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Chapter 21
Anatomy of Blood Vessels
▪Structure of Vessel Walls- walls of arteries and veins contain three distinct layers—tunica intima,
tunica media, and tunica externa.
▫Tunica intima▫Tunica media▫Tunica externa▪Differences Between Arteries and Veins
Sectional View
▫Walls of arteries are thicker than those of veins
▫Arteries tend to keep a circular shape, veins looked distorted and collapsed.
▫Endothelial lining of an artery appears pleated, veins have smooth endothelium
Gross Dissection
▫Arteries are more resilient
▫Veins contain valves
▪Arteries- blood vessels that carry blood away from the heart
▫Thick muscular walls make arteries elastic and contractile
-Elasticity allows for passive changes in the diameter of the vessel in response to
changes in blood pressure
-Contractility allows for change in diameter actively
-Vasoconstriction-Vasodilation-Vasoconstriction and vasodilation affect
1)
2)
3)
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▫Elastic Arteries- also called conducting arteries.
▫Muscular Arteries- also called medium-sized arteries.
▫Arterioles- smallest of the arteries
▪Capillaries- smallest of all blood vessels
▫Capillaries are the only blood vessels whose walls permit exchange between the blood
and surrounding interstitial fluid.
▫Contains an endothelial tube inside a delicate basal lamina, no tunica media or externa
is present.
▫Average diameter is around 8 m, very close to that of a RBC.
-Continuous Capillaries- most of the body is supplied by these
-Fenestrated Capillaries- found in the choroid plexus, some endocrine organs,
absorptive areas of the digestive tract, and filtration sites in the kidneys.
-Sinusoids- similar to fenestrated capillaries
-Capillary Beds- interconnected network of capillaries
-A single arteriole generally gives rise to dozens of capillaries that empty
into several venules.
-Precapillary sphincter-Metarteriole-Thoroughfare channel-Anastomoses- arterial and ateriovenous-Angiogenesis-
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▪Veins- Collect blood from all tissues and organs and return it to the heart
▫Venules- smallest of the veins, collect blood from the capillary beds
▫Medium-Sized Veins
▫Large Veins
▫Venous Valves- allow for one way movement of blood towards the heart, against
gravity.
▪Distribution of Blood- see fig. 21-6
Cardiovascular Physiology
▪Pressure- force exerted against a liquid generates hydrostatic pressure (HP)
▫Circulatory pressure- pressure gradient in the systemic cardiovascular system.
Divided into three components.
-Blood Pressure-Capillary Hydrostatic Pressure (CHP)-Venous Pressure-
▪Resistance- any force that opposes movement.
▫Total peripheral resistance▫Peripheral resistance (PR)▫Flow is directly proportional to the pressure gradient, and inversely proportional to the
resistance.
▫Vascular Resistance-Vessel length- the longer the vessel, the more resistance
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-Vessel diameter- friction affects the blood that is close to the vessel walls.
▫Viscosity- resistance to flow caused by interactions among molecules suspended in a
liquid.
▫Turbulence- a resistance in flow caused by swirling blood and eddies.
▪Overview of Cardiovascular Pressures
▫Arterial Blood Pressure- maintains blood flow through capillaries, rises and falls during
ventricular systole and diastole, respectively.
-Systolic pressure
-Diastolic pressure
-Pulse pressure
-Mean arterial pressure (MAP)-Hypertension and Hypotension-
▫Elastic Rebound- force applied to blood as arteries recoil during ventricular diastole.
▫Venous pressure determines venous return.
-Venous pressure averages about 2 mmHg at the entrance to the heart.
-Although venous pressures are low, veins offer little resistance
-As veins merge, they become larger, blood velocity increases.
-Two factors assist the low venous pressures in propelling blood back to the
heart; muscular compression of peripheral veins, and the respiratory pump.
-Muscular compression-
-Respiratory Pump-
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▪Capillary Pressures and Capillary Exchange- fluids and solutes leave and return to the capillaries.
Has 4 important functions:
1. Ensures that plasma and interstitial fluid are in constant communication
2. Accelerates distribution of nutrients, hormones, and dissolved gases throughout
tissues.
3. Assists in transport of insoluble lipids and tissue proteins that cannot enter the
bloodstream by crossing the capillary walls.
4. Flushing action that carries bacterial toxins and other chemical stimuli to lymphoid
tissues.
▫Most important processes that move materials across typical capillaries are diffusion,
filtration, and reabsorption.
-Diffusion- occurs best when distances are small, gradients are high, and solutes
are small. Occurs by five routes:
1.
2.
3.
4.
5.
-Filtration- hydrostatic pressure drives capillary filtration
-Reabsorption- movement of water back into the capillary
▫Interplay between Filtration and Reabsorption
*Study fig. 21-11 and the relative text on pages740-742*
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Cardiovascular Regulation
▪Important that groups of cells that become active receive the appropriate amount of blood for
nutrient, gas, and waste exchange.
▪Tissue perfusion▪Three factors involved in the regulation of cardiovascular function are; autoregulation, neural
mechanisms, and endocrine mechanisms.
▫Autoregulation- local factors change the pattern of blood flow within capillary beds in
response to chemical changes in the interstitial fluids.
▫Neural Mechanisms- Controlled by cardiac centers and vasomotor centers located in the
medulla oblongata.
-Cardiac centers have both cardioacceleratory and cardioinhibitory centers which
use the sympathetic and parasympathetic divisions respectively.
-Vasomotor centers have two populations of neurons
1) large group responsible for widespread vasoconstriction
-release of NE at peripheral blood vessels causes the stimulation
of the smooth muscle in the walls of the arterioles.
2) smaller group responsible for the vasodilation of arteries in skeletal
muscle and the brain.
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-stimulation of these neurons will relax the smooth muscle cells
in the walls of the arterioles.
-Vasomotor Tone- blood vessels are partially constricted at all times.
-Baroreceptor Reflexes- specialized receptors that monitor the stretch in the
walls of expandable organs.
-carotid sinuses, aortic sinuses, wall of right atrium; locations of
baroreceptors associated with cardiovascular regulation.
*see fig. 21-13 for the relationships of blood pressure and the
baroreceptor reflexes.*
-Chemoreceptor Reflexes- respond to changes in CO2, O2 or pH levels of blood
and CSF.
-chemoreceptors are located in the carotid bodies, near the carotid sinus
and the aortic bodies, near the aortic arch.
*see fig 21-14 for the relationships of blood pressure and the
chemoreceptor reflexes*
▫Hormones and Cardiovascular Regulation
-ADH- released in response to decreased blood volume, increased osmotic
concentration of plasma, or circulating angiotensin II
-Immediate result is peripheral vasoconstriction that elevates blood
pressure
-Also stimulates the conservation of water at the kidneys, preventing a
reduction in blood volume
-Angiotensin II- appears in blood after renin is released by kidneys. Renin is
released in response to a fall in renal blood pressure. Angiotensin II has four
important functions
1. stimulates adrenal production of aldosterone
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2. stimulates the secretion of ADH
3. stimulates thirst
4. stimulates cardiac output and vasoconstriction
-Erythropoietin- released at the kidneys in response to decreased blood pressure
and low oxygen content.
-stimulates the production and maturation of red blood cells, increasing
volume and viscosity of blood and oxygen carrying capacity
-Natriuretic Peptides; atrial natriuretic and brain natriuretic peptides
-Atrial natriuretic peptide- produced by cardiac muscle cells in the wall of
the right atrium in response to excessive stretching during diastole.
-Brain natriuretic peptide- produced by ventricular muscle cells to similar
stimuli as ANP.
-These peptide hormones reduce blood volume and pressure by:
1. increasing sodium ion excretion at kidneys
2. promoting water loss by increasing urine volume.
3. reduces thirst
4. blocking the release of ADH, aldosterone, E, and NE.
5. stimulates peripheral vasodilation.
Patterns of Cardiovascular Response
▪Exercise and the Cardiovascular System
▫Light exercise- lower levels of physical exertion, three interrelated changes take place:
1.
2.
3.
▫Heavy exercise- higher levels of exertion, other physiological adjustments occur.
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▫Exercise, Cardiovascular Fitness, and Health
-Trained athletes have bigger hearts and larger stroke volumes
-can maintain sufficient blood flow with lower heart rate
▫Exercise and Cardiovascular Disease
-Regular exercise, balanced diet, no smoking has many benefits:
*lower total blood cholesterol
*reduces stress
*lowers blood pressure
*slows the formation of plaque
▪Cardiovascular Response to Hemorrhaging
▫Short-Term Elevation of Blood Pressure
▫Long-Term Restoration of Blood Volume
Distribution of Blood Vessels
▪Three general functional patterns to keep in mind as we cover the vessels:
1. peripheral distributions of arteries and veins on the left and right sides are generally
identical, except near the heart
2. a single vessel may have several names as it crosses specific anatomical boundaries.
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3. arteries and their corresponding veins usually travel together to reach their target
organs.
The Pulmonary Circuit
▪Use fig. 21-18 to identify the following vessels:
▫pulmonary trunk
▫left and right pulmonary arteries
▫left and right pulmonary veins
The Systemic Circuit
▪Use figs. 21-19 through 21-31 to identify the vessels on your vessel list.
Fetal Circulation
▪Major differences in fetal and adult circulations reflect different sources of respiratory and
nutritional support.
▪Embryonic lungs and digestive tract are nonfunctional.
▪Placental Blood Supply
▫Blood flow to the placenta is provided by a pair of umbilical arteries, which arise from
the internal iliac arteries.
▫Blood returns from the placenta in the single umbilical vein, bringing oxygen and
nutrients to the developing fetus
▫Ductus venosus- a vascular connection to an intricate network of veins within the
developing liver.
▪Circulation in the Heart and Great Vessels
▫Use fig. 21-32 to understand fetal circulation
▪Cardiovascular Changes at Birth
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▫An infants first breath expands the lungs and pulmonary vessels, causing blood to rush
into the pulmonary vessels
▫Rising O2 levels stimulate the constriction of the ductus arteriosus, isolating the
pulmonary and aortic trunks
▫Rising pressure in the left atrium causes the foramen ovale to close
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