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
Blood Vessel Structure & Function
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
Blood Vessel Structure & Function
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
Structure of Blood Vessel Walls
Blood Vessel Walls
The walls of blood vessels are composed of three distinct layers or tunics
The tunics surround a central opening called a lumen
Blood Vessel Walls
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
Blood Vessel Walls
In blood vessels larger than 1 mm in diameter, a sub- endothelial layer of loose connective tissue, subendothelial layer, (basement membrane) supports the endothelium
Blood Vessel Walls
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
Blood Vessel Walls
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
Blood Vessel Walls
The outermost layer of a blood vessel is the tunica externa
This tunic is composed largely of loosely woven collagen fibers that protect blood vessels and anchor it to surrounding structures
Blood Vessel Walls
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
Arteries are vessels that carry blood away from the heart
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
Elastic (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 (Conducting) Arteries
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
Muscular (Distributing) Arteries
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
Muscular (Distributing) Arteries
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
Muscular (Distributing) 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
Muscular (Distributing) Arteries
The elastin in muscular arteries, like that in elastic arteries, helps dampen the pulsatile pressure produced by the heartbeat
Arterioles
Arterioles have a lumen diameter from 0.3 mm to 10 m, 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
Arterioles
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
Capillaries
The microscopic capillaries are the smallest blood vessels
In some cases, one endothelial cell forms the entire circum- ference of the capillary wall
The average length of a capillary is 1 mm and the average diameter is 8-10 m
Capillaries
Capillaries have a lumen just large enough for blood cells to slip through in single file
Capillaries
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
Capillaries
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
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
Capillary Beds
In most body regions, a capillary bed consists of two types of vessel a vascular shunt (meta- arteriole) and true capillaries
Capillary Beds
The terminal arteriole leads into a metarteriole which is directly continuous with the thorough- fare channel
Capillary Beds
The thoroughfare channel joins the post- capillary venule that drains the capillary bed
Capillary Beds
The true capillaries number 10 to 100 per capillary bed, depending on the organ served
Branch from metarteriole to thoroughfare channel
Capillary Beds
A cuff of smooth muscle fibers, called a pre- capillary sphincter surrounds the root of each capillary at the metarteriole and acts as a valve to regulate the flow of blood into the capillary
Capillary Beds
When the precapillary sphincters are relaxed, blood flows through the true capillaries and takes part in exchanges with tissue cells
Capillary Beds
When the precapillary sphincters are contracted, blood flows through the shunts and bypasses the tissue cells
Capillary Beds
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
Capillary Beds
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
Capillary Permeability
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
Capillary Permeability
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
Capillary Permeability
External to the endothelial cells, the delicate capillary is strengthened and stabilized by scattered pericytes
Capillary Permeability
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
Capillary Permeability
Structurally there are three types of capillaries
Continuous
Fenestrated
Sinusoidal
Continuous Capillaries
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
Continuous Capillaries
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 occur only where there are exceptionally high rates of exchange of small molecules between blood and the surrounding tissue
Fenestrated Capillaries
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
Fenestrated capillaries are found in the small intestine to receive digested nutrients
These capillaries are also found in the synovial membranes of joints to allow water molecules to exit the blood to form synovial fluid
Routes of Capillary Permeability
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
Routes of Capillary Permeability
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
Low Permeability Capillaries
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
Vital capillaries that must cross brain capillaries are “ushered through” by highly selective transport mechanisms in the plasma membranes of the endothelial cells
Sinusoidal Capillaries
Some organs contain wide, leaky capillaries called sinusoids
Each sinusoid follows a twisted path and has both expanded and narrowed regions
Sinusoidal Capillaries
Sinusoids are usually fenestrated and their endothelial cells have fewer cell junctions than do ordinary capillaries
Sinusoidal 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
Sinusoidal Capillaries
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
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
Veins
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
Venules, ranging from 8 to 100 m in diameter are formed when capillaries unite
The smallest venules, the postcapillary venules, consist of endothelium on which lie pericytes
Venules
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
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
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
Veins
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
Veins
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
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
Veins
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
Veins
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
Vascular Anastomoses
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
Vascular Anastomoses
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
Vascular Anastomoses
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
Vascular Anastomoses
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
Vasa Vasorum
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
