20.1 Structure and Function of Blood Vessels

Blood vessels are classified into 3 primary types based on

function

Arteries

transport blood AWAY from the heart to the capillaries.

Capillaries

Microscopic, relatively porous blood vessels from the exchange of substances between blood and tissues.

Veins

drain blood from the capillaries, transporting it back towards the heart.

Vessel walls are composed of layers called

tunics.

The tunics surround

the lumen

Lumen

is the inside space of the vessel through which blood flows.

3 tunics of the vessel

• tunica intima

• tunica media

• tunica externa

Tunica intima

The innermost layer of a blood vessel wall.

• composed of an endothelium(a simple squamous epithelium) that is adjacent to the blood vessel lumen and a thin sub endothelial layer of areolar connective tissue.

The endothelium of the tunica intima

provides a smooth surface as the blood moves through the lumen of the blood vessel and releases substances(nitric oxide, endothelin) to regulate contraction and relaxation of smooth muscle within the tunica media.

Tunica media

The middle layer of the vessel wall.

Composed predominantly of a circularly arranged layers of smooth muscle cells that are supported by elastic fibers.

contraction of smooth muscle in the tunica media results in vasoconstriction and relaxation of the smooth muscle causes vasodilation or widening of the blood vessel lumen.

Tunica externa/tunica adventitia

The outermost layer of the blood vessel walls

composed of areolar connective tissue that contains elastic and collagen fibers.

Helps anchor the vessel to other structure.

Very large blood vessels require their own blood supply to the tunica externa in the form of a network of small arteries called

the vasa vasorum, it extends through the tunica externa.

Companion vessels

Arteries and veins that supply the same body region and tend to lie next to one another.

Arteries have _______

• thicker tunica media

• a narrower lumen

• more elastic and collegen fiber

These differences mean that arterial walls can expand and then recoil and are more resilient and resistant to changes in blood pressure than are veins.

Artery also remain patent even without blood in it.

Veins have ____

• thicker tunica externa

• wider lumen

• less elastic and collagen fibers

than a companion artery

The walls of a vein is typically

collapsed if no blood is in the vessel.

Capillaries are unique in that they

contain only the tunica initima composed of an endothelium and its underlying basement membrane; there is no subendothelial layer.

Having this thin barrier allows for rapid gas and nutrient exchange between the blood in capillaries and the tissues.

Arteries progressively branch into ___ as they extend from the heart to the capillaries

smaller vessels.

There is both a corresponding decrease in decrease in diameter and a change in the composition of the tunic wall that includes both a decrease in relative amount of elastic fibers and an increase in the relative amount of smooth muscle.

3 classifications for arteries:

• elastic arteries

• muscular arteries

• arterioles

Elastic arteries

• largest arteries

• vessel diameters (widths) ranging from 2.5 to 1 centimeters

• Also called conducting arteries because they conduct blood away from the heart to the smaller muscular arteries.

• Have a large proportion of elastic fibers; present throughout all 3 tunics, especially in the tunica media

• the elastic fibers allow the artery to stretch and accommodate the blood when a heart ventricle ejects blood into it during ventricular systole (contraction) and then recoil, which helps propel the blood through the arteries during ventricular diastole (relaxation)

The largest arteries close to the heart

• aorta

• pulmonary trunk

• brachiocephalic

• common carotid

• subclavian

example of elastic arteries

the common iliac arteries

elastic arteries branch into

muscular arteries

Muscular arteries

• diameters ranging from 1 centimeter to 0.3 millimeter.

• also called disturbing arteries because they distribute blood to specific body regions and organs

• have proportionately thicker tunica media, which multiple layers of smooth muscle cells.

• the elastic fibers in muscular arteries are confined to 2 circumscribed sheets, which include the internal elastic lamina and the external elastic lamina.

• the relatively greater amount of smooth muscle and lesser amount of elastic fibers result in better ability to vasoconstriction and vasodilate, although with a lessened ability to stretch in comparison to elastic arteries.

examples off muscular arteries

• branchial

• anterior tibal

• coronary

• inferior mesenteric

Muscular arteries branch into

arterioles

Arterioles

• smallest arteries

• diameters ranging from 0.3 millimeters to 10 micrometers

• vessels are not named

• have fewer than 6 layers of smooth muscle in their tunica media.

• Larger arterioles have all 3 tunics

• smallest arterioles may have a tunica intima surrounded by a single layer of smooth muscle cells.

Smooth muscle in small muscular arterioles and the arterioles is

slightly contracted.

Vasomotor tone

The contracted state in blood vessels

• regulated by the vasomotor center in the medulla oblongata

Motor neurons of the _____ of the autonomic nervous system (ANS) extend from the ____ to the smooth muscle in the tunica media of small muscular arteries and arterioles.

sympathetic division; vasomotor center

Sympathetic tone

the continuous relaying of motor output along sympathetic neurons, and is responsible in maintaining vasomotor tone.

From this slightly contracted state the vasomotor center can

cause the blood vessels to be either vasoconstrictor to a greater degree to decrease blood flow to allow less blood into a tissue or vasodilator to allow more blood into a tissue.

Capillaries (In detail)

• smallest blood vessels

• connect arterioles to venues (the smallest veins)

• is approximately 1mm in length

• diameter of 9 to 10 micrometers, just slightly larger than the diameter of a single erythrocyte.

• consist solely of the endothelium, which is composed of a simple squamous epithelium resting on a basement membrane.

• The narrow vessel diameter and the thin wall are optimal for exchange of substances between blood and body tissues.

Capillaries are differentiated based on

relative degree of permeability

Types of capillaries

• continuous capillaries

• fenestrated capillaries

• sinusoids

Continuous capillaries

• the most common type of capillary.

• Endothelial cells form a complete, continuous lining around the lumen that rests on a complete basement membrane.

• Tight junctions secure endothelial cells to one another; however, they do not form a complete “seal”. The gaps between endothelial cells are called intercellular clefts.

• Materials can move into or out of the blood either through endothelial cells by membrane transport processes(diffusion, pinocytosis) or between endothelial cells through intercellular clefts by diffusion and bulk flow.

Intercellular clefts

The gaps between the endothelial cells

The size of intercellular clefts prevents

the movement of large substances from the blood, including formed elements(erythrocytes, leukocytes, and platelets) and plasma proteins, while allowing the movement of plasma containing small substances (smaller than 5 nanometers), such as water, glucose, amino acids, and ions.

Continous capillaries are found in

most tissues

ex: in muscle, the skin, the thymus, the lungs, and the brain and spinal cord.

Fenestrated capillaries

• composed of a complete, continuous lining of endothelial cells and a complete basement membrane with intercellular clefts between the cellls

• small regions of the endothelial cells (typically 10 to 100 nanometers in diameter) are extremely thin; these thin areas are called fenestrations or pores.

• Are found where a great deal of fluid transport between the blood and interstitial tissue occurs.

Fenstrations

thin areas in the endothelial cells.

they are small enough to prevent formed elements from passing through the wall yet large enough to allow the movement of some smaller plasma proteins.

Fenestrated capillaries are found in

• the small intestine for the absorption of nutrients

• the ciliary process of the eye in the production of aqueous humor

• the choroid plexus of the brain in the production of cerebrospinal fluid

• most the endocrine glands to facilitate the movement of hormones into the blood.

• the kidneys for the filtering of blood

Sinusoid capillaries

• also called discontinuous capillaries

• significantly more permeable than both continuous capillaries and fenestrated capillaries.

• Capillaries that have an incomplete lining of the endothelial cells with large openings, or gaps, and the basement membrane is either discontinuous or absent.

• These openings allow for movement of large substances (formed elements, large plasma proteins) as well as plasma between the blood and tissues.

• common feature is their reddish color.

Sinusoid capillaries are found in

• red bone marrow for the entrance of formed elements into the blood

• the liver for monitoring of blood content(for potentially harmful substances)

• the spleen for the removal of aged erythrocytes from the cardiovascular circulation

• some endocrine glands (anterior pituitary, adrenal glands, parathyroid glands) for the movement of hormone molecules into the blood.

• 
  

Capillary bed

A group of capillaries (10 to 100) that function together

A capillary bed has blood delivered by

a metarteriole

Metarteriole

A branch of an arteriole

The proximal part of the metarteriole is encircled by

scattered smooth muscle cells

Thoroughfare channel

the distal part of the metarteriole, has no smooth muscle cells.

The thoroughfare channel connects to

a postcapillary venule

postcapillary venule

drains blood from the capillary bed.

True capillaries

Vessels that branch from the metarteriole and make up the bulk of the capillary bed.

For some capillary beds, a smooth muscle ring called the ___ controls blood flow into the true capillaries.

precapillary sphincter

Precapillary sphincter

a smooth muscle ring

Relaxation of the pre capillary sphincter permits

blood to flow into the true capillaries

Contraction of the precapillary sphincter causes

blood to flow directly from the metarteriole and thoroughfare channel into the postcapilary venule with blood bypassing the capillary bed.

The pre capillary sphincters open to

allow blood to enter the capillary bed to supply the tissue with needed oxygen and nutrients.

When the tissue’s needs are met, it closes and blood is routed through the metarteriole, bypassing the capillaries.

Vasomotion

The cycle of contracting and relaxing which is a rate of 5 to 10 cycles per minute that the precapillary sphincters go through.

Only about ___ of the capillary beds are open, because

one-quarter; there are over 60,000 miles of capillaries and only about 250 to 300 milliliters (mL) of blood (5% of total blood volume) moving through the capillaries. There is simply not enough blood available to fill all capillaries at the same time.

Perfusion

The specific amount of blood entering capillaries per unit time per gram of tissue.

typically expressed in milliliters per minute per gram (mL/min/g)

Venules

• Smallest veins

• measuring from 8 to 100 micrometers in diameter

• companion vessels with arterioles

• smallest venues called postcapillary venues.

• Smaller venules merge into larger venules

• largest venules have all 3 tunics

• venules merge to form veins

postcapillary venules

• smallest venules

• drain blood from capillaries.

A venule becomes a vein when

its diameter is greater than 100 micrometers

Small and medium-sized veins are companion vessels with

muscular arteries

Largest veins are companion vessels with

elastic arteries

Most veins contain numerous __ to prevent blood from pooling in the limbs.

valves.

The valves are formed primarily of

tunica intima and strengthened by elastic and collagen fibers.

Valves have an anatomic structure similar to

semilunar valves of the heart.

What is the percentage of the total blood that is moving through different components of the cardiovascular system while at rest?

• 18% within pulmonary circulation

• 12% within the heart

• about 70% within the systemic circulation, 55% of that 70% within the body’s systemic veins

Blood reservoirs

The relatively large amount of blood within veins that allows veins to function as blood reservoirs.

Blood may be shifted from venous reservoirs to circulate more quickly within the cardiovascular system through ___ of veins, when more blood is needed with increased physical exertion.

vasoconstriction.

Blood is shifted back into venous reservoirs through ___ of veins, when less blood is needed at rest

vasodilation.