Chapter 21: Blood Vessels and Circulation Primary sources for figures and content: Marieb, E. N. Human Anatomy & Physiology. 6 th ed. San Francisco: Pearson Benjamin Cummings, 2004. Martini, F. H. Fundamentals of Anatomy & Physiology. 6 th ed. San Francisco: Pearson Benjamin Cummings, 2004.
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Chapter 21: Blood Vessels and Circulation Primary sources for figures and content: Marieb, E. N. Human Anatomy & Physiology. 6 th ed. San Francisco: Pearson.
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Chapter 21:
Blood Vessels and Circulation
Primary sources for figures and content:
Marieb, E. N. Human Anatomy & Physiology. 6th ed. San Francisco: Pearson Benjamin Cummings, 2004.
Martini, F. H. Fundamentals of Anatomy & Physiology. 6th ed. San Francisco: Pearson Benjamin Cummings, 2004.
The types of blood vessels, their structures
and functions.
6 Classes of Blood Vessels
1. Arteries:– carry blood away from heart– Branch and decrease in diameter
2. Arterioles:– Are smallest branches of arteries– Connect to capillaries
3. Capillaries:– are smallest blood vessels– location of exchange between blood and interstitial
fluid
6 Classes of Blood Vessels
4. Venules:– Smallest veins– collect blood from capillaries
5. Veins:– return blood to heart– Converge and increase in diameter
6. Anastomoses:- Bypass connection between vessels
The Largest Blood Vessels
• Attach to heart• Pulmonary trunk:
– carries blood from right ventricle– to pulmonary circulation
• Aorta:– carries blood from left ventricle– to systemic circulation
The Smallest Blood Vessels
• Capillaries• Have small diameter and thin walls • Chemicals and gases diffuse
across walls
The Anatomy of Blood Vessels
• Arteries, veins, and capillaries • Have different functions • Have different structures
• From heart to capillaries, arteries change:– from elastic arteries – to muscular arteries – to arterioles
Elastic Arteries
• Also called conducting arteries• Diameter up to 2.5cm• Elastin in all three tunics
– Elasticity evens out pulse force
• Stretch (ventricular systole) and rebound (ventricular diastole)
• Not involved in systemic vasoconstriction
Muscular Arteries
• Also called distribution arteries• Are medium-sized (most arteries)• Transport blood to organs and tissues• Diameter 10mm – 0.3mm• More smooth muscle and less elastin in
tunica media than elastic arteries• Involved in systemic vasoconstriction via
sympathetic stimulation
2. Arterioles
• Also known as resistance vessels• Connect blood supply to capillary beds• Are small – diameters 300µm – 10µm• All three tunics thin with few elastic
fibers• Involved in local vasoconstriction via
endocrine or sympathetic stimulation
Health Problems with Arteries
1. Aneurysm:– Pressure of blood exceeds elastic
capacity of wall– Causes bulge or weak spot prone to
rupture– Caused by chronic high blood
pressure or arteriosclerosis
Health Problems with Arteries
2. Arteriosclerosis:– Variety of pathological conditions
causing changes in walls that decrease elasticity (“thickenings”)• Focal calcification = smooth muscle
degenerates, replaced by calcium salts
• Atherosclerosis
3. Atherosclerosis: lipid deposits
Health Problems with Arteries
4. Stroke = cerebrovascular accident (CVA)– Interruption of arterial supply to portion
of brain due to embolism or atherosclerosis
– Brain tissue dies and function is lost
3. Capillaries
• Only vessels with thin enough wall structure to allow complete diffusion– Designed to allow diffusion to/from the tissue
• Diameter 8 µm– Consists of tunica intima only – endothelium + basal lamina
• Human body contains 25,000 miles of capillaries
Capillary Structure
Figure 21-4
Capillary Function
• Location of all exchange functions of cardiovascular system
• Materials diffuse between blood and interstitial fluid
Capillary Structure
• Endothelial tube, inside thin basal lamina
• No tunica media• No tunica externa• Diameter is similar to red blood
cell
Capillaries
Types of Capillaries
1. Continuous capillaries- Normal diffusion to all tissues except
epithelium and cartilage- Complete endothelium, tight junctions- Functions:
• Permit diffusion of: water, small solutes, lipid-soluble materials
• Block: blood cells and plasma proteins
• e.g., the blood–brain barrier
Types of Capillaries
2. Fenestrated capillaries- High volume fluids or large solute
transfer- Pores/fenestrations span endothelium– Permit rapid exchange of water and
larger solutes between plasma and interstitial fluid
Fenestrated Capillaries
• Are found in:– choroid plexus– endocrine organs– kidneys– intestinal tract
Sinusoids
• Areas in:– liver– spleen– bone marrow– endocrine organs
• Have gaps between adjacent endothelial cells
Types of CapillariesSinusoids
3. Sinusoids– Cell or large protein exchange– Gaps between endothelial cells– Permit free exchange of water and large
plasma proteins between blood and interstitial fluid
– Phagocytic cells monitor blood at sinusoids– Found: liver, bone marrow, lymphoid tissues
Capillary Networks
Figure 21-5
Capillaries Networks
• Organized into Capillary bed or capillary plexus
• Connect 1 arteriole and 1 venule• Not enough total blood to fill all
capillaries at once– Flow through capillary bed must be
controlled based on need via precapillary spincters
Capillary Sphincter
• Guards entrance to each capillary • Opens and closes, causing
capillary blood to flow in pulses
Vasomotion
• Contraction and relaxation cycle of capillary sphincters – Spincter relaxed = flow in capillary
bed– Spincter constricted = capillary bed
empty, flow through anastomoses
• Causes blood flow in capillary beds to constantly change routes
Structure of Blood Vessels
Figure 21-2
Veins vs. Arteries
• Are larger in diameter • Have thinner walls • Carry lower blood pressure
4. Veins
• Collect blood from capillaries in tissues and organs
• Return blood to heart• Can serve as blood reservoir• Thin walls but large lumens• Thin tunica media = little smooth muscle or
elastin• Tunica externa = elastin and smooth muscle• Tunica intima = valves to prevent back-flow
3 Vein Categories
1. Venules (5th type of vessel):– very small veins
• Average diameter 20 µm
– collect blood from capillary beds– Small venules lack tunica media
2. Medium-sized veins:- Diameter 2-9 mm
3. Large Veins:- Diameters up to 3 cm
Valves in the Venous System
Figure 21-6
Valves in tunicaintima insure oneway movement
Vein Valves
• Valves = Folds of tunica intima • Prevent blood from flowing backward• Pressure from heart drives blood flow in
arteries, but pressure in veins often too low to oppose gravity
• Compression pushes blood toward heart – Skeletal muscle movement required to
• Blood pressure changes throughout body– Greatest in arteries leaving heart, lowest
in veins returning to heart
• Person’s BP measured at arteries near heart– Systolic pressure/diastolic pressure (from
ventricles, squeeze/rest)– “Normal” = 110/70 mmHg
Vessel Diameter and Cardiac Pressure
Figure 21-9a
Figure 21-10
Pressures in the Systemic Circuit
Pressures in the Systemic Circuit
• Systolic pressure:– peak arterial pressure during ventricular
systole
• Diastolic pressure:– minimum arterial pressure during
diastole
• Pulse pressure: – difference between systolic pressure and
diastolic pressure
Abnormal Blood Pressure
• Hypertension: – Arterial pressure > 150/90 mmHg– abnormally high blood pressure– Causes incr. workload for heart– Untreated = enlarged left ventricle
requires more O2 heart can fail
• Hypotension: – abnormally low blood pressure
Blood Pressure
• As arteries branch, area for blood increases, pressure decreases and becomes constant
• Blood at arterioles ~35mmHg capillaries Blood at venules ~18mmHg
• Pressure continues to decline as veins increase diameter
In a healthy individual, where would the blood pressure be greater, at the aorta or
at the inferior vena cava?
A. aorta
B. inferior vena cava
While standing in the hot sun, Sally begins to feel light headed and faints.
Explain.
A. Blood has pooled in her lower limbs.
B. Cardiac output has decreased, sending less blood to the brain.
C. Sweating has reduced blood volume.
D. All of the above have occurred.
The mechanisms and pressures involved
in the movement of fluids between capillaries and
interstitial spaces.
Capillary Exchange• Vital to homeostasis• Functions to feed tissues and remove wastes• Due to filtration and diffusion• Dependent on good blood flow and pressure• Moves materials across capillary walls by:
1. Diffusion2. Filtration3. Reabsorption
1. Diffusion
• Movement of ions or molecules:– from high concentration to lower concentration
1. Small ions transit through endothelial cells – e.g. Na+
2. Large ions & small organics pass between endothelial cells– E.g. glucose, amino acids
3. Lipids pass through endothelial membrane – e.g. steroid hormones
1. Diffusion
4. Large water soluble compounds diffuse at fenestrated capillaries– e.g. in intestine
5. Large plasma proteins diffuse only at sinusoids– e.g. in liver
2. Filtration
• The removal of large solutes through a porous membrane
• Pressure forces substances through membrane• Blood hydrostatic pressure in capillaries drives
water and solutes out of plasma to tissues– 24L/day
• Most recollected by osmosis (plasma proteins) back into capillary – filtered at arteriole end– absorbed at venule end
2. Filtration
• 3.6 L/day flows through interstitial spaces, recollected by lymphatic system– Accelerates distribution of nutrients– Flushes out toxins and pathogens
•Will be removed/detoxified by immune cells in lymphatic system
3. Reabsorption
• The result of osmosis
Hydrostatic pressure:– forces water out of solution
Osmotic pressure:– forces water into solution
**Both control filtration and reabsorption through capillaries
Figure 21-12
Forces Across Capillary Walls
Net Hydrostatic Pressure
• The difference between:– capillary hydrostatic pressure (CHP)– and interstitial fluid hydrostatic
pressure (IHP)
• Pushes water and solutes:– out of capillaries– into interstitial fluid
Net Colloid Osmotic Pressure
• The difference between:– blood colloid osmotic pressure (BCOP)– and interstitial fluid colloid osmotic
pressure (ICOP)
• Pulls water and solutes:– into capillary– from interstitial fluid
Capillary Exchange
• At arterial end of capillary:– fluid moves out of capillary– into interstitial fluid
• At venous end of capillary:– fluid moves into capillary– out of interstitial fluid
Edema
• Buildup of fluid in the tissues, due to too much diffusion or filtration, not enough osmosis, or blocked lymphatics
KEY CONCEPT• Total peripheral blood flow equals cardiac output• Blood pressure overcomes friction and elastic
forces to sustain blood flow• If blood pressure is too low:
– vessels collapse– blood flow stops– tissues die
• If blood pressure is too high:– vessel walls stiffen– capillary beds may rupture
Cardiovascular Regulation
Cardiovascular Regulation• Flow, BP, and resistance must be controlled to
insure delivery of nutrients and removal of wastes in tissues
• Changes blood flow to a specific area:– at an appropriate time and area– without changing blood flow to vital organs
• Blood flow to active vs. inactive tissues must be differentially controlled
• This is accomplished by autoregulation, neural regulation, and hormone release
Cardiovascular Response to Hemorrhages
• Short term (aimed at incr. BP and incr. Flow)– Blood flow to brain kept constant while other
systems adjust, can compensate for ~20% blood loss
1.Incr. cardiac output = trigger peripheral vasoconstriction to incr. BP
2.Venoconstrict to moblize venous reserve to incr. blood volume
3.Release NE, ADH, Angiotensin II to incr. BP
Cardiovascular Response to Hemorrhages
• Long term (aimed at restoring normal blood volume after hemorrhage)1. Recall fluid from interstitial spaces2. Release Incr. ADH for fluid retention at kidney3. Increase thirst4. Release EPO to Incr. RBCs
Shock• Low BP and inadequate blood flow• Due to:
– Loss of > 30% blood volume– Damage to heart– External pressure on heart– Extensive vasodilation
• Result in:– Hypotension, rapid weak pulse clammy skin, confusion– Incr. heart rate– Decr. urine production and blood pH
*Body focuses on supplying blood to brain at expense of other tissues
Circulatory Collapse
• Blood flow stops completely as muscles in vessels no longer contract due to lack of oxygen