Circulation

1. 23-1
Human Anatomy of Vessels
and Circulation

2. 23-2
Blood Vessels
 Naming:
 Often share names with either the body region they traverse
or the bone next to them.
 Some are named for the structure they supply.
 Arteries and veins that travel together sometimes share the
same name.
 Systemic circulation
 consists of the blood vessels that extend to and from the
body tissues.
 Pulmonary circulation
 consists of the vessels that take the blood to the lungs for
gas exchange.
 Work continuously with each other.

3. 23-3
Three Main Classes of Blood
Vessels
 Arteries convey blood away from the
heart to the body tissues.
 Arteries branch, or bifurcate, into
smaller and smaller vessels (arterioles)
until they feed into the capillaries,
where gas and nutrient exchange
occurs.
 From the capillaries, veins return blood
to the heart.

4. 23-4
Blood Vessels
 An efficient mode of transport for oxygen, nutrients,
and waste products to and from body tissues.
 Heart is the mechanical pump that propels the blood
through the vessels.
 Heart and blood vessels form a closed-loop system.
 Blood is continuously pumped to and from the
tissues.
 Are not rigid and immobile.
 Can pulsate and change shape in accordance with
the body’s needs.

5. 23-5
Three Main Classes of Blood
Vessels
 Arteries become progressively smaller as
they divide and get further from the heart.
 Veins become progressively larger as they
merge and get closer to the heart.
 Anastomosis: Site where two or more
vessels merge to supply the same body
region.
 arterial anastomoses: alternate route
 Veins tend to form many more anastomoses than
do arteries.

6. 23-6
Three Main Classes of Blood
Vessels
 End arteries
 Arteries that do not form anastomoses
Only one route, e.g., renal artery, splenic
artery
 Functional end arteries
 Have small anastomoses,e.g.,
coronary arteries

7. ANASTOMOSIS
(G. Anestomosis : opening outlet)
23-7
Anastomosis could be defined as follows:
(a) Anatomically speaking, it is a communication between
two vessels by collateral channels.
(b) Clinically speaking, it is a communication created by
surgical, traumatic or pathological means between the two
normally distinct spaces or organs, e.g.:
(i) Ureterotubal anastomosis an anastomosis between
the ureter and uterine tube.
(ii) Intestinal Anastomosis: An anastomosis between the
two portions of intestinal tract.

8. TYPES OF
ANASTOMOSES
 There are three types of anastomoses:
 I. Arterial Anastomosis:The anastomoses between various arteries
of the body are of great importance in medicine and surgery and it has
following sub-varities :
 (a) Actual Anastomosis: It is a communication between arterial trunks
of nearly equal size.
 Examples:
 i) Labial branches of facial artery.
 ii) Anterior and posterior intercostal arteries.
 iii) Uterine arteries.
 iv) Ovarian arteries
 v) Gastric arteries
 vi) Gastroepiploic arteries
 Test: The cut vessel spurts from both ends. 23-8

9. (b) Potential Anastomosis
 It is a communication established by union of terminal
arterioles.
 Examples:
 i) Coronary arteries.
 ii) Limb arteries in the regions of joints
 iii) Cortical arteries of cerebral hemispheres. Significance:
 With gradual occlusion of the artery, the arterioles can dilate
sufficiently to provide adequate nutrition. However, if occlusion
of main vessel is sudden, then the potential anastomosis will
not be able to dilate sufficiently in order to provide adequate
nourishment. As a result of this, the death of the area supplied
by the artery will occur (Necrosis / Infarction). 23-9

10. (c) End Anastomosis or no
Anastomosis
 In certain regions of the body, there are arteries, which have no
anastomosis with their neighbouring vessels. These arteries are, therefore,
called as End-Arteries.
 True Example:Central Artery of the Retina.
 Significance:
 If an artery of this type is occluded, then the area supplied by the artery will
undergo necrosis (It means microscopic death of tissue). That is why the
occlusion of central artery of retina will lead to blindness.
 False examples:
 Here the communication takes place at capillary level only and practically
results in the formation of End-Anastomosis, e.g.
 i) Central Arteries of the brain.
 ii) Arteries of spleen.
 iii) Arteries of kidney.
 iv) Arteries of liver.
 v) Vasa recta of mesenteric arteries.
 vi) Metaphyseal arteries of long bones.
 vii) Arteries of lung.
 Interruption of arterial flow will result in necrosis or infraction or gangrene
of the area supplied.
23-10

11. II. Venous Anastomosis
 It is a communication between veins. It is much more
frequent and highly variable and is of lesser
significance.
 Examples:
 i) Dorsal venous arch of hand.
 ii) Dorsal venous arch of foot.
 iii) Anastomosis between cephalic and basalic veins
through median cubital vein.
 iv) Azygos vein which connects the inferior vena
cava with the superior vena cava.
23-11

12. III. Arterio-venous Anastomosis
or Vascular Shunts
 In many tissues of the body, communication exists between
arterial and venous sides of circulation. When these short
circuiting channels open then the blood will pass directly from
the arterioles into the venules and will not pass through the
capillaries. The reason for their existence and the mechanism
of their control are both ill-understood.
 The arterio-venous shunts are divided into three sub-varieties
according to site, dimensions and complexity.
 A. Prefrential thoroughfare channels:
 Grant and Wright (1968 and 1970) have demonstrated that
true capillaries are also given out from thoroughfare channel,
which connects the terminal arteriole and vennule.
23-12

13. Characteristics
 I) the caliber is larger than true capillaries.
 2) The structure resembles the continuous
capillaries but few smooth muscle cells are
present around the endothelium.
 3) The thoroughfare channel and its
associated capillaries form a functional micro-
circulatory unit. Each capillary from the
thoroughfare channel is surrounded by a pre-
capillary sphincter at its origin. 23-13

14. 4) The size of micro-circulatory unit varies in different tissues,e.g.:
i) In striated muscle, each thoroughfare channel may give rise to about 2
true capillaries.
ii) In glandular tissue, only two capillaries are given out from thoroughfare
channel.
5) The form of capillary network also varies with the type of the tissue, e.
a) The capillary network is rounded or angular at
Following situations:
i) Lungs.
ii) Mucous membrances.
iii) Skin.
b) The capillary network is elongated in:
i) Muscles.
ii) Nerves.
c) The capillaries form a looped arrangement at following locations:23-14

15. B. Simple arteriovenous
anastomosis
 It is formed by union of smaller arteries and veins. The connecting
vessels are straight or coiled, have a thick muscular coat and
average diameter of 20 gm. These vessels are richly supplied with
unmylenated sympathetic nerve fibres. Under the influence of
sympathetic activity vessels remain closed and blood passes
through capillary bed in an ordinary way and when they are open
the blood passes directly from arteries into veins.
 Sites:
 i) Skin of Nose.
 ii) Skin of Lips.
 iii) Skin of External Ear.
 iv) Mucous membrane of Nose.
 v) Mucous membrane of alimentary canal.
 vi) Erectile tissue of sexual organs.
 vii) Thyroid gland.
 viii) Coccygeal body.
 ix) Sympathetic ganglia. 23-15

16. C. Complex arterio-venous
Anastomosis or Glomus
 Site: They are seen in skin of hands and feet especially the digital
pads and nail beds. They are situated in dermis of skin.
 An arteriole, just before it gives rise to its capillary network, also
gives rise to 2-4 short tortuous vessels. These tortuous vessels are
anastomotic channels of glomus. They show following peculiarities.
 1) The endothelium forms a cushion like elevation near their origin
from the arteriole. The smooth muscle cells are also developed. This
mechanism serves as a valve.
 2) There is no elastic tissue.
 3) Collagenous & reticulin fibres and short contractile epitheloid
pale staining cells are common.
 4) They open directly into the large venous segments (funnel
shaped vein) which lead in to a vein (collecting vein).
 Functions:
 They are concerned with local regulation of peripheral circulation and
thereby control the temperature and tissue nutrition.
23-16

17. Clinical Significance
 Certain vascular ailments of limbs are due to abnormality in
structure or functioning of glomus mechanism.
 Overall Functions of Arteriovenous Anastomoses:
 1. They regulate blood pressure.
 2. They regulate the secretions of epitheloid cells.
 3. They control pressor reception.
 4. They control local body temperature by relaxation.
(either the temperature is raised or lowered).
 5. They control local nutrition.
 6. They help to increase the pressure in portal vein when
absorption is not occuring in intestine. In this situation
blood directly starts flowing from arterioles to venule in the
villi and by-pass the capillary network.
 7. They help in absorption from intestine. During
absorption the direct arteriovenous connection is closed
and blood passes through capillary network present at the
23-17

18. 23-18
SIGNIFICANCE OF ANASTOMOSES:
1. They help in maintenance of equal pressure in the area supplied by
them.
2. They provide alternate channels of supply to a particular area.
3. Clinically, the anastomoses provide, by their enlargement, the basis of
collateral circulation, when a vessel is occluded by ligation or disease.
MECHANISM OF DILATATION OF ANASTOMOTIC CHANNELS OR
COLLATERAL CHANNELS:
The mechanism of dilatation of anastomotic channel is ill-understood. However,
following factors may play some role:
1. Decrease in peripheral resistance.
2. Hypoxia.
3. Nervous factors.
4. Accumulated metabolites.
5. Younger age.

19. 19

20. 23-20
Blood Vessel Tunics
 Tunica Intima, or Tunica Interna
 innermost layer
 composed of:

an endothelium (simple squamous epithelium)

subendothelial layer (areolar CT)
 Tunica Media
 middle layer of the vessel wall
 composed of:

circularly arranged smooth muscle cells
 Sympathetic innervation:

Increase: vasoconstriction (narrowing of the blood vessel
lumen)

Decrease: vasodilation (widening of the blood vessel lumen)

21. 23-21
Blood Vessel Tunics
Tunica Externa, or Tunica Adventitia
 outermost layer
 composed of:

areolar connective tissue that contains elastic and collagen fibers
 helps anchor the vessel to other tissues
 Term adventitia is used to specify outer layer in blood vessels that
are buried in CT
 Vasa vasorum : blood vessels that supply large blood vessels
 In the externa
 Arteries vs Veins:
 Media largest in arteries, externa largest in veins
 Lumen is smallest in arteries
 Artery wall have more elastic and collagen fibers
 Capillaries: only the Interna

22. 22

23. 23-23
Arteries
 In the systemic circulation, carry oxygenated
blood to the body tissues.
 Pulmonary arteries carry deoxygenated blood
to the lungs.
 Three basic types of arteries:
 elastic arteries, muscular arteries, and arterioles
 as an artery’s diameter decreases

corresponding decrease in the amount of elastic fibers

relative increase in the amount of smooth muscle

24. 24

25. 23-25
Capillaries
 Contain only the tunica intima, but this layer consists
of a basement membrane and endothelium only.
 Allow gas and nutrient exchange between the blood
and the body tissues to occur rapidly.
 Smallest blood vessels, connect arterioles to venules.
 Are called the functional units of the cardiovascular
system.
 A group of capillaries (10–100) functions together and
forms a capillary bed.

26. 26

27. 23-27
The Three Basic Kinds of
Capillaries
 Continuous capillaries
 the most common type
 Fenestrated capillaries
 Sinusoids, or discontinuous capillaries

28. 28

29. 23-29
Veins
 Drain capillaries and return the blood to the heart.
 Walls are relatively thin and the vein lumen is larger.
 Systemic veins carry deoxygenated blood to the right
atrium of the heart, while pulmonary veins carry
oxygenated blood to the left atrium of the heart.
 Blood pressure is substantially reduced by the time
blood reaches the veins.
 Hold about 60% of the body’s blood at rest.
 Veins function as blood reservoirs.

30. 23-30
From Venules to Veins
 Venules merge to form veins.
 Venule becomes a “vein” when its diameter is greater than 100
micrometers.
 Blood pressure in veins is too low to overcome the forces of
gravity.
 To prevent blood from pooling in the limbs, most veins contain
one-way numerous valves to prevent blood backflow in the
veins.
 As blood flows superiorly in the limbs, the valves close to
prevent backflow.
 Numerous valves along its length to assist in moving blood back
to the heart.

31. 23-31
From Venules to Veins
Many deep veins pass between skeletal
muscle groups.
 As the skeletal muscles contract, veins
are squeezed to help pump the blood
toward the heart.
 This process is called the skeletal
muscle pump.

32. 32

33. 23-33
Blood Pressure
 Force/unit area blood places on the
inside wall of a blood vessel.
 Measures in mmHg
 Sphygmomanometer: device to
measure blood pressure.
 Systolic blood pressure
 Diastolic blood pressure
 120/80 mmHg

34. 34

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39. 23-39
Circle of Willis
 An important anastomosis of arteries around the sella turcica.
 Formed from posterior cerebral arteries and posterior
communicating arteries (branches of the posterior cerebral
arteries), internal carotid arteries, anterior cerebral arteries, and
anterior communicating arteries (which connect the two anterior
cerebral arteries).
 Equalizes blood pressure in the brain and can provide collateral
channels should one vessel become blocked.

40. 40

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44. 23-44
Hepatic Portal System
 A venous network that drains the GI tract and shunts the blood
to the liver for processing and absorption of transported
materials.
 Blood exits the liver through hepatic veins that merge with the
inferior vena cava.
 Is needed because the GI tract absorbs digested nutrients, and
these nutrients must be processed and/or stored in the liver.

45. 45

46. 23-46
Pulmonary Circulation
 Responsible for carrying deoxygenated blood from the right side
of the heart to the lungs, and then returning the newly
oxygenated blood to the left side of the heart.
 Blood is pumped out of the right ventricle into the pulmonary
trunk.
 This vessel bifurcates into a left pulmonary artery and a right
pulmonary artery that go to the lungs.

47. 47

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50. 23-50
Aging and the Cardiovascular
System
 Heart and blood vessels become less resilient.
 Elastic arteries are less able to withstand the forces
from the pulsating blood.
 Systolic blood pressure may increase with age.
 Apt to develop an aneurysm, whereby part of the
arterial wall thins and balloons out.
 Wall is more prone to rupture, which can cause
massive bleeding and death.
 Incidence and severity of atherosclerosis increases.

51. 23-51
Fetal Circulation
 Oxygenated blood from the placenta enters through the
umbilical vein.
 Blood is shunted away from the liver and directly toward the
inferior vena cava through the ductus venosus.
 Oxygenated blood in the ductus venosus mixes with
deoxygenated blood in the inferior vena cava.
 Blood empties into the right atrium.
 Most of the blood is shunted to the left atrium via the foramen
ovale.
 Blood flows into the left ventricle and out the aorta.
 A small amount of blood enters the right ventricle and
pulmonary trunk, but much of this blood is shunted to the aorta
through ductus arteriosus.
 Blood travels to the rest of the body, and the deoxygenated
blood returns to the placenta through umbilical arteries.

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