Blood vessels and blood pressure

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Blood vessels and blood pressure

Transcript Of Blood vessels and blood pressure

Blood vessels and blood pressure I. Introduction
- distribution of CO at rest
II. General structure of blood vessel walls
- walls are composed of three distinct layers:
1. Tunica intima is the innermost layer; it is composed of single layer of endothelial cells and a thin layer of loose connective tissue (basement membrane, BM)
2. Tunica media is the middle layer; it is composed of a mixture of circularly arranged smooth muscle cells and sheets of elastin, the proportion of each depending on artery type; the smooth muscle cell layer is innervated by vasomotor fibers (ANS), innervation can produce vasoconstriction
3. Tunica adventitia is the outermost layer; it is composed of loosely woven connective tissue infiltrated by nerves, blood vessels and lymphatics
III. Basic organization of the CV system • elastic arteries -- conducting vessels • muscular arteries -- distributing vessels • arterioles -- resistance vessels • capillaries -- sites of nutrient, fluid, and gas excahnge with tissues • venules • small veins --> large veins -- capacitance vessels
IV. Hemodynamics overview
A. Blood flow, blood pressure, resistance
- blood flow: volume of blood flowing through vessel/organ/ circulation per minute; as far as systemic circulation, blood flow = CO
- blood pressure: pressure gradient between 2 points in vasculature
-resistance: opposition to flow due to friction • Flow (F) = Pressure (P)/ Resistance (R)

B. Factors influencing resistance
- R = 8ηL/πr4 • viscosity (h) -- friction of fluid molecules as they slide over one another o hematocrit
o plasma protein concentration
o constant for CV system • length -- longer the vessel, greater surface area, greater resistance to flow
o constant for CV system
• radius -- changing radius greatly alters surface area of vessel exposed to a given volume of blood o decreasing radius -- tremendously increases resistance o increasing radius -- tremendously decreases resistance
- by simplification: R = 1/r4
V. Arteries -- functional characteristics
A. Low-resistance vessels -- blood rapidly moves from heart to tissues
B. Pressure reservoirs -- provide driving force for blood during diastole, secondary pumps
- note that despite contraction-relaxation cycles, blood pressure and blood flow through capillaries does not fluctuate -- not pulsatile
• during systole more blood enters arteries from heart than leaves them due to resistance of smaller vessels downstream
o arteries expand temporarily, hold "excess" ejected blood
• during diastole heart does not pump blood into arteries, stretched arterial walls recoil, "excess" blood pushed to vessels downstream
• thus arteries play role in dampening pressure fluctuations occurring during cardiac cycle in ventricles

C. Arterial pressure
- arterial pressure not constant as volume of blood entering arteries during systole is 1/3 greater to volume of blood leaving arteries during diastole
• systolic pressure: highest pressure in arteries at peak of ejection (120 mm Hg) o only 1/3 of blood that enters arteries during this period leaves these vessels
• diastolic pressure: lowest pressure in arteries during cardiac cycle (70 mm Hg)
o lowest pressure achieved in arteries as blood is draining into remainder of vessels during diastole
• pulse pressure: systolic pressure - diastolic pressure
• mean arterial pressure: (map) average pressure in artery throughout 1 turn of the cardiac cycle o (diastolic + 1/3PP)
VI. Arterioles
A. Functional characteristics
- media proportionately the predominant layers, composed primarily of smooth muscle
- are the major resistance vessels of the vascular tree • mean arterial pressure before arterioles is 93 mm Hg; pressure of blood leaving arterioles is 37 mm Hg
• arteriolar resistance also converts pulsatile systolic-diastolic pressure swings in arteries to non-pulsatile pressure seen in capillaries
• resistance changes achieved by varying radius of vessels
o small change in radius, large change in resistance to blood flow and thus blood pressure § vasodilation § vasoconstriction
o thus arterioles are prime controllers and regulators of blood pressure - arterioles display a state of partial constriction, vascular tone -- establishes a baseline resistance to blood flow

- state of partial constriction largely due to: • sympathetic fibers innervate media -- vasomotor fibers
o tonically discharge
o release norepinephrine -- in most beds maintains basal vascular tone
o no parasympathetic innervation to arterioles § vasoconstriction -- increase sympathetic discharge § vasodilation -- decrease sympathetic discharge
B. Local control of arteriolar radius -- autoregulation: capacity of tissues to regulate own blood flow
- variably distributes cardiac output among various systemic beds so that blood flow matches tissues' metabolic needs
• metabolic hypothesis
o accumulation/absence of metabolites produces vasodilation/vasoconstriction of arterioles
o the following produce relaxation of arteriolar smooth muscle (arteriolar dilation): § increased pCO2 § decreased pO2 § lincreased actic acid § adenosine release § increased K+ § increased temperature
• myogenic hypothesis
o vessel responds to increased stretch by reflex contraction
o vessel responds to decreased blood flow by myogenic relaxation -increases blood flow through area
• example of reactive hyperemia -- response of blood vessel to occlusion o what happens when occlusion removed
o what is role of myogenic and metabolic autoregulation processes in response?

C. Systemic control of arteriolar radius
1. control by hormones- systemic regulation of arteriolar diameter • norepinephrine/epinephrine o norepinephrine § released by vasomotor fibers in arteriole media § high affinity for a receptors -- generalized vasoconstrictor effect § can bind b receptors -- vasodilatory effect o epinephrine o most abundant of medullary hormones o high affinity for b receptors -- vasodilatory effect § dilates vessels in skeletal muscle • atrial natriuretic factor (ANF) o decreases blood pressure by promoting fluid loss from plasma • vasopressin (ADH) -- elevates blood pressure o promotes water reabsorption in kidneys o vasoconstrictor • angiotensin II o part of renin-angiotensin-aldosterone cascade o important in maintenance of blood pressure during hemorrhage and shock • histamine o inflammatory response
2. Neural regulation - systemic regulation of cardiovascular function - Flow (F) = Pressure (P)/ Resistance (R) - CO = BP/R --> CO = BP x r4 - since resistance is varied by altering arteriolar diameter, resistance is peripheral in circulation -- total peripheral resistance (TPR) - CO = BP/TPR --> BP = CO x TPR

- thus can vary blood pressure by changing cardiac output and varying resistance of arterioles • vasomotor tone maintains vascular tone of arterioles
o maintains adequate driving pressure of blood to all systemic beds
§ if all arterioles dilate, blood pressure falls substantially, lose adequate driving force for blood flow
o individual beds can use autoregulatory and local mechanisms to fine adjust amount of blood flow -- however need pressure head to drive flow
IV. Capillaries
- sites of exchanges (solutes and fluids) between blood and the tissues
- exchanges between blood and the tissues are passive • diffusion -- solutes • bulk flow -- fluid
- capillary structure permits such functions: • diffusing molecules travel very short distances between blood and ISF and cells • capillaries very narrow • capillaries are very thin -- 1 mm diameter o single layer of flattened endothelial cells
• total surface area of capillaries is tremendous o influence on velocity of blood flow: recall that velocity is displacement per unit time (cm/s) while flow is volume per unit time (cm3/s)
o velocity (V) is proportional to flow (F) divided by area o V=F/A (cm/s = cm3/s/cm2)
• structure of capillary wall o exchanges possible across cell § diffusion § vesicular transport
o exchanges possible between cell junctions
§ exact amount regulated by state of junction -- tight junction integrity and dynamics

o exchanges possible via "pores" in cells, fenestrations - a capillary bed and regulation of capillary perfusion:
• arteriole
• metarteriole -- thoroughfare channel
• true capillaries
o precapillary sphincters -- open or close in response to metabolic status of tissue; work with arteriole autoregulation in control of perfusion through vascular bed
• post-capillary venule - capillary exchanges -- diffusion of solutes across capillary wall
• exchanges occur between plasma and ISF (80% ECF)
o composition of ISF reflects composition of plasma (20% ECF)
§ thus regulate composition of plasma to regulate composition of ISF (most ECF)
• exchanges of solutes by simple or facilitated diffusion - capillary exchanges -- bulk flow
• movement of protein- free plasma out of capillary into ISF (filtration) at arterial end of capillary; movement of protein- free fluid from ISF into capillary (reabsorption) at venule end of capillary
• occurs because of differences between hydrostatic and osmotic pressures of plasma and ISF
o outward pressures
§ capillary hydrostatic pressure
§ ISF osmotic pressure
o inward pressures
§ plasma osmotic pressure
§ ISF hydrostatic pressure
• in most capillaries outward pressures prevail and arteriolar end and inward pressure greater at venule end
o some capillaries reabsorption along full length
o some capillaries filtration along full length

• note that on average more fluid filters out at arteriole end than at venule end o this fluid returned to circulation by lymphatics o other roles of lymphatics -- immune, GI absorption of fat
- clinical example of capillary dynamics -- edema • reduced concentration of plasma proteins o renal failure o liver failure o protein deficient-diet • increased permeability of capillary walls • increased venous pressure o pregnancy -- edema in legs • blockage of lymph vessels -- elephantiasis
V. Veins
- veins are capacitance vessel -- on average 64% of blood in circulatory system at one time found in veins
- pressure gradient that drives flow through veins very small; veins have structural adaptation that allow them to perform their function -- return blood to heart -- despite this low gradient:
• very thin walls, little elastin • little myogenic tone • large radii -- offer very little resistance to flow • have valves -- unidirectional flow of blood through veins
o valve dysfunction § varicose veins § hemorhoids
- factors that affect venous capacity will influence venous return and thus cardiac output (Starling's law):
• effect of vasomotor sympathetic tone on venous return o vasoconstriction decreases venous capacity and increases venous return o vasodilation increases venous capacity and decreases venous return

• effect of skeletal muscle activity on venous return
o increased skeletal muscle activity milks veins -- increases venous return
• effect of respiratory pump
o inspiration -- intra-thoracic pressure less than intra-abdominal -- suction of blood to heart
• cardiac suction VI. Regulation of blood pressure
1. Short term regulatory mechanisms: neural regulation of BP
- cardiovascular center (CV) in the medulla: • Vasomotor center (VM): gives rise to sympathetic fibers that innervate smooth muscles of arterioles and veins; tonically discharges, arterioles always partially constricted, vasomotor tone; increased sympathetic activity will increase vasomotor tone (vasoconstriction); decreased sympathetic activity will decrease vasomotor tone (vasodilation)
• Cardioaccelerator center (CA): gives rise to sympathetic fibers that when activated increase HR and contractility of cardiac muscle
• Cardioinhibitory center (CI): gives rise to parasympathetic fibers that cause a decrease in HR. 1. innervation of blood vessels (sympathetic)
-adrenergic fibers -originate in VM center (VC)
2. innervation of heart (sympathetic)
-originate in VM center (CA) 3. innervation of heart (PS)
-originate in CI center
-examine tonic discharge of each
• tonic discharge of VC- affects to veins and arterioles • tonic discharge of CA vs CI- which one predominates 4. Afferents to cardioregulatory center
a. baroreceptors
b. chemoreceptors -- role in blood pressure regulation
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