Chapter 20– Blood Vessels & Circulation



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Chapter 20–


Critically read Chapter 20 pp. 756-774 right before 20.4 “Venous return and circulatory shock” section. Also read Table 20.3 (p.782) and Insight 20.5 (p.808) in the textbook.

  • Critically read Chapter 20 pp. 756-774 right before 20.4 “Venous return and circulatory shock” section. Also read Table 20.3 (p.782) and Insight 20.5 (p.808) in the textbook.

  • Comprehend Terminology (those in bold)

  • Study-- Figure questions, Think About It questions, and Before You Go On (section-ending) questions

  • Do Testing Your Recall— 1-8, 10-12, 14-16, 18

  • Do True or False– 1-2, 4-5, 8-9

  • Do Testing Your Comprehension-- #5



§ 20.1—General Anatomy of Blood Vessels

  • § 20.1—General Anatomy of Blood Vessels



1A. Closed circulatory system– Def. Blood flows in a continuous circuit through the body under pressure generated by the heart.

  • 1A. Closed circulatory system– Def. Blood flows in a continuous circuit through the body under pressure generated by the heart.

    • 1B. Open circulatory system-- In what animals?
  • 2. Three principal categories of blood vessels:

    • Arteries: efferent vessels
    • Capillaries:
    • Veins: afferent vessels
  • Fig. 20.x





1. Innermost layer (tunica interna/intima)

  • 1. Innermost layer (tunica interna/intima)

    • A. Structures: lines the inside of the vessel and is exposed to the blood; consists of--
      • Endothelial cells– histology?
      • Basement membrane
      • Connective tissue (sparse)
    • B. Functions of the endothelial cells—
      • Selectively permeable barrier
      • Secrets chemicals--?
      • Repels blood cells and platelets
    • Fig. x




2. Middle layer (tunica media)—thickest layer

  • 2. Middle layer (tunica media)—thickest layer

  • Structures:

    • Smooth muscle cells--
    • Collagen fibers
    • Elastic fibers (in arteries)
  • B. Functions of this layer:

    • Strengthen the vessel
    • Provide vasomotion--?


3. Outermost layer (tunica externa or advertitia)—

  • 3. Outermost layer (tunica externa or advertitia)—

  • A. Structures:

    • Largely loose connective tissue (collagen fibers)
  • B. Functions:

    • Protection & anchoring
    • Provide passage for--
      • Vasa vasorum— vessels of the vessels
  • Fig. 20.2





More muscular

  • More muscular

  • Able to resist high blood pressure

    • Thus called resistance vessels
  • Retain their round shape even when empty

  • Divided into three categories by size (next slide)



1. Conducting (elastic/large) arteries - largest

  • 1. Conducting (elastic/large) arteries - largest

    • Ex. aorta, common carotid, subclavian, common iliac, and pulmonary trunk (Fig. 20.23)
    • Structure– (Slide #10)
      • tunica media-- 40-70 layers of smooth muscle alternating with elastic tissue
      • Internal/external elastic lamina— not obvious
      • tunica externa– vasa vasorum
    • Function--
      • Able to expand/recoil--
      • But not so in atherosclerosis– aneurysms and rupture (Slides #15-16)






Def.– a balloon-like outpocketing of an artery wall (Fig. Y)

  • Def.– a balloon-like outpocketing of an artery wall (Fig. Y)

  • Risk– for rupture, most often reflects gradual weakening of the artery

  • Causes– OFTEN chronic hypertension or atherosclerosis

  • Common sites– abdominal aorta, renal arteries, and the arterial circle at base of brain

  • Fig. Y





2. Distributing (muscular, medium) arteries

  • 2. Distributing (muscular, medium) arteries

    • Distribute blood to specific organs
    • Ex. brachial, femoral, renal, and splenic arteries etc.
    • Structure--
      • tunica media– up to 40 layers of smooth muscle
      • Internal/external elastic lamina— conspicuous/not conspicuous (circle one)
    • Fig. 20.34, 29, 30, 36


3. Resistance (small) arteries

  • 3. Resistance (small) arteries

    • Up to 25 layers of smooth muscle
    • Elastic tissue little
    • ARTERIOLES (smallest of these); 1-3 smooth m. layers
      • Empty blood into capillaries through ____________________
      • Here individual muscle cells form a precapillary sphincter encircling the entrance to capillary; function?
      • Fig. 20.3




Where– structures in major arteries above heart

  • Where– structures in major arteries above heart

  • Function– to monitor blood pressure/chemistry

  • Three kinds (2 categories): Fig. 20.4

  • Carotid sinuses (Baroreceptors)—Details next

    • Location-- in walls of ascending aorta etc.
    • monitors BP – a rise in BP signals brainstem . . .
  • Carotid bodies (Chemoreceptors)

    • Location-- oval bodies near carotids
    • monitor blood chemistry
      • adjust respiratory rate to stabilize pH, CO2, and O2
  • Aortic bodies (Chemoreceptors)





Material exchanges– between blood and tissue fluids

  • Material exchanges– between blood and tissue fluids

    • Locations-- _____________ and smallest of the venules
  • Structure– endothelium + ____________

    • Fig. X next
  • Close vicinity to all cells— Exceptions

    • Scarce in: tendons, ligaments, & cartilage
    • Absent from (3 locations): -__________________________(Epi. & Eyes)




1. Continuous capillaries- occur in most tissues, ex. Skeletal muscle

  • 1. Continuous capillaries- occur in most tissues, ex. Skeletal muscle

    • endothelial cells have tight junctions with intercellular clefts (allow passage of solutes)
    • What molecules can pass– ex. glucose
    • What molecules can not– protein, formed elements of the blood
    • Fig. 20.5




2. Fenestrated capillaries

  • 2. Fenestrated capillaries

    • Structure – have _____________ on endothelial cells
    • filtration pores – spanned by very thin glycoprotein layer - allows passage of molecules such as _____________
    • Locations-- organs that require rapid absorption or filtration - kidneys, small intestine etc.
    • Fig. 20.6 a and b






3. Sinusoids (discontinuous) capillaries-

  • 3. Sinusoids (discontinuous) capillaries-

    • Structure– endothelial cells separated by wide gaps; no basal lamina
    • Conform to the shape of the surrounding tissue
    • Molecules can pass– proteins and blood cells
    • Locations-- liver, bone marrow, spleen, lymphatic organs
    • Fig. 20.7




b/c Greater capacity for blood containment than arteries do (Fig. 20.8)

    • b/c Greater capacity for blood containment than arteries do (Fig. 20.8)
    • thinner walls—due to less muscular and elastic tissue; why?
    • lower blood pressure: 10 mm Hg with little fluctuation
    • ____________ aid skeletal muscles in upward blood flow




Postcapillary venules-- only tunica intima

  • Postcapillary venules-- only tunica intima

    • Receive blood from capillaries
    • more porous than capillaries
  • Muscular venules-- receive blood from #1

    • have tunica media (1-2 layers of smooth muscle) + thin tunica externa
  • Medium veins–



Venous sinuses--

  • Venous sinuses--

    • veins with thin walls, large lumens, no smooth muscle; vasomotion– yes/no? (Circle one)
    • Examples– coronary sinus of the heart and the dural sinuses of the brain
  • Large veins--

    • Greater than 10 mm (diameters)
    • Venae cavae, pulmonary veins, internal jugular veins


Most common route

  • Most common route

    • heart  arteries  arterioles  capillaries  venules  veins
  • Portal system

    • blood flows through two consecutive capillary networks before returning to heart
    • 3 places in human body–




Def. Point where 2 blood vessels merge

  • Def. Point where 2 blood vessels merge

  • Arteriovenous shunt

    • artery flows directly into vein; fingers etc.
  • Venous anastomosis

    • most common type
    • alternate drainage of organs; Fig. 20.33
  • Arterial anastomosis





§ 20.2— Blood Pressure, Resistance, and Flow

  • § 20.2— Blood Pressure, Resistance, and Flow



Importance– deliver oxygen and nutrients and to remove wastes at a rate keeps pace with tissue metabolism

  • Importance– deliver oxygen and nutrients and to remove wastes at a rate keeps pace with tissue metabolism

  • Blood flow (F)– is the amount of blood flowing through an organ, tissue, or blood vessel in a given time

  • Hemodynamics: Blood Flow (F) = ΔP/R

    • Where ΔP is the pressure difference and R is the resistance


Blood pressure (BP)– Def. the force per unit area exerted by the blood against a vessel wall

    • Blood pressure (BP)– Def. the force per unit area exerted by the blood against a vessel wall
    • In what vessels can you find BP?
    • Figure 20.10 has the answer




BP is understood to mean the pressure in the _________________

    • BP is understood to mean the pressure in the _________________
    • BP rises and falls in a pulsatile fashion in the arteries and arterioles
    • Figure Z (what BP do we measure?)




Systolic P.– the maximum p. exerted in the arteries when blood is ejected into them during ventricular ejection, averages 120 mm Hg (Mercury)

    • Systolic P.– the maximum p. exerted in the arteries when blood is ejected into them during ventricular ejection, averages 120 mm Hg (Mercury)
      • Physiology– during ventricular systole, a volume of blood enters the arteries from the ventricle. How much actually moves to the arterioles?
      • Status of the semilunar valves in this particular cardiac cycle? (open or close)


Diastolic P.– the arterial p. when blood is draining off into the arterioles during diastole, averages ________ Hg. Lowest during cardiac cycle.

    • Diastolic P.– the arterial p. when blood is draining off into the arterioles during diastole, averages ________ Hg. Lowest during cardiac cycle.
      • Physiology– during ventricular diastole, the semilunar valves close, no blood enters the arteries but the arteries moves the blood forward. Why?


Pulse P.– is the difference between systolic and diastolic pressure

    • Pulse P.– is the difference between systolic and diastolic pressure
    • The Mean Arterial P. (MAP)— is the average blood pressure throughout the cardiac cycle
      • is monitored and regulated by BP reflexes
      • MAP = diastolic p. + 1/3 pulse p.
      • Figure Z






Def.– high blood pressure; a chronic resting blood pressure higher than 140/90-- (hypertension)

    • Def.– high blood pressure; a chronic resting blood pressure higher than 140/90-- (hypertension)
    • Results– aneurysms, atherosclerosis, heart failure, stroke, etc.
    • Hypotension– a chronic low resting BP (90/50 or lower);
    • Causes– blood loss, dehydration, anemia, in people approaching death


Resistance depends on three variables below: (Note: Blood Flow = ΔP/Resistance)

  • Resistance depends on three variables below: (Note: Blood Flow = ΔP/Resistance)

  • Blood viscosity inversely relates to blood flow—

    • Anemia & hypoproteinemia -- ___ blood flow
    • Polycythemia & dehydration -- ___ blood flow
    • Vessel length– pressure and flow decline with distance (farther end of the vessel)
      • The above two variables usually quite stable
    • Vessel radius on blood flow— proportional to the fourth power of radius
    • Blood Flow α radius4
    • Table 20.2




Neural control–

  • Neural control–

    • Baroreflex autonomic regulation--
      • BP increases –baroreceptors firing rate increases
      • Figure 20.13
    • Chemoreflex– response to changes in blood chemistry
    • Medullary ischemic reflex– an automatic response to a drop in perfusion of the brain




Hormonal control--

  • Hormonal control--

    • Angiotensin II-- ↑ BP
    • Aldosterone– ↑ BP
    • Atrial natriuretic peptide-- ↓ BP
    • Antidiruetic hormone-- ↑ BP
    • Epinephrine and Norepinephrine-- ↑ BP


Blood flow= Δ Blood Pressure/Resistance

  • Blood flow= Δ Blood Pressure/Resistance

    • BP = Blood Flow x Resistance (R)
    •  BP = Blood Flow x 1/(Radius)4
  • Why vasodilation causes resistance to decrease?

  • BP = Blood Flow x R = Cardiac output x R

    •  BP = Heart rate (beats/min) x stroke volume (ml/beat) x R
    • Thus, heart rate and stroke volume impact BP
    • Fig. 20.13 again




Diffusion: (1a, 1b, and 1c of Fig. 20.16)

  • Diffusion: (1a, 1b, and 1c of Fig. 20.16)

  • Transcytosis: (2 of Fig. 20.16)

    • Pinocytosis/endocytosis then exocytosis
    • Fatty acids, albumin, insulin etc.
  • Fig. 20.16





Hydrostatic pressure– due to liquid

  • Hydrostatic pressure– due to liquid

    • Mainly caused by the blood pressure
    • 30 mm Hg at arterial end and 10 mm Hg at the venous end
  • Colloid osmotic pressure– due to protein

    • Mostly by albumin etc.
  • Difference of 1-2 above is Net Filtration or Reabsorption Pressure

  • Fig. 20.17





Def.– accumulation of fluid in a tissue.

  • Def.– accumulation of fluid in a tissue.

  • Three causes:

    • Increased capillary filtration: hypertension etc.
    • Reduced capillary reabsorption: due to albumin-- hypoproteinemia
    • Obstructed lymphatic drainage
  • Edema’s consequences:

    • Oxygen delivery/waste removal are impaired
    • Tissue death (necrosis)


Watch a video— Baroreceptor reflex control of blood pressure

  • Watch a video— Baroreceptor reflex control of blood pressure

  • Watch a video— Fluid exchange across the capillary




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