Circulation

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Human Anatomy of Vessels and Circulation

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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.

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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.

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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.

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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.

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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

ANASTOMOSIS (G. Anestomosis : opening outlet)

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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.

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

(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

(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.

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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.

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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

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.

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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

25 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.g.: 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: i) Vascular papillae of skin. ii) Tongue.

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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.

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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

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 tips of the villi and hence promotes absorption of nutrient material.

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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.

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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)

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Blood Vessel TunicsTunica 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

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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

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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.

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The Three Basic Kinds of Capillaries Continuous capillaries

the most common type Fenestrated capillaries Sinusoids, or discontinuous capillaries

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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.

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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.

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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.

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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

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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.

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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.

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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.

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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.

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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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