Chapter 18: The CVS II: The Blood Vessels

Chapter 18: The CVS II: The Blood Vessels
2. Describe the three layers of a blood vessel.
Blood vessels have three layers except the capillaries:
(I) Tunica intima: (i) Endothelium: simple squamous
epithelium; (ii) Internal elastic lamina for recoil (in arteries).
(II) Tunica media: (i) Smooth muscle: vasoconstriction
(Sympathetic stimulation) and vasodilation (sympathetic inhib.)
Influence blood flow and blood pressure. (ii) External elastic
membrane for recoil (in arteries). (III)Tunica externa (tunica
adventitia); Collagen fibers: protect the blood vessel; contains
nerve fibers, lymphatic vessels and blood vessels.
3. Compare various types of arteries and veins.
(i) Elastic Arteries (conducting arteries): with elastin in all three
tunics; Aorta and its major branches; Act as pressure reservoirs
that expand and recoil as blood ejected from heart.
(ii) Muscular Arteries (distributing arteries): Deliver blood to
body organs; more smooth muscle; active in vasoconstriction.
(iii) Arterioles: Smallest arteries; lead to capillary beds; control
flow into capillary beds via vasodilation and vasoconstriction.
(iv) Veins: compared to arteries: Have thinner walls; larger
lumens; lower BP; lack internal and external elastic
membranes; venous valves prevent backflow of blood.
4. Describe blood flow, blood pressure, resistance.
Blood flow: Volume of blood flowing through vessel, organ, or
entire circulation in given period (Equivalent to cardiac output
(CO) for entire vascular system). Blood pressure (BP): Force per
unit area exerted on wall of blood vessel by blood. Pressure
gradient provides driving force that keeps blood moving from
higher to lower pressure areas. Resistance (peripheral
resistance): Opposition to flow. Measure of amount of friction
blood encounters with vessel walls. Three important sources of
resistance: (i) Blood viscosity (remain relatively constant):
thickness of blood. Increased viscosity = increased resistance.
(ii) Total blood vessel length (remain relatively constant):
Longer vessel = greater resistance encountered. (iii) Blood
vessel diameter: Greatest influence on resistance; smaller the
vessel, more resistance.
Blood flow (F) is directly proportional to blood pressure
gradient ( P); If  P increases, blood flow speeds up. Blood
flow (F) is inversely proportional to peripheral resistance (R). If
R increases, blood flow decreases.
6. Blood pressure differs in the arteries, capillaries, and veins.
Arterial Blood Pressure: (i) Systolic pressure: Pressure
generated by ventricular contraction; (ii) Diastolic pressure:
During diastole; (iii) Pulse pressure: Difference between
systolic and diastolic pressure. (iii) Mean arterial pressure
(MAP): MAP is calculated as: MAP = diastolic pressure + 1/3
pulse pressure
Capillary Blood Pressure: Low capillary pressure is desirable
because: High BP would rupture fragile, thin-walled capillaries.
Venous Blood Pressure: Functional adaptations important to
venous return: (i) Muscular pump: contraction of skeletal
muscles "milks" blood toward heart; valves prevent backflow.
(ii) Respiratory pump: pressure changes during breathing move
blood toward heart. As we inhale, abdominal pressure
increases, squeezing the local veins and forcing blood towards
heart. At the same time pressure in chest decreases, allowing
thoracic veins to expand and speed up blood entry to right
atrium. (iii) Sympathetic venoconstriction: under sympathetic
control pushes blood toward heart
7. Describe how blood pressure is regulated.
Main factors influencing blood pressure: (i) Cardiac output (CO
= SV × HR); (ii) Peripheral resistance (PR) (diameter of blood
vessels); (iii) Blood volume; [I] Long-term renal regulation:
Long-term controls alter blood volume; Low blood pressure
leads to release of renin by kidneys; Renin-angiotensin-
aldosterone mechanism: Angiotensin II acts in four ways to
stabilize arterial blood pressure: (a) aldosterone (b) ADH, (c)
sensation of thirst, (d) angiotensin II is a potent vasoconstrictor.
[II] Short-term neural and hormonal controls
Change blood pressure by altering peripheral resistance and
CO: (a) Neural Controls: Drop in arterial pressure is detected by
baroreceptors (stretch reflexes) located in carotid sinuses,
aortic arch, walls of large arteries of neck and thorax; impulses
from baroreceptors stimulate medullary cardiac and vasomotor
centers. This increase heart rate, & force of contraction (SV)
(increase CO) and vasoconstriction (increase PR); BP return to
normal range. (ii) All these are reversed when BP increase;
Cardiac centers: Cardioaccelatory center (Sympathetic
stimulus: increases heart rate and increases force of heart
contraction); Cardioinhibitory center (Parasympathetic:
decreases heart rate); Vasomotor center: Control the diameter
of blood vessels.
(b) Hormonal control: Cause increased blood pressure [most];
Cause lowered blood pressure [Atrial natriuretic peptide]
8. Discuss disorders of blood pressure.
[I] Primary or Essential Hypertension (90%; no underlying
cause identified; Risk factors include heredity, diet, obesity,
age, diabetes mellitus, stress, and smoking; No cure but can be
controlled); [II] Secondary hypertension (due to identifiable
disorders such as hyperthyroidism and Cushing's syndrome;
Treatment focuses on correcting underlying cause)
Circulatory shock: Any condition in which blood vessels are
inadequately filled and blood cannot circulate normally. If
circulatory shock persists, cell die and organ damage follows. (i)
Hypovolemic shock: results from large scale of blood or fluid
loss (acute hemorrhage). (ii) Vascular shock: Blood volume is
normal, but circulation is poor as a result of vasodilation
(anaphylactic shock as a result of allergic reaction). (iii)
Cardiogenic shock: Pump failure occurs when heart is so
inefficient to sustain adequate circulation (myocardial
infarction-heart attack).
9. Compare the three types of capillaries.
Capillaries: Their walls consist of just a thin tunica intima;
Functions: Exchange of gases, nutrients, wastes, hormones,
between blood and interstitial fluid; Three structural types:
All have intercellular clefts, which allow passage of fluids and
small solutes. (i) Continuous capillaries: (least permeable and
most common) Abundant in skin, muscles, and CNS (blood
brain barrier) (ii) Fenestrated capillaries: large fenestrations
(pores) increase permeability. Occurs in areas of active
filtration, absorption, or secretion (kidney, small intestine).

(iii) Sinusoid capillaries (sinusoids): Most permeable (allows
large molecules and even blood cells to pass); Larger
fenestrations; larger intercellular clefts; large lumens.
Occurs in limited locations (liver, bone marrow, spleen).
Capillary beds: Capillary beds consists of two types of vessels:
(i) Vascular shunt: short vessel that directly connects terminal
arteriole and postcapillary venule. (ii) True capillaries: Branch
off metarteriole and return to thoroughfare channel;
Precapillary sphincters (cuff of smooth muscle fibers) at the
root of each true capillary at the metarteriole regulate blood
flow into true capillaries (constrict / relax); Blood may go into
true capillaries or to shunt depending on needs of tissues.
11. Explain net filtration pressure across the capillary wall.
Fluid Movements: Fluid leaves capillaries through the clefts at
arterial end; most returns to blood at venous end. Hydrostatic
pressure in capillary “pushes” fluid; Osmotic pressure in
capillary “pulls” fluid. (i) At arterial end: NFP= (HPc + OPif) –
(HPif + OPc) = (35 + 3) – (0 + 25) = 13 mm Hg (net outward
pressure). (ii) At venous end: NFP = (HPc + OPif) – (HPif + OPc) =
(15 + 3) – (0 + 25) = –7 mm Hg (net inward pressure)