The blood vessels of the body form a closed delivery system that begins and ends at the heart

Often compared to a plumbing system, it is a far more dynamic system of structures that pulse, constrict and relax and even proliferate to meet changing body needs

The major types of blood vessels are:◦ Arteries

The large distributing vessels that bring blood to the body ◦ Capillaries

The tiny vessels that distribute blood to the cells◦ Veins

The large collecting vessels that bring blood back to the heart

Intermediate vessels connect◦ Arterioles bring blood to the capillaries◦ Venules drain blood from the capillaries

The pattern of distribution starts with arteries to arterioles to capillaries to venules to veins

The blood vessels in the adult human body carry blood in a distribution network that is approximately 60,000 miles in length

Only capillaries come into intimate contact with tissue cells and serve cellular needs

• The walls of blood vessels are composed of three distinct layers or tunics 1.Tunica Intima 2.Tunica Media 3.Tunica Adventitia The tunics surround a central opening called a lumen

The innermost tunic is the tunica intima This tunic contains the endothelium, the simple

squamous endothelium that lines all vessels Its flat cells fit closely together, forming a slick

surface that minimizes friction as blood moves through the vessel lumen

In blood vessels larger than 1 mm in diameter, a sub- endothelial layer of loose connective tissue, subendothelial layer, (basement membrane) supports the endothelium

The middle tunic, the tunica media, is mostly circularly arranged smooth muscle cells and sheets of elastin

The activity of the smooth muscle is regulated by vasomotor nerve fibers of the sympathetic division of the autonomic nervous system

Tunica Media

Depending on the needs of the body, the vasomotor fibers can cause vaso-constriction or vasodilation

The activities of the tunica media are critical in regulating circulatory dynamics

Generally, the tunica media is the bulkiest layer in arteries, which bear the chief responsibility for maintaining blood pressure and continuous blood circulation

The outermost layer of a blood vessel is the tunica externa or tunica adventitia

This tunic is composed largely of loosely woven collagen fibers that protect blood vessels and anchor it to surrounding structures

Tunicaexterna

The tunica externa is infiltrated with nerve fibers and lymphatic vessels and, in larger vessels, a system of tiny blood vessels

These vessels, the vasa vasorum nourish the external tissues of the blood vessel wall

Arteries are vessels that carry blood away from the heart

All arteries carry oxygen rich blood with the exception of those in the pulmonary circuit

Blood proceeds to the tissues through these arteries, which are classified into three types: ◦ Elastic arteries◦ Muscular arteries◦ Arterioles

Elastic arteries are thick walled arteries near the heart - the aorta and its major branches

These arteries are the largest in diameter and the most elastic

A large lumen allows them to serve as low resistance pathways that conduct blood from the heart to medium-sized arteries and thus are called conducting arteries

The elastic arteries contain more elastin than any other type of vessel

While present in all three layers, the tunica media contains the most

The abundant elastin enables these arteries to withstand and smooth out large pressure fluctuations by expanding when the heart forces blood into them and then recoiling to propel blood onward into the circulation when the heart relaxes

Elastic arteries also contain substantial amounts of smooth muscle, but they are relatively inactive in vasoconstriction

Because elastic arteries expand and recoil passively to accommodate changes in blood volume, the blood is kept under pressure

Thus, blood flows continuously rather than starting and stopping with each heart beat

The muscular distributing arteries deliver blood to specific body organs and account for most of the named arteries

Proportionately, they have the thickest media of all vessels

Their tunica media contains relatively more smooth muscle and less elastic tissue than that of elastic arteries

They are more active in vasoconstriction and are less distensible

As in all vessels, concentric sheets of elastin occur within the tunica media of muscular arteries although these sheets are not as thick or abundant as those of elastic arteries

A feature unique to muscular arteries, especially thick sheets of elastin lie on each side of the tunica media

An external elastic lamina lies between the tunica media and tunica externa

The elastin in muscular arteries, like that in elastic arteries, helps dampen the pulsatile pressure produced by the heartbeat

Arterioles have a lumen diameter from 0.3 mm to 10 µm, and are the smallest of the arteries

Larger arterioles exhibit all three tunics, but their tunica media is chiefly smooth muscle with a few scattered muscle fibers

The smaller arterioles that lead into capillary beds, are little more than a single layer of smooth muscle cells spiraling around the endothelial lining

The diameter of each arteriole is regulated in two ways:◦ Local factors in the tissues signal the smooth

musculature to contract or relax, thus regulating the amount of blood sent downstream to each capillary bed◦ Sympathetic nervous system adjusts the diameter of

arterioles throughout the body to regulate systemic blood pressure

The microscopic capillaries are the smallest blood vessels

In some cases, one endothelial cell forms the entire circumference of the capillary wall

The average length of a capillary is 1 mm and the average diameter is 8-10 µm

Capillaries have a lumen just large enough for blood cells to slip through in single file

Capillaries are the body’s most important blood vessels because they renew and refresh the surrounding tissue fluid (interstitial fluid) with which all cells in the body are in contract

Capillaries deliver to interstitial fluid the oxygen and nutrients that cells need while removing carbon dioxide and nitrogenous wastes that cells deposit in the fluid

Given their location and the thinness of their walls capillaries are ideally suited for their role of providing access to nearly every cell

Along with the universal functions just described some capillaries also perform site-specific functions◦ Lungs: gas exchanges◦ Endocrine glands: pick up hormones◦ Small intestine: nutrients◦ Kidneys: removal of nitrogenous wastes

A capillary bed is a network of the body’s smallest vessels that run throughout almost all tissues, especially the loose connective tissue

This flow is also called a “microcirculation”

The capil lary bed consist of two types of vessels: Shunt vessels(met-arterioles) and true capillaries

The terminal arteriole leads into a metarteriole which is directly continuous with the thorough- fare channel

The thoroughfare channel joins the post- capillary venule that drains the capillary bed

The true capillaries number 10 to 100 per capillary bed, depending on the organ served

Branch from metarteriole to thoroughfare channel

A cuff of smooth muscle fibers, called a pre- capil lary sphincter surrounds the root of each capillary at the metarteriole and acts as a valve to regulate the flow of blood into the capillary

When the pre-capillary sphincters are relaxed, blood flows through the true capillaries and takes part in exchanges with tissue cells

When the pre-capillary sphincters are contracted, blood flows through the shunts and bypasses the tissue cells

Most tissues have a rich supply, but there are a few exceptions◦ Tendons and ligaments / poorly vascularized◦ Cartilage / from adjacent connective tissue◦ Epithelia / from adjacent connective tissue◦ Cornea / nourished by aqueous humor

The relative amount of blood entering a capillary bed is regulated by vasomotor nerve fibers and local chemical conditions

A capillary bed may be flooded with blood or almost completely bypassed, depending on conditions in the body or in that specific organ

Example of shunting blood from digestive organs to skeletal muscles

The structure of capillaries is well suited for their function in the exchange of nutrients and wastes between the blood and the tissues through the tissue fluid

A capillary is a tube consisting of thin endothelial cells surrounded by a basal lamina

The endothelial cells are held together by tight junctions and occasional desmosomes

Tight junctions block the passage of small molecules, but such junctions do not surround the whole perimeter of the endothelial cells

Instead, gaps of unjoined membrane called intercellular clefts occur through which small molecules exit and enter the capillary

External to the endothelial cells, the delicate capillary is strengthened and stabilized by scattered pericytes

The pericytes are spider shaped cells whose thin processes form a network that is widely spaced so as to not to interfere with capillary permeability

Structurally there are three types of capillaries◦ Continuous◦ Fenestrated◦ Sinusoidal

Continuous capillaries are abundant in the CNS, skin and muscles and are the most common

They are continuous in the sense that their endothelial cells provide an uninterrupted lining

Adjacent cells are joined laterally by tight junctions

However, these are usually incomplete and leave gaps of unjoined membrane called intracellular clefts that are just large enough to allow limited passage of fluids

Fenestrated capillaries have fenestrations (pores) spanning the endothelial cells

Fenestrated capillaries occur only where there are exceptionally high rates of exchange of small molecules between blood and the surrounding tissue

The fenestrations are usually covered by a thin diaphragm but this variety has much greater permeability to fluids and small solutes

Fenestrated capillaries are found where active capillary absorption or filtrate formation occurs

Fenestrated capillaries are found in the small intestine to receive digested nutrients

Intercellularclefts

These capillaries are also found in the synovial membranes of joints to allow water molecules to exit the blood to form synovial fluid

Molecules pass into and out of capillaries via four routes◦ Direct diffusion through endothelial cell membranes◦ Through the intercellular clefts◦ Through cytoplasmic vesicles or caveolae◦ Through fenestrations in fenestrated capillaries

Most exchange of small molecules is thought to occur through intercellular clefts

Caveolae apparently transport a few larger molecules, such as small proteins

Carbon dioxide and oxygen seem to be the only important molecules that diffuse directly through endothelial cells because these uncharged molecules easily diffuse through lipid containing membranes of cells

The blood-brain barrier prevents all but the most vital molecules(even leukocytes) from leaving the blood and entering brain tissue

The blood-brain barrier derives its structure from the capillaries of the brain

Brain capillaries have complete tight junctions, so intercellular clefts are absent

Brain capillaries are continuous, not fenestrated and they also lack caveolae

Vital capillaries that must cross brain capillaries are “ushered through” by highly selective transport mechanisms in the plasma membranes of the endothelial cells

Some organs contain wide, leaky capillaries called sinusoids

Each sinusoid follows a twisted path and has both expanded and narrowed regions

Sinusoids are usually fenestrated and their endothelial cells have fewer cell junctions than do ordinary capillaries

In some sinusoids the intercellular cleft is wide open

Sinusoids occur wherever there is an extensive exchange of large materials, such as proteins or cells, between the blood and surrounding tissue

Sinusoids are found in only in bone marrow and spleen, where many blood cells move through their walls

The large diameter and twisted course of sinusoids ensure that blood slows when flowing through these vessels, allowing time for the many exchanges that occur across their walls

Veins are the blood vessels that conduct blood from the capillaries back to the heart

Because blood pressure declines substantially while passing through the high-resistance arterioles and capillary beds, blood pressure in the venous part of the circulation is much lower than in the arterial part

Because they need not withstand as much pressure, the walls of veins are thinner than those of comparable arteries

The venous vessels increase in diameter, and their walls gradually thicken as they progress from venules to the larger and larger veins leading to the heart

Venules, ranging from 8 to 100 µm in diameter are formed when capillaries unite

The smallest venules, the postcapillary venules, consist of endothelium on which lie pericytes

Venules join to form veins With their large lumens and thin walls, veins can

accommodate a fairly large blood volume Up to 65%of the body’s total blood supply is found

in the veins at any one time although the veins are normally only partially filled with blood

Veins have three distinct tunics, but their walls are always thinner and their lumens larger than those of corresponding arteries

There is little smooth muscle even in the largest veins

The tunica externa is the heaviest wall layer and is often several times thicker than the tunica media

In the venae cavae, the largest veins, which return blood directly to the heart the tunica externa is further thickened by longitudinal bands of smooth muscle

Tunica externa

Veins have less elastin in their walls than do arteries, because veins do not dampen any pulsations (these have been smoothed out by the arteries)

Because blood pressure within veins is low, they can be much thinner walled than arterioles without danger of bursting

Low-pressure conditions demand some special adaptations to help return blood to the heart at the same rate as it was pumped into circulation

One structural feature that prevents the backflow of blood away from the heart is the presence of valves within veins

Venous valves are formed from folds of the tunica intima and they resemble the semilunar valves of the heart in structure and function

Venous valves are most abundant in the veins of the limbs, where the upward flow of blood is opposed by gravity

A few valves occur in the veins of the head and neck, but none are located in veins of the thoracic and abdominal cavities

A functional mechanism that aids the return of venous blood to the heart is the normal movement of our body and limbs

Another mechanism of venous return is called the skeletal muscular pump

Here contracting muscles press against the thin-walled veins forcing valves proximal to the contraction to open and propelling the blood toward the heart

Where vessels unite or interconnect, they form vascular anastomoses

Most organ receive blood from more than one arterial branch and arteries supplying the same area often merge, forming arterial anastomoses

Arterial anastomoses provide alternative pathways called collateral channels for blood to reach a given body region

If one arterial branch is blocked arterial anastomoses provide the region with an adequate blood supply

Arterial anastomoses are abundant in abdominal organs and around joints, where active movement may hinder blood flow through one channel

Anastomoses are also prevalent in the abdominal organs, brain, and heart

Because of the many anastomoses among the smaller branches of the coronary artery in the heart wall, a coronary artery can be 90% occluded by atherosclerosis (plaque) before a myocardial infarction (heart attack) occurs

Arteries that do not anastomose, or which have a poorly developed collateral circulation (retina, kidneys, spleen) may be vulnerable if their blood flow is interrupted

Veins anastomoses much more freely than arteries and because of abundant collateral circulation occlusion of a vein rarely blocks blood flow leading to tissue death

The wall of the blood vessels contain living cells and therefore require a blood supply of their own

For this reason the larger arteries and veins have tiny arteries, capillaries and veins in their tunica externa

These tiny vessels the vasa vasorum nourish the outer half of the wall of a large vessel with the inner half being nourished by the blood in the lumen