BIOL 3542 Lecture Notes - Lecture 15: Vascular Smooth Muscle, Metarteriole, Elastic Fiber
26 Jun 2018
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Human Physiology 2
Chapter 15: Blood Flow and the Control of Blood Pressure
The Blood Vessels
blood vessel walls composed of layers of smooth muscle, elastic connective tissue and
fibrous connective tissue
inner lining of blood vessels is thin layer of endothelium (type of epithelium)
endothelial cells secrete paracrine signals, help regulate blood pressure, blood vessel growth,
absorption of materials
Tunica Intima (Intima): endothelium and adjacent elastic connective tissue
thickness of smooth muscle and connective tissue layers varies in different vessels
Blood Vessels Contain Vascular Smooth Muscle
Vascular Smooth Muscle: smooth muscle of blood vessels arranged in circular or spiral layers
vasoconstriction narrows diameter of vessel lumen
vasodilation widens it
smooth muscle cells maintain state of partial contraction at all times, creating condition
called muscle tone
contraction depends on calcium entry from extracellular fluid
signal molecules including neurotransmitters, hormones, paracrine signals, influence vascular
smooth muscle tone
vasoactive paracrine molecules secreted by endothelial cells lining blood vessels or tissues
surrounding vessels
Arteries and Arterioles Carry Blood Away from the Heart
arteries have thick smooth muscle layer, large amounts of elastic and fibrous connective
tissue
because of stiffness of fibrous tissue, substantial energy required to stretch artery walls
outward
required energy stored by stretched elastic fibres, released through elastic recoil
arteries, arterioles characterized by divergent pattern of blood flow
as major arteries divide, walls of smaller arteries become less elastic, more muscular
arteriole walls contain several smooth muscle layers that contract, relax under influence of
chemical signals
Microcirculation: arterioles, capillaries, venules (post-capillary vessels)
some arterioles branch into metarterioles which are only partially surrounded by smooth
muscle whereas true arterioles have continuous smooth muscle layer
if precapillary sphincters relaxed, blood flowing into metarteriole directed into adjoining
capillary beds
if precapillary sphincters constricted, metarteriole blood bypasses capillaries, goes directly to
venous circulation
Exchange Takes Place in the Capillaries
capillaries are smallest vessels in cardiovascular system
primary site of exchange between blood and interstitial fluid
to facilitate exchange, lack smooth muscle and elastic or fibrous tissue reinforcement
walls consist of layer of endothelium one cell thick, supported on acellular matrix called
basal lamina (basement membrane)
highly branched, contractile pericyte cells form mesh-like outer layer between capillary
endothelium and interstitial fluid
the more pericytes, the less leaky the capillary endothelium
pericytes secrete factors the influence capillary growth and can differentiate to become new
endothelial or smooth muscle cells
loss of pericytes around capillaries of retina hallmark of diabetic retinopathy
Blood Flow Converges in the Venules and Veins
blood flows from capillaries into smaller venules
smallest venules similar to capillaries with thin exchange epithelium and little connective
tissue
distinguished from capillaries by convergent pattern of blood flow
smooth muscle present in walls of larger venules
blood flows from venules to veins that become larger in diameter as they travel to heart
venae cavae (largest veins) empty into right atrium
some veins have internal one-way valves to ensure blood cannot flow backward
no valves in venae cavae
more veins than arteries
veins have larger diameter
veins hold more than half the blood in circulatory system, making them the volume reservoir
veins lie closer to surface of body than arteries
veins have thinner walls than arteries, with less elastic tissue, so they expand easier when
filled with blood
Angiogenesis Creates New Blood Vessels
Angiogenesis: process by which new blood vessels develop, especially after birth
blood vessel growth in children necessary for development
in adults, angiogenesis takes place as wounds heal, uterine lining grows after menstruation
and with endurance exercise training, enhancing blood flow to heart muscle and skeletal
muscles
growth of malignant tumors requires angiogenesis
cancer cells instruct host tissue to develop new blood vessels to feed growing tumor
angiogenesis controlled by balance of angiogenic, antiangiogenic cytokines
related growth factors including vascular endothelial growth factor (VEGF) and fibroblast
growth factor (FGF) promote angiogenesis
growth factors are mitogens (promote mitosis)
produced by smooth muscle cells and pericytes
cytokines that inhibit angiogenesis include angiostatin made from blood protein
plasminogen, and endostatin
Coronary Heart Disease (coronary artery disease): condition in which blood flow to
myocardium decreased by fatty deposits that narrow lumen of coronary arteries
new blood vessels develop spontaneously in some people to form collateral circulation
that supplements flow through partially blocked artery
Blood Pressure
as blood is ejected from left ventricle, aorta and arteries expand to accommodate it
when ventricle relaxes and aortic valve closes, elastic arterial walls recoil, propelling blood
forward into arteries, arterioles
arteries keep blood flowing by sustaining driving pressure for blood flow
Blood Pressure Is Highest in Arteries and Lowest in Veins
blood pressure highest in arteries, decreases continuously as blood flows through circulatory
system
aortic pressure reaches average high of 120mmHg during ventricular systole (systolic
pressure) the falls to low of 80mmHg during ventricular diastole (diastolic pressure)
high diastolic arterial pressure reflects ability of arteries to capture, store energy in their
elastic walls
rapid pressure increase that occurs when left ventricle pushes blood into aorta can be felt as a
pulse, or pressure wave, transmitted through fluid-filled arteries
travels 10x faster than blood itself
pulse felt occurs slightly after ventricular contraction that created wave
amplitude of pressure wave decreases over distance because of friction, disappears at
capillaries
Pulse Pressure: measure of strength of pressure wave
systolic pressure – diastolic pressure = pulse pressure
by time blood reaches veins, pressure has decreased from friction, pressure wave no longer
exists
venous blood flow steady rather than pulsatile (in pulses), pushed along by continuous
movement of blood out of capillaries
blood return to heart (venous return) aided by valves, skeletal muscle pump and respiratory
pump
when muscles contract, they compress veins, which forces blood upward past valves
Arterial Blood Pressure Reflects the Driving Pressure for Blood Flow
arterial blood pressure, or simply “blood pressure” reflects driving pressure created by
pumping action of heart
customary to assume arterial blood pressure reflects ventricular pressure
arterial pressure is pulsatile so we use single value – mean arterial pressure (MAP) – to
represent driving pressure
MAP = diastolic P + 1/3(systolic P – diastolic P)
MAP closer to diastolic pressure than systolic pressure because diastole lasts twice as long as
systole
Document Summary
Chapter 15: blood flow and the control of blood pressure. Tunica intima (intima): endothelium and adjacent elastic connective tissue thickness of smooth muscle and connective tissue layers varies in different vessels. Arteries and arterioles carry blood away from the heart arteries have thick smooth muscle layer, large amounts of elastic and fibrous connective tissue because of stiffness of fibrous tissue, substantial energy required to stretch artery walls outward. Exchange takes place in the capillaries capillaries are smallest vessels in cardiovascular system primary site of exchange between blood and interstitial fluid to facilitate exchange, lack smooth muscle and elastic or fibrous tissue reinforcement. Cancer cells instruct host tissue to develop new blood vessels to feed growing tumor angiogenesis controlled by balance of angiogenic, antiangiogenic cytokines related growth factors including vascular endothelial growth factor (vegf) and fibroblast growth factor (fgf) promote angiogenesis. Produced by smooth muscle cells and pericytes cytokines that inhibit angiogenesis include angiostatin made from blood protein plasminogen, and endostatin.
