Download Human Physiology

Chapter 13
Blood, Heart
and Circulation
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Components of Circulatory System
Include
cardiovascular and lymphatic systems
Heart pumps blood thru cardiovascular system
Blood vessels carry blood from heart to cells and back
Includes arteries, arterioles, capillaries, venules, veins
Lymphatic system picks up excess fluid filtered out in
capillary beds and returns it to veins
Its lymph nodes are part of immune system
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Structure of Heart
 Heart
has 4 chambers
 2 atria receive blood from venous system
 2 ventricles pump blood to arteries
 2 sides of heart are 2 pumps separated by muscular septum
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Cardiac Cycle
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Cardiac Cycle
Is
repeating pattern of contraction and relaxation of heart
Systole refers to contraction phase
Diastole refers to relaxation phase
Both atria contract simultaneously; ventricles follow 0.10.2 sec later
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Cardiac Cycle
End-diastolic volume
is volume of blood in ventricles at end
of diastole
Stroke volume is amount of blood ejected from ventricles
during systole
End-systolic volume is amount of blood left in ventricles at
end of systole
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Cardiac Cycle continued
 As
ventricles contract, pressure rises, closing AV valves
 Called isovolumetric contraction because all valves are closed
 When pressure in ventricles exceeds that in aorta, semilunar
valves open and ejection begins
 As pressure in ventricle falls below that in aorta, back pressure
closes semilunars
 All valves are closed and ventricles undergo isovolumetric
relaxation
 When pressure in ventricles falls below atria, AVs open and
ventricles fill
 Atrial systole sends its blood into ventricles
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Electrical Activity of Heart
Myocardial
cells are short, branched, and interconnected by
gap junctions
Entire muscle that forms a chamber is called a myocardium
or functional syncytium
Because APs originating in any cell are transmitted to all
others
Chambers separated by nonconductive tissue
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SA Node Pacemaker
In
normal heart, SA
node functions as
pacemaker
Depolarizes
spontaneously to
threshold
(= pacemaker
potential)
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SA Node Pacemaker continued
 Membrane
voltage begins at 60mV and gradually
depolarizes to -40 threshold
 Spontaneous depolarization
is caused by Na+ flowing
through channel that opens
when hyperpolarized (HCN
channel)
 At threshold V-gated Ca2+
channels open, creating
upstroke and contraction
 Repolarization is via opening
of V-gated K+ channels
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Conducting Tissues of Heart continued
 In
septum of ventricles,
His divides into right and
left bundle branches
 Which give rise to
Purkinje fibers in walls
of ventricles
 These stimulate
contraction of
ventricles
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Conduction of APs
APs
from SA node spread at rate of 0.8 -1 m/sec
Time delay occurs as APs pass through AV node
Has slow conduction of 0.03– 0.05 m/sec
AP speed increases in Purkinje fibers to 5 m/sec
Ventricular contraction begins 0.1–0.2 sec after
contraction of atria
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Refractory Periods
 Heart
contracts as
syncytium and thus
cannot sustain force
 Its AP lasts about 250
msec
 Has a refractory period
almost as long as AP
 Cannot be stimulated to
contract again until has
relaxed
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Arteries
Large
arteries are muscular and elastic
Contain lots of elastin
Thicker compared to veins
Part of autonomic nervous system
Expand during systole and recoil during diastole
Helps maintain smooth blood flow during diastole
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Arteries
 Small
arteries and arterioles are muscular
 Provide most resistance in circulatory system
 Arterioles cause greatest pressure drop
 Mostly connect to capillary beds
 Some connect directly to veins to form arteriovenous
anastomoses
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Capillaries
Provide
extensive surface area for exchange
Blood flow through a capillary bed is determined by state of
precapillary sphincters of arteriole supplying it
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Types of Capillaries
 In
continuous capillaries, endothelial cells are tightly joined
together
 Have narrow intercellular channels that permit exchange of
molecules smaller than proteins
 Present in muscle, lungs, adipose tissue
 Fenestrated capillaries have wide intercellular pores
 Very permeable
 Present in kidneys, endocrine glands, intestines.
 Discontinuous capillaries have large gaps in endothelium
Are large and leaky
Present in liver, spleen, bone marrow
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Veins
Contain
majority of blood in circulatory system
Very compliant (expand readily)
Contain very low pressure (about 2mm Hg)
Insufficient to return blood to heart
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Veins
 Blood
is moved toward
heart by contraction of
surrounding skeletal
muscles (skeletal
muscle pump)
 And pressure drops
in chest during
breathing
 1-way venous
valves ensure blood
moves only toward
heart
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Heart Disease
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Atherosclerosis
 Is
most common form of
arteriosclerosis (hardening
of arteries)
 Accounts for 50% of
deaths in US
 Localized plaques
(atheromas) reduce flow
in an artery
 And act as sites for
thrombus (blood clots)
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Atherosclerosis
Plaques
begin at sites
of damage to
endothelium
E.g. from
hypertension,
smoking, high
cholesterol, or
diabetes
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Atherosclerosis
Plaques
begin at sites
of damage to
endothelium
E.g. from
hypertension,
smoking, high
cholesterol, or
diabetes
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Cholesterol and Plasma Lipoproteins
 High
blood cholesterol is
associated with risk of
atherosclerosis
 Lipids, including
cholesterol, are carried in
blood attached to LDLs
(low-density lipoproteins)
and HDLs (high-density
lipoproteins)
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Cholesterol and Plasma Lipoproteins
LDLs
and HDLs are produced in liver and taken into cells
by receptor-mediated endocytosis
In cells LDL is oxidized
Oxidized LDL can injure endothelial cells facilitating
plaque formation
Arteries have receptors for LDL but not HDL
Which is why HDL isn't atherosclerotic
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Lymphatic System
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Lymphatic System
Has
3 basic functions:
Transports interstitial fluid (lymph) back to blood
Transports absorbed fat from small intestine to blood
Helps provide immunological defenses against pathogens
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Lymphatic System continued
Lymphatic
capillaries
are closed-end tubes
that form vast
networks in
intercellular spaces
Very porous, absorb
proteins,
microorganisms, fat
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Lymphatic System continued
Lymph
is carried
from lymph
capillaries to
lymph ducts to
lymph nodes
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Lymphatic System continued
 Lymph
nodes filter lymph
before returning it to
veins via thoracic duct or
right lymphatic duct
 Nodes make lymphocytes
and contain phagocytic
cells that remove
pathogens
 Lymphocytes also made
in tonsils, spleen, thymus
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