Cardiovascular System- Blood Vessels (Pt. 1)

Five main types of blood vessels:

• arteries

• arterioles

• capillaries

• venules

• veins

Arteries

Carry blood away from heart to other organs

Arterioles

• branches from arteries divide into smaller branches to make these

• delivers blood to capillaries

Blood Capillaries

• As arterioles enter tissues, they branch out smaller into these

• located between arterioles and venules

• thin walls allow substances to exchange between blood and body tissues

Venules

• small veins (reunited groups of capillaries within tissue)

• collects blood from capillaries to deliver to veins

Veins

• formed by venules

• convey blood from tissues back to heart

Walls of blood vessels have three layers:

1. epithelial inner lining (Tunica intima)

2. middle smooth muscle and elastic CT (Tunica media)

3. CT external layer (Tunica externa)

Tunica Intima (interna)

• in direct contact with blood as it flows through lumen (interior opening) of vessels

• Innermost layer is endothelium

• middle layer is basement membrane

• outermost layer is internal elastic membrane

Endothelium (intima)

• continuous within endothelial lining of heart

• thin layer of flattened cells

• lines inner surface of entire cardiovascular system

• physically influences blood flow

• allows secretions of locally acting chemical mediators (contractile state of vessels)

• assists with capillary permeability

• smooth luminal surface facilitates efficient blood flow by reducing surface friction

Basement Membrane (intima)

• deep to endothelium (anchors to endothelium)

• provides structural support base for epithelial layer

• framework of collagen fibers gives it tensile strength

• stretching, recoil and resilience 

• guides cell movements during tissue repair of vessel walls

Internal elastic membrane (intima)

• forms boundary between intima and media

• thin sheet of elastic fibers with openings (looks like Swiss cheese)

• openings facilitate diffusion of materials through tunica intima into tunica media

Tunica Media

• Muscular and CT layer

• greatest variation among vessel types

• in most vessels it is relatively thick comprised mainly of smooth muscle cells and many elastic fibers

Primary role of smooth muscle cells in tunica media is to…

Regulate diameter of lumen for blood flow and blood pressure

• increase in sympathetic stimulation causes smooth muscle to contract (vasoconstriction)

• decrease in sympathetic stimulation causes smooth muscle to relax (vasodilation)

Smooth muscle will also contract when…

small arteries or arterioles are damaged (vascular spasm)

• limits loss of blood flow through injured vessels

• It can also help produce elastic fibers (helps with stretch and recoil)

Tunica Externa

• consists of elastic and collagen fibers

• contains numerous nerves

• contains ESPECIALLY in large blood vessels, tiny blood vessels that supply tissue of the vessel wall

• anchors vessels to surrounding tissues

Vasa Vasorum

Small vessels that supply blood to tissues of larger vessels

Arteries have a thick…

muscular-to-elastic tunica media.

• they normally have high compliance, allowing their walls to stretch easily or expand without tearing in response to small increases in pressure

Elastic Arteries

• largest arteries in body ranging from aorta and pulmonary trunk to finger sized branches of aorta

• largest diameter among arteries

• relatively thin walls compared to size of the vessel itself

• well-defined internal and external elastic membranes with thick tunica media dominated by elastic fibers (called elastic lamellae)

Elastic arteries include two major trunks that exit the heart:

aorta and pulmonary trunk

Aortas major initial branches include:

• brachiocephalic

• subclavian

• common carotid

• common iliac

Function of Elastic arteries

Help propel blood onward while ventricles relax

• as blood ejects from heart into elastic arteries, the walls stretch to accommodate surge of blood

• Stretching of walls causes elastic fibers to store mechanical energy (pressure reservoir)

• Elastic fibers recoil and convert stored energy in vessel into kinetic energy of blood

• blood continues to move through arteries even when ventricles are relaxed

• also called conducting arteries because they conduct blood from heart to more muscular arteries

Muscular Arteries

• medium sized arteries

• tunica media contains more smooth muscle and fewer elastic fibers

• capable of greater vasoconstriction and vasodilation to adjust rate of blood flow

• well-defined internal elastic membrane and thin external elastic membrane

Size of Muscular arteries

• pencil-sized femoral and axillary

• string-sized arteries that enter organs

• vessel wall comprises larger percentage (25%) of total diameter

Muscular arteries are also called…

Distributing arteries, because they distribute blood to each of the organs

• Ex: Brachial artery in arms

• Ex: Radial artery in forearm

Tunica externa of muscular arteries

• often thicker than tunica media

• contains fibroblasts, collagen fibers, and elastic fibers (oriented longitudinally)

• loose structure allows changes in diameter, but prevents shortening or retraction when cut

There is less elastic tissue in the muscular artery walls, so…

the vessels do not propel blood or recoil like elastic arteries can

• instead, thick muscular tunica media is primarily responsible for functions of muscular arteries

Vascular tone

ability of muscle to contract and maintain state of partial contraction

• stiffens vessel wall and helps maintain vessel pressure and blood flow

Anastomosis

Union of branches of two or more arteries supplying the same region of the body

• alternative route of blood flow through anastomoses are called collateral circulation

• arteries that do not anastomose are called end arteries

Arterioles

• small arteries that are very abundant

• regulate blood flow into capillary networks of body tissues

Layers of arterioles

• Thin tunica intima with thin fenestrated internal elastic membrane disappearing at terminal end

• Tunica media with one to two layers of smooth muscle cells having circular orientation in vessel wall

• Tunica externa consists of areolar CT containing abundant unmyelinated sympathetic nerves (can alter diameter of arterioles)

Metarteriole

• region located at terminal end of arteriole

• tapers toward capillary junction

• empties into venules

Precapillary Sphincter

• formed at the metarteriole-capillary junction by the distal-most muscle cell

• regulates blood flow into capillaries

• other muscle cells regulate resistance to blood flow

Arterioles are also known as…

Resistance vessels, because they regulate blood flow from arteries into capillaries

• resistance is the opposition to blood flow due to friction between blood and walls of vessels

• contraction of smooth muscle (causing vasoconstriction) increases resistance and decreased blood flow into capillaries

• Relaxation of smooth muscle (causing vasodilation) decreases resistance and increases blood flow to capillaries

Vasoconstriction of arterioles causes an…

increase in blood pressure

Vasodilation of arterioles causes a…

decrease in blood pressure

Capillaries

• smallest of blood vessels

• forms U-turns that connect arterial outflow to venous return

• have thin walls

• found near almost every cell in the body

Microcirculation

Flow of blood from metarterioles through capillaries into a postcapillary venule

Function of Capillaries

Exchange of substances between blood and interstitial fluid

• called exchange vessels

Body tissues with high metabolic requirements like the muscles, brain, liver, kidneys, and nervous system use more…

O2 and nutrients and thus have extensive capillary networks

Tissues with lower metabolic requirements, such as tendons and ligaments, contain…

fewer capillaries

Capillaries are absent in a few tissues such as…

all covering and lining epithelia, cornea and lens of eyes, and cartilage.

Capillaries lack…

the tunica media and tunica externa layers

Capillary walls are composed of…

a single layer of epithelial cells and a basement membrane

• substances in blood only have to pass through one cell layer to reach interstitial fluid and tissue cells

Exchange of materials only occurs through…

walls of capillaries and the beginning of venules

• walls of arteries, arterioles, most venules and veins are too thick

In most tissues, blood flows through only…

a small part of the capillary network when metabolic needs are low

• when tissues are active, like a contracting muscle, the entire capillary network fills with blood

Capillary bed

network consisting of 10-100 capillaries that arise from a single metarteriole

Blood flows through the capillary, from an arteriole into a venule in two ways (READ)

1. Capillaries: In this route, blood flows from an arteriole into capillaries and then into venules. At the junctions between the metarteriole and the capillaries are rings of smooth muscle fibers called precapillary sphincters that control the flow of blood through the capillaries. When the precapillary sphincters are relaxed (open), blood flows into the capillaries. When precapillary sphincters contract (close or partially close), blood flow through the capillaries ceases or decreases. Typically, blood flows intermittently through capillaries due to alternating contraction and relaxation of the smooth muscle of metarterioles and the precapillary sphincters. This intermittent contraction and relaxation, which may occur 5 to 10 times per minute, is called vasomotion. In part, vasomotion is due to chemicals released by the endothelial cells; nitric oxide is one example. At any given time, blood flows through only about 25% of the capillaries.

2. Thoroughfare channel: The proximal end of a metarteriole is surrounded by scattered smooth muscle fibers whose contraction and relaxation help regulate blood flow. The distal end of the vessel has no smooth muscle; it resembles a capillary and is called a thoroughfare channel. Such a channel provides a direct route for blood from an arteriole to a venule, thus bypassing capillaries.

Three types of capillaries:

1. Continuous

2. fenestrated

3. sinusoids

Continuous Capillaries

• Most capillaries are these

• plasma membranes of endothelial cells form a continuous tube only interrupted by intercellular clefts

• found in central nervous system, lungs, muscle tissue and skin

Fenestrated Capillaries

• plasma membranes of endothelial cells have many small pores (fenestrations)

• found in kidneys, intestinal villi, choroid plexuses of ventricles in brain, ciliary processes of eyes and most endocrine glands

Sinusoids

• wider and more winding than other capillaries

• endothelial cells may have large fenestrations

• have an incomplete or absent basement membrane

• very large intercellular clefts that allow proteins and blood cells to pass from a tissue into bloodstream

Newly formed blood cells enter bloodstream through…

sinusoids of red bone marrow

Sinusoids in liver contain…

phagocytic cells that remove bacteria and other debris from blood

• spleen, anterior pituitary, parathyroid and suprarenal glands also have sinusoids

Sequence that blood passes

From the heart→ arteries→ arterioles→ capillaries→ venules→ veins→ back to the heart

Portal system

Circulation of blood from one capillary network into another via portal veins

• names of portal systems give the location of the second capillary location (hepatic portal circulation is in the liver)

Venules

• have thin walls which do not maintain shape

• Drain the capillary blood and begin return of blood flow back to the heart

Venules receive blood initially from…

Capillaries called postcapillary venules

Postcapillary venules

• smallest of the venules 

• have loosely organized intercellular junctions (weakest endothelial contacts on vascular tree)

• very porous

• significant sites of nutrient and waste exchange

• also, a site of white blood cell emigration

Postcapillary venules acquire one or two layers of…

circular smooth muscle fibers, making them muscular venules

Muscular Venules

• thicker walls 

• cannot exchange with interstitial fluid anymore

The thin walls of postcapillary and muscular venules are the most…

Distensible elements of the vascular system, allowing them to serve as reservoirs to hold a lot of blood

• blood volume increases 360%

Veins

• Tunica intima is thinner than arteries with few smooth muscle and elastic fibers

• Tunica externa is thickest layer, consisting of collagen and elastic fibers

• lack the internal or external elastic membrane

• not designed to withstand high pressure

• lumen is larger than artery

Pumping action of the heart is what…

brings venous blood back to the heart

• contractions in skeletal muscle also helps boost the venous blood

Difference in pressure can be determined by a cut in the vessel:

• in veins, blood will slowly move out of the vessel

• in arteries, blood spurts out of the vessel

• this is because the walls of veins are not as strong as the walls of arteries

Many veins contain…

valves, which are thin folds of tunica intima that form flaplike cusps

• Valve cusps will project into lumen, pointing toward heart

• prevents backflow of blood

Vascular sinus

vein with a thin endothelial wall with no smooth muscle

• surrounding dense CT replaces tunica media and externa in providing support

Some veins are…

paired and accompany medium or small muscular arteries

• the double sets of veins escort arteries to connect with others by venous channels called anastomotic veins

• majority of paired veins are found in the limbs

Anastomotic veins

cross the accompanying artery to form ladderlike rungs between paired veins

Superficial veins

• found in subcutaneous tissue deep to skin

• course through the subcutaneous tissue unaccompanied by parallel arteries

• form small connections with deep veins, which travel between skeletal muscles

Connections of deep and superficial veins…

allows communication between deep and superficial flow of blood

Superficial veins in the upper limb…

• are much larger than the deep veins

• serve as major pathways from capillaries of upper limb back to heart

Deep veins in the lower limb…

return blood from capillaries back to the heart

One-way valves in small anastomosing vessels allow…

blood to pass from superficial veins to the deep veins, but prevent it from flowing the opposite direction

Percentages of blood volume at rest:

• systemic veins and venules (64%)

• systemic arteries and arterioles (13%)

• systemic capillaries (7%)

• pulmonary blood vessels (9%)

• heart (7%)

Venoconstriction

Constriction of veins

• reduces volume of blood in reservoirs

• allows greater blood flow to skeletal muscles when needed

• In cases of hemorrhages, when blood volume and pressure decrease, it helps counteract drop in blood pressure

Principal blood reservoirs are veins of the:

• abdominal organs (liver and spleen)

• skin

Capillary exchange

movement of substances between blood and interstitial fluid

• 7% of blood continually exchanges materials with interstitial fluid 

• substances enter and leave capillaries by diffusion, transcytosis and bulk flow

Diffusion in capillary exchange

O2, CO2, glucose, amino acids, and hormones enter and leave capillaries

• O2 and nutrients are highly concentrated in blood then diffuse down concentration grad. into interstitial fluid and then into body cells

• CO2 and other wastes released by body cells are highly concentrated in interstitial fluid, so they diffuse into blood