mmHg (1.5 kPa), roughly 10% of
arterial pressures.

3. • The walls of veins are
→much thinner
→contain less smooth muscle
→and are less elastic than arterial walls
→also contain more collagen than elastic fibers
• As a result, the walls of veins are easily stretched and exhibit much less recoil
than occurs in arteries.
• The large diameter and low pressure of veins permits the venous system to
function as a storage reservoir for blood.
• If venous pressures were high, very high wall tensions would develop,
requiring the walls to be very strong to prevent them from tearing.

4. • In the event of blood loss, venous blood volume, not arterial volume,
decreases in order to maintain arterial pressure and capillary blood flow.
• The decrease in the venous blood reservoir is compensated for by a
reduction in venous volume.
• The walls of many veins are covered by smooth muscle innervated by
sympathetic adrenergic fibers.
• Stimulation of these nerves cause vasoconstriction and a reduction in the
size of the venous reservoir.
• This reflex allows some bleeding to take place without a drop in venous
pressure.
• Blood donors actually lose part of their venous reservoir; the loss is
temporary, however, and the venous system gradually expands as blood is
replaced due to fluid retention.

5. Venous blood flow
• Blood flow in veins is affected by a number of factors other than
contractions of the heart.
• Contraction of limb muscles and pressure exerted by the diaphragm on
the gut both result in the squeezing of veins in those parts of the body.
• Because veins contain pocket valves that allow flow only toward the
heart, this squeezing enhances the return of blood to the heart.
• Thus venous return to the heart increases during exercise, as skeletal
muscle contractions squeeze veins and drive blood towards the heart.
• The increase in venous return will increase cardiac output.

6. • Breathing in mammals also contributes to the return of venous blood
to the heart.
• Expansion of the thoracic cage reduces pressure within the chest and
draws air into the lungs.
• This pressure reduction sucks blood from the veins of the head and
abdominal cavity into the heart and large veins situated within the
thoracic cavity.

7. • In sharks contractions of the ventricle reduce pressure in the
pericardial cavity, so blood from the venous system is sucked into the
atrium.
• Peristaltic contractions of the smooth muscle of venules, can promote
venous flow toward the heart.
• Such peristaltic activity has been observed in the venules of the bat
wing.

8. Blood distribution in veins
• Venous smooth muscle also aids in regulating the distribution of blood in the
venous system.
• When a person shifts from a sitting position to a standing position, the change
In the relative positions of heart and brain with respect to gravity activates
sympathetic adrenergic fibers that Innervate limb veins, causing contraction
of venous smooth muscle and thereby promoting the redistribution of pooled
blood.
• Such venoconstriction is inadequate, however; to maintain good circulation if
the standing position is held for long periods in the absence of limb
movements, as when soldiers stand immobile during a review.
• Under such circumstances, venous return to the heart, cardiac output, arterial
pressure, and flow of blood to the brain are all reduced, which can result In
fainting.

9. • Similar problems affect bedridden patients who attempt to stand after
several days of inactivity and astronauts returning to Earth after a long
period of weightlessness.
• The reflex control of venous volume is normally reestablished with use.
• The effects of gravity on blood distribution are not important in aquatic
animals because the densities of water and blood are not very different.
• For this reason, pooling of blood due to gravity does not occur in aquatic
animals.
• Because of the large difference between the density of air and blood,
pooling became a problem with the evolution of terrestrial forms.

10. Countercurrent exchangers
• In many animals arteries and veins run next to each other with the
blood flows moving in opposite directions (i.e., countercurrent blood
flow).
• In many such instances, especially if the vessels are small, there will
be exchange of heat between the countercurrent blood flows.
• Because heat is transferred much more easily than gas, it is possible to
have heat exchange with little gas transfer.
• Countercurrent heat exchangers are common in the limbs of birds and
mammals and are used to regulate the rate of heat loss via the limbs.

11. • A countercurrent arrangement of small arterioles and venules is referred to
as a rete mirabile.
• Before entering a tissue, an artery divides into a large number of small
capillaries that parallel a series of venous capillaries leaving the
tissue.
• The "arterial" capillaries are surrounded by "venous" capillaries, and vice
versa, forming an extensive exchange surface between inflowing and
outflowing blood.
• These retial capillaries serve to transfer heat or gases between arterial blood
entering a tissue and venous blood leaving it.
• In humans, this type of countercurrent exchanger is found only in the
kidney.

12. • Tuna have a large number of retia mirabile, which are used to regulate
the temperature of the brain, muscles, and eyes.
• The rete mirabile leading to the physoclist swim bladder of other fish
such as the eel function as a carbon dioxide countercurrent exchanger.

13. Capillaries and the Microcirculation
• Most tissues have such an extensive network of capillaries that any
single cell is not more than three or four cells away from a capillary.
• This is important for the transfer of gases, nutrients, and waste
products, because diffusion is an exceedingly slow process.
• Capillaries are usually about 1mm long and 3-10 pm in diameter, just
large enough for red blood cells to squeeze through.
• Large leukocytes, however, may become lodged in capillaries,
stopping blood flow.
• The leukocytes are either dislodged by a rise in blood pressure or
migrate slowly along the vessel wall until they reach a larger vessel
and are swept into the bloodstream.

14. Microcirculatory beds
• Small terminal arteries subdivide to form arterioles, which in turn
subdivide to form metarterioles and subsequently capillaries, which then
rejoin to form venules and veins.
• The arterioles are invested with smooth muscle that becomes
discontinuous in the metarterioles and ends in a smooth muscle ring, the
precapillary sphincter.
• Capillary walls, which are completely devoid of connective tissue and
smooth muscle, consist of a single layer of endothelial cells surrounded
by a basement membrane of collagen and mucopolysaccharides.
• The capillaries are often categorized as arterial, middle, or venous
capillaries, the latter being a little wider than the other two types.

16. • A few elongated cells with the ability to contract, called pericyte cells,
are found wrapped around capillaries.
• The venous capillaries empty into pericytic venules, which in turn join
the muscular venules and veins.
• The venules and veins are valved, and the muscle sheath appears after
the first postcapillary valve.
• Even though the walls of capillaries are thin and fragile, they require
only a small wall tension to resist stretch in response to pressure
because of their small diameter.

17. • Most arterioles are innervated by the sympathetic nervous system; a
few arterioles (e.g., those in the lungs) are innervated by the
parasympathetic nervous system.
• Different tissues have varying numbers of capillaries open to flow and
show some variation in the control of blood flow through the capillary
bed.
• In some tissues, opening and closing of the precapillary sphincters,
which are not innervated and appear to be under local control, alter
blood distribution within the capillary bed.

18. • In other tissues, however, most, if not all, of the capillaries tend to be
open (e.g., in the brain) or closed (e.g., in the skin) for considerable
periods.
• All capillaries combined have a potential volume of about 14% of an
animal's total blood volume.
• At any one moment, however, only 30%-50% of all capillaries are
open, and thus only 5%-7% of the total blood volume is contained in
the capillaries.

19. Material transfer across capillary walls
• Transfer of substances between blood and tissues occurs across the
walls of capillaries, pericytic venules, and, to a lesser extent,
metarterioles.
• The endothelium composing the capillary wall is several orders of
magnitude more permeable than epithelial cell layers, allowing
substances to move with relative ease in and out of capillaries.
• However, the capillaries in various tissues differ considerably in
permeability.
• These permeability differences are associated with marked changes in
the structure of the endothelium.

20. Types of capillaries
• Based on their wall structure, capillaries are classified into three
types.
1) Continuous capillaries: which are the least permeable, are
located in muscle, nervous tissue, the lungs, connective tissue,
and exocrine glands.
2) Fenestrated capillaries: which exhibit intermediate
permeability, are found in the renal glomerulus, intestines, and
endocrine glands.
3) Sinusoidal capillaries: which are the most permeable, are
present in the liver, bone marrow, spleen, lymph nodes, and
adrenal cortex.

21. Continuous capillaries
• In the continuous capillaries of skeletal muscle, the endothelium is
about 0.2-0.4 pm thick and is underlain by a continuous basement
membrane.
• The endothelial cells are separated by clefts, which are about 4 nm
wide at the narrowest point.
• Most of the cells contain large numbers
of pinocytotic vesicles about 70 nm in
diameter.

22. • Substances can move across the wall of continuous capillaries either
through or between the endothelial cells.
• Lipid-soluble substances diffuse through the cell membrane, whereas
water and ions diffuse through the water filled clefts between cells.
• In brain capillaries, there are transport mechanisms for glucose and
some amino acids.
• The reduced permeability of brain capillaries, however, is also
considered to result from the tight junctions between endothelial cells.

23. • The continuous capillaries in the lung are less permeable than those in
other tissues.
• In these less-permeable capillaries, the pressure pulse may play a role
in augmenting movement of substances (e.g., oxygen) through the
endothelium.
• As pressure rises, fluid is forced into the capillary wall, but as pressure
drops, fluid returns to the blood.
• This tidal flushing of the capillary wall should enhance mixing in the
endothelial barrier and effectively augment transfer.

24. Fenestrated capillaries
• In the capillaries of the renal glomerulus and gut, the inner and outer
plasma membranes of endothelial cells are closely apposed and
perforated by pores in some regions, forming a fenestrated
endothelium.
• Not surprisingly, these fenestrated
capillaries are permeable to nearly
everything except large proteins and
red blood cells.

25. • The kidney ultra filtrate is formed across such an endothelial barrier.
• The basement membrane of fenestrated endothelia normally is
complete and may constitute an important barrier to the movement of
substances across fenestrated capillaries.
• These capillaries contain only a few vesicles, which probably play no
role in transport.

26. Sinusoidal capillaries
• The endothelium in sinusoidal capillaries is characterized by large
para-cellular gaps that extend through the basement membrane and an
absence of vesicles in the cells.

27. • Liver and bone capillaries always contain large para-cellular gaps, and
most substance transfer across these capillaries occurs between the cells.
• As a result, the fluid surrounding the capillaries in liver has much the same
composition as plasma.
• The clefts, pores, and para-cellular gaps through which substances can
freely diffuse across capillary walls are about 4 nm wide.
• But only molecules much smaller than 4 nm can get through, indicating the
presence of some further sieving mechanism.
• The diameter of these openings varies within a single capillary network and
usually is larger in the pericytic venules than in the arterial capillaries.

28. • This is of functional significance because blood pressure, which
is the filtration force for moving fluid across the wall, decreases from
the arterial to the venous end of the capillary network.
• Inflammation or treatment with a variety of substances (e.g.,
histamine, bradykinin, and prostaglandins) increases the size of the
openings at the venous end of the capillary network, making it very
permeable.

29. Capillary pressure and flow
• The arrangement of arterioles and venules is such that all capillaries are
only a short distance from an arteriole, so that pressure and flow are fairly
uniform throughout the capillary bed.
• Transmural pressures of about 10 mm Hg have been recorded in capillaries.
• High pressures inside a capillary result in the filtration of fluid from the
plasma into the interstitial space.
• This filtration pressure is opposed by the plasma colloid osmotic pressure,
which results largely from the higher concentration of proteins in the blood
than in the interstitial fluid.
• Because of their large size, these plasma proteins are retained in the blood
and not transported across the capillary wall.

30. • To visualize the relationship of these two pressures, consider the
schematic situation depicted in Figure 12-39.
• Generally, blood pressure is higher than the colloid osmotic pressure at
the arterial end of a capillary bed, so fluid moves into the interstitial
space (area 1).
• The blood pressure steadily decreases along the length of the capillary,
while the colloid osmotic pressure remains constant.
• Once the blood pressure falls below the colloid osmotic pressure, fluid in
the interstitial space is drawn back into the blood by osmosis (area 2).

32. • Thus the net movement of fluid at any point along the capillary is
determined by two factors:
a) the difference between blood pressure and colloidal osmotic pressure and
b) the permeability of the capillary wall, which tends to increase toward the venous
end.
• This concept is sometimes referred to as the Starling hypothesis, after its
initial proponent, Ernest Starling (1866- 1927), whose prolific research also
included studies on the relationship between ventricle work output and
venous filling pressure.
• In most capillary beds, the net loss of fluid at the arterial end is somewhat
greater than the net uptake at the venous end of the capillary.

33. • Net filtration of fluid across capillary walls will result in an increase in
tissue volume, termed edema, unless the excess fluid is carried away
by the lymphatic system.
• In the kidney, capillary pressure is high and filtration pressures exceed
colloild osmotic pressures; hence, an ultra filtrate is formed in the
kidney tubule, eventually to form urine.
• The kidney is encapsulated to prevent swelling of the tissue in the face
of ultra filtratlon.
• In most other tissues, there is only a small net movement of fluid
across capillary walls and tissue volume remains constant.

34. • A rise in capillary pressure, due to a rise in either arterial or venous
pressure, will result in increased loss of fluid from the blood and tissue
edema.
• A drop in colloid osmotic pressure can result from a loss of protein
from the plasma by starvation or excretion or by increased capillary
wall permeability, leading to movement of plasma proteins into the
interstitial space.
• If filtration pressure remains constant, a decrease in colloid osmotic
pressure will also result in an increase in net fluid loss to the tissue
spaces.