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Circulatory
Systems
Chapter 8; 25-27.10.2010
Overview
 Most metazoans larger than a few cells have
circulatory systems
 Major function
 Transport oxygen, carbon dioxide, nutrients, waste
products, immune cells, and signaling molecules
throughout the body
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Components of Circulatory Systems
 Circulatory systems move fluids by increasing the
pressure of the fluid in one part of the body
 Fluid flows through the body, “down” the pressure
gradient
 Three main components are needed
 Pump or propulsive structures
 For example, a heart
 System of tubes, channels, or spaces
 Fluid that circulates through the system
 For example, blood
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Types of Fluid
 Interstitial fluid
 Extracellular fluid that directly bathes the tissues
 Blood
 Fluid that circulates within the vessels of a closed
circulatory system
 Lymph
 Fluid that circulates in the secondary circulatory
system of vertebrates - the lymphatic system
 Hemolymph
 Fluid that circulates in an open circulatory system
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Types of Pumps
 Chambered hearts
 Contractile chambers
 Blood enters atrium
 Blood is pumped out by
ventricle
 Skeletal muscle
 Squeeze on vessels to
generate pressure
 Pulsating blood vessels
 Peristalsis
 Rhythmic contractions of
vessel wall pumps blood
Figure 8.2
 One-way valves help to
ensure unidirectional flow
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Closed and Open Circulatory Systems
(System of tubes, channels, or spaces)
 Closed
 Circulatory fluid remains within vessels and does not
come in direct contact with the tissues
 Circulating fluid is distinct from interstitial fluid
 Molecules must diffuse across vessel walls
 Open
 Circulatory fluid comes in direct contact with the
tissues in spaces called sinuses
 Circulating fluid mixes with interstitial fluid
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Closed and Open Circulatory Systems
Gastrovascular cavity
thin body walls
flagella stir fluid
Fig. 9.1
Open circulatory system
hemolymph
large volume
low pressure
most invertebrates
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Closed circulatory system
blood
small volume
high pressure
vertebrates
some invertebrates
1. OPEN CIRCULATORY SYSTEM
(most invertebrates)
Low pressure (< 1.5 kPa)
High volume (30% body vol.)
Slow velocity
Fig. « 9.7 »
2. CLOSED CIRCULATORY SYSTEM
(cephalopods, vertebrates (fish, amphibian,
reptiles, birds, mammals))
High pressure (>12 kPa)
Low volume (5-10%)
High velocity
Distribution
Ultrafiltration
Fig. 9.10
Circulation in fish (water breathing)
respiratory and systemic circulations in series
sinus venosus – precursor of SA node in mammals
Amphibians and Reptiles
 Heart is only partially divided
 Two atria and one ventricle
 A three-chambered heart
 Blood from both atria flow into the ventricle
 Oxygenated and deoxygenated blood can mix
 Oxygenated and deoxygenated blood are kept fairly
separate by a mechanism that is not completely
understood
 Ventricle pumps blood into pulmonary and systemic
circuits
 Blood can be diverted between pulmonary and systemic
circuits
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Circulation in birds and mammals
respiratory and systemic circulations
in parallel
Four chambered heart
Two completely separate circuits
pulmonary circuit
systemic circuit
(low pressure)
(high pressure)
 Muscular, chambered heart contracts to increase
the pressure of the blood
 Blood flows away from the heart in arteries
 Arteries branch to form more numerous, but
smaller diameter, arteries
 Small arteries branch into arterioles within
tissues
 Blood flows from arterioles into capillaries
 Capillaries are the site of diffusion of
molecules between blood and interstitial
fluid
 Capillaries coalesce to form venules
 Venules coalesce to form veins
 Veins carry blood to the heart
Evolution of Circulatory Systems
 First evolved to transport nutrients to body cells
 Very early began to serve respiratory function
 Closed systems evolved independently in jawed
vertebrates, cephalopods, and annelids
 Increased blood pressure and flow
 Increased control of blood distribution
 Closed systems evolved in combination with
specialized oxygen carrier molecules
 High metabolic rates
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Structure of Vertebrate Blood Vessels
 A complex wall surrounding a central lumen
 Wall composed of up to three layers
 Tunica intima (internal lining)
 Smooth, epithelial cells (vascular endothelium)
 Tunica media (middle layer)
 Smooth muscle
 Elastic connective tissue
 Tunica externa (outermost layer)
 Collagen
 Thickness of the wall varies among vessels
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Structure of Vertebrate Blood Vessels
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Figure 8.10
Capillaries
 Lack tunica media and tunica externa
 Continuous
 Cells held together by tight junctions
 Skin and muscle
 Fenestrated
 Cells contain pores
 Specialized for exchange
 Kidneys, endocrine organs, and
intestine
 Sinusoidal
 Few tight junctions
 Most porous for exchange of large
proteins
 Liver and bone marrow
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Figure 8.11
Anatomy of the mammalian heart
adult heart
four chambers (two atria, thin-walled; two
ventricles, thick-walled), complete
separation of left and right heart (ventricles
separated by intraventricular septum)
fetal heart
foramen ovale
ductus arteriosus
pulmonary circuit not
functional
CARDIAC VALVES
•thin flaps of fibrous tissue
•move passively in response to
differential pressures
•prevent flow of blood
Atrioventricular valves
Between atria and ventricles
Tricuspid on right
Bicuspid on left
Semilunar valves
Between ventricles and arteries
Aortic between left ventricle and aorta
Pulmonary between right ventricle and pulmonary artery
Heart Murmurs
Valve deformities  abnormal blood flow  murmurs
CORONARY CIRCULATION
•heart receives 4-5% of blood pumped
•coronary arteries arise at base of aorta
•blood enters only during ventricular
relaxation due to force of elastic recoil of aorta
•blockage of coronary arteries
causes heart failure
•coronary bypass
Vertebrate Hearts
 Complex walls with four main parts
 Pericardium
 Sac of connective tissue that surrounds heart
 Outer (parietal ) and inner (visceral) layers
 Space between layers filled with lubricating fluid
 Epicardium
 Outer layer of heart, continuous with visceral pericardium
 Contain nerves that regulate heart and coronary arteries
 Myocardium
 Layer of heart muscle cells (cardiomyocytes)
 Endocardium
 Innermost layer of connective tissue covered by epithelial
cells (called endothelium)
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Myocardium
 Two types of myocardium
 Compact
 Tightly packed cells arranged in regular pattern
 Spongy
 Meshwork of loosely connected cells
 Relative proportions vary among species
 Mammals
 Mostly compact
 Fish and amphibians
 Mostly spongy
 Arranged as trabeculae that extend into chambers
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Myocardium
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Figure 8.17
Conducting Pathways
 Modified cardiomyocytes
 Cells with elongated, pale appearance
 Do not contract
 Spread action potential rapidly throughout
myocardium
 Can undergo rhythmic depolarizations
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Electrical conduction in the mammalian heart
depolarization (contraction)
repolarization (relaxation)
Normal pacemaker is the
SINOATRIAL NODE
Heart beat:
begins at SA node
spreads through atria
delayed at AV node
spreads through ventricles
Fig. 9.23
Control of Contraction
 Vertebrate hearts are myogenic
 Cardiomyocytes produce spontaneous rhythmic
depolarizations
 Do not require nerve signal
 Cardiomyocytes are electrically coupled via gap
junctions to ensure coordinated contractions
 Action potential passes directly from cell to cell
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Control of Contraction
 Pacemaker
 Cells with fastest intrinsic rhythm
 In the sinus venosus in fish
 In the right atrium of other vertebrates
 Sinoatrial (SA) node
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Pacemaker Cells
 Derived from
cardiomyocytes
 Characteristics of
pacemaker cells
 Small with few myofibrils,
mitochondria, or other
organelles
 Do not contract
 Have unstable resting
membrane potential
(pacemaker potential) that
drifts upwards until it
reaches threshold and
initiates an action potential
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Figure 8.23
Control of Pacemaker Potentials
 Increasing heart rate
 Norepinephrine released from sympathetic neurons
and epinephrine released from the adrenal medulla
 More Na+ and Ca2+ channels open
 Rate of depolarization and frequency of action
potentials increase
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Increasing Heart Rate
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Figure 8.24
Control of Pacemaker Potentials
 Decreasing heart rate
 Acetylcholine released from parasympathetic neurons
 More K+ channels open
 Pacemaker cells hyperpolarize
 Time for depolarization takes longer, frequency of
action potentials decreases
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Decreasing Heart Rate
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Figure 8.25
Fig. 9.25 Electrocardiogram (ECG) tracings
P wave
depolarization of atrium
QRS
depolarization of ventricle
T wave
repolarization of ventricle
(repolarization of atrium masked by QRS)
Mammalian Cardiac Cycle
 Cardiac cycle – pumping action of the heart
 Two phases
 Systole
 Contraction
 Blood is forced out into the circulation
 Diastole
 Relaxation
 Blood enters the heart
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Mammalian Cardiac Cycle
 Atria and ventricles alternate systole and diastole
 The two atria contract simultaneously
 There is a slight pause
 The two ventricles contract simultaneously
 Atria and ventricles relax while the heart fills with
blood
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CARDIAC CYCLE
cardiac contraction (systole, 0.3 sec in human)
cardiac relaxation (diastole, 0.5 sec in human)
Ventricular Pressure
 Left ventricle contracts more forcefully and
develops higher pressure
 Resistance in pulmonary circuit low due to high
capillary density in parallel
 Large cross-sectional area
 Less pressure needed to pump blood through
pulmonary circuit
 Low pressure protects delicate blood vessels of
lung
 Systemic and pulmonary circuits have same blood
flow
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Ventricular Pressures
Pressure changes in the heart and arteries of mammals
•greater pressure in left heart (supplies systemic circuit)
•lower pressure in right heart (supplies pulmonary circuit)
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Figure 8.22
Electrical and Mechanical Events in the
Cardiac Cycle
 Heart functions as an
integrated organ
 Electrical and
mechanical events are
correlated
 Changes in pressure and
volume of chambers
 Blood flow through
chambers
 Heart sounds
 Opening and closing of
valves
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Physics of Blood Flow
Law of bulk flow: Q = DP/R
Q = flow
DP = pressure drop
R = resistance
R = 8Lh /pr4
L = length of the tube
h = viscosity of the fluid
r = radius of the tube
vasoconstriction / vasodilation
Poiseuille’s equation: Q = DPp r4 / 8Lh
 More detailed version of law of bulk flow
relationship between pressure and flow in small terminal arteries,
capillaries, and veins; flow & resistance most influenced by vessel diameter
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Physics of Blood Flow
R = resistance
R = 8Lh /pr4
L = length of the tube
h = viscosity of the fluid
r = radius of the tube
vasoconstriction / vasodilation
Poiseuille’s equation: Q = DPp r4 / 8Lh
resistance =
(P1 - P2)
Q
= 8Lη
pr4
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STROKE VOLUME (SV) - Volume of blood pumped with each
beat
= (end diastolic vol. - end systolic vol.) in healthy humans at rest
stroke volume ~ (140 - 60) ml = 80 ml
both ventricles eject same volume of blood (total blood volume in humans 5-6 L)
SV
regulated by:
end diastolic volume
SV  EDV
mean arterial pressure
SV  1/MAP
contractility
SV  C
End-diastolic volume determined by:
venous filling pressure
venoconstriction, skeletal muscle pump
atrial pressure
ventricular distensibility
filling time
Blood Pressures in human circulation
usually reported as mm Hg ( = torr)
normal range 120-130/80-85 mm Hg
(systolic/diastolic pressure)
CARDIAC OUTPUT (CO) - Volume of blood pumped
per unit time
= stroke volume x heart rate
in healthy humans at rest, heart rate ~70 beats/min
cardiac output = 80 ml x 70 = 5.6 L/min
Cardiac Output
CO = HR  SV
 Cardiac output can be modified by regulating heart
rate and/or stroke volume
 Heart rate
 Modulated by autonomic nerves and adrenal medulla
 Decreased HR (bradycardia)
 Increased HR (tachycardia)
 Stroke volume
 Modulated by various nervous, hormonal, and physical
factors
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Distribution of cardiac output during exercise
Supplying O2 during exercise
in human
increase O2 delivery via:
increased Hb saturation
increase O2 extraction
increased cardiac output
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Control of Stroke Volume
 Nervous and endocrine systems can cause the heart to contract more
forcefully and pump more blood with each beat
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Figure 8.30
Frank-Starling Law of the Heart
Contractility  EDV
Increase in EDV causes:
•  in myocardial stretch
•  in contractile tension
•  in ventricular systolic pressure
 ventricular filling during diastole,
 ejection during systole
heart receives & ejects given
volume of blood each cardiac
cycle.
Level of sympathetic activity shifts the position of
the cardiac muscle length-tension relationship
Fig. 9.28
CIRCULATION
How do velocity and pressure change as blood flows
from heart through various blood vessels:
Geometry of Blood Vessels (dog)
Type
D(mm)
Number
Aorta
10
1
Arteries
3
40
Arterioles 0.02
40000000
Capillaries 0.008
1200000000 600
Venules
0.03
80000000
Veins
6
40
Vena Cava 12.5
1
Total area (cm2)
0.8
3
125
60
570
11
1.2
Total volume (mL)
30
60
25
110
220
50
Laminar and turbulent flow
in vessels
Generally in the body, blood flow
is laminar. However, under
conditions of high flow,
particularly in the ascending
aorta, laminar flow can be
disrupted and become turbulent.
When this occurs, blood does
not flow linearly and smoothly in
adjacent layers, but instead the
flow can be described as being
chaotic. Turbulent flow also
occurs in large arteries at branch
points, in diseased and
narrowed (stenotic) arteries, and
across stenotic heart valves.
http://www.cvphysiology.com/Hemodynamics/H007.htm
Modeling Circulatory Systems
 Like electrical
resistors, blood vessels
can be arranged in
series or parallel
 Resistors in series
 RT = R1 + R2…
 Resistors in parallel
 1/RT = 1/R1 + 1/R2…
 Because of the law of
conservation of mass,
the flow through each
segment of the system
must be equal
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Figure 8.14
COMPLIANCE
increased P  stretch  increased volume
increased r  decreased resistance  increased flow
Compliance = D volume/D pressure
Venous system:
very compliant \volume reservoir
large volume changes result in small pressure changes
Arterial system:
less compliant \ pressure reservoir
maintain capillary flow
Compliance of veins 24x greater than arteries:
(except elastic aortae which dampen pressure oscillations)
Veins Act as a Volume Reservoir
 Veins have thin, compliant
walls
 Small increases in blood
pressure lead to large
changes in volume
 In mammals, veins hold
more than 60% of the
blood
 Vein volume (and venous
return) controlled by
sympathetic nerves
 Venomotor tone
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Figure 8.36
Blood Velocity
 Flow (Q)
 Volume of fluid transferred per unit time
 Velocity
 Distance per unit time
 Blood velocity = Q/A
 A = cross-sectional area of the channels
 Velocity of flow is inversely related to total crosssectional area
 For example, total cross-sectional area of capillaries is very
large  velocity is slow  long time for diffusion
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Transmural Pressure
 Pressure exerts a force across vessel wall
 Law of LaPlace
 T = aPr
 T = tension in vessel wall
 a = constant
 0.5 for a vessel
 1.0 for a chamber
 P = transmural pressure
 difference between
internal and
external pressure
 r = vessel radius
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Figure 8.15
Pressure in Vertebrate Circulatory Systems
 Blood pressure in left
ventricle changes with
systole and diastole
 Pressures decreases as
blood moves through
system
 Pressure and pulse decrease
in arterioles due to high
resistance
 Velocity of blood highest
in arteries, lowest in
capillaries, and
intermediate in veins
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Figure 8.33
Arteries Act as Pressure Dampeners
 Pressure fluctuations in
arteries are smaller than
those in left ventricle
 Aorta acts as a pressure
reservoir
 Elasticity of vessel wall
 Expands during
systole
 Elastic recoil during
diastole
 Dampens pressure
fluctuations
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Figure 8.34
Mean Arterial Pressure (MAP)
 Average arterial pressure over time
 MAP = 2/3 diastolic pressure + 1/3 systolic pressure
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Table 8.2
Moving Blood Back to the Heart
 Blood in veins is under
low pressure
 Two pumps assist in
moving blood back the
heart
 Skeletal muscle
 Contraction (shortening
and thickening) of
muscle squeezes vein
 Respiratory pumps
 Pressure changes in
thoracic cavity during
ventilation
 Valves in veins assure
unidirectional flow
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Figure 8.35
ARTERIOLES
•control blood distribution to tissues
•surrounded by smooth muscle
•vasoconstriction, vasodilation
Q (flow rate) = (P1- P2) pr4
8Lh
nutritional flow
waste disposal
regulation of cell activities
Regulation of Blood Flow
 Arterioles control blood distribution
 Because arterioles are arranged in parallel, they can
alter blood flow to various organs
 Vasoconstriction and vasodilation
 Changes in resistance alter flow
 Control of vasoconstriction and vasodilation
 Autoregulation
 Direct response of the arteriole smooth muscle
 Intrinsic factors
 Metabolic state of the tissue
 Extrinsic factors
 Nervous and endocrine systems
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Myogenic Autoregulation of Flow
 Some smooth muscle cells in arterioles are
sensitive to stretch and contract when blood
pressure increases
 Acts as negative feedback loop
 Prevents excessive flow of blood into tissue
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Metabolic Activity of Tissues
 Smooth muscle cells in
arterioles sensitive to
conditions of extracellular fluid
 Levels of metabolites alter
vasoconstriction/vasodilation
 Blood flow matched to
metabolic requirements
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Figure 8.32
Neural and Endocrine Control of Flow
 Norepinephrine from sympathetic neurons causes
vasoconstriction
 Decreased sympathetic tone causes vasodilation
 Other hormones affect vascular smooth muscle
 Vasopressin (ADH) from the posterior pituitary causes
generalized vasoconstriction
 Angiotensin II produced in response to decreased
blood pressure causes generalized vasoconstriction
 Arterial natriuretic peptide (ANP) produced in
response to increased blood pressure promotes
generalized vasodilation
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Control of Vasoconstriction/Vasodilation
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Table 8.1
Regulation of Blood Pressure
 Pressure is the primary driving force for blood flow
through organs
 Rearrange the equation: CO = MAP / TPR
 MAP = CO  TPR
 Body varies cardiac output (CO) and total
peripheral resistance (TPR) to maintain a near
constant mean arterial pressure (MAP)
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Regulation of Blood Pressure
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Figure 8.37
Kidneys Help Maintain Blood Volume
 Increases in blood volume
leads to increase in blood
pressure, and vice versa
 Kidneys excrete or retain
water to adjust blood
volume (and pressure)
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Figure 8.39
Regulation of circulation
Priorities:
maintain continuous perfusion of brain & heart
then supply other organs as needed
maintain ECF volume & composition
hyperemia

increased capillary flow
ischemia

cessation of capillary flow
 tissue metabolism   vessel dilation   flow
brain and heart continuously perfused, last to be deprived of
capillary flow
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Medullary cardiovascular center
receives inputs from
Baroreceptors
carotid sinus
aortic arch
subclavian
common carotid
pulmonary artery
Mechanoreceptors
atrial
ventricular
Chemoreceptors
arterial
ventricular
Skeletal muscle afferent fibres
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Baroreceptors
Monitor blood pressure
e.g.
carotid sinus baroreceptors
spontaneous resting AP rate
 blood pressure =  stretch vessel wall   AP rate
 decrease CO via bradycardia and
 decrease peripheral vascular resistance
\ blood pressure (negative feedback)
Mechanoreceptors
Monitor stretch e.g. atrial myelinated B-fibres
spontaneous resting AP rate sensitive to atrial filling rate&volume
 blood volume   venous volume =  venous P 
 atrial filling   AP rate
 increase heart rate and
 increase diuresis
\  blood volume (negative feedback feedback)
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Baroreceptor
reflex
Fig. 9.41
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Peripheral Chemoreceptors
Monitor O2, CO2, pH in arterial blood
primary effect on regulation of ventilation
During normal breathing:
decrease O2, increase CO2 = decrease pH
hyperventilation
peripheral vasodilation (except lungs)
increased cardiac output
During apnea (e.g. diving)
decreased O2
peripheral vasoconstriction (except brain and heart)
bradycardia
decreased cardiac output
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Net Filtration Pressure (NFP)
 Blood pressure forces fluid out of capillaries
 Starling principle: NFP = (Pcap – Pif) – (pcap – pif)
 NFP = net filtration pressure
 Pcap = hydrostatic pressure of blood in the capillary
 Pif = hydrostatic pressure of interstitial fluid
 pcap = osmotic pressure in the capillary
 pif = osmotic pressure interstitial fluid
 May result in net filtration or net reabsorption
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Capillary Filtration
Fig.9.36
Oncotic P = colloid osmotic P due to
[protein] plasma> [protein] interstitial fluid
Blood P>Oncotic P
Oncotic P> P Blood
Consequences of capillary filtration
bulk fluid flow in interstitial spaces a  v
carrying small organic molecules & ions
~85% of filtered fluid is uptaken (except in kidney) but 15% not recovered
gradual net loss of fluid from blood and edema
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LYMPHATIC SYSTEM
- parallels veins in structure, function, and topography
- no connection with arterial system
- low pressure
FUNCTION
·return lymph to circulatory
system (3 L/day in humans)
·filter lymph at lymph nodes
·lymphocytes secrete antibodies
and destroy cells
·carry chylomicrons from
intestine to circulatory system
·carry albumin from liver to
circulatory system
Fig. 9.37
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Causes of Edema
Increased blood pressure
increases filtration pressure at arterial end of capillaries
\ more fluid is filtered
Increased tissue protein
increases solutes in tissue interstitial fluid
\less fluid reabsorbed at venous end of capillaries
usually localized edema due to leakage of plasma protein
Decreased plasma protein
decreases solutes in plasma
\less fluid reabsorbed at venous end of capillaries caused by:
liver disease (decreased protein production)
kidney disease (leakage into urine)
protein malnutrition
Obstruction of lymph vessels
lymph accumulates
e.g. infections of filaria round worm
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Composition of Blood
 Primarily water, containing
 Dissolved ions and organic solutes
 Blood cells (hemocytes)
 Dissolved proteins
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Blood Proteins
 Invertebrates
 Primarily respiratory pigments
 Vertebrates
 Carrier proteins such as albumin and globulins
 Proteins involved in blood clotting
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Blood Cells (Hemocytes)
 Functions
 Oxygen transport or storage
 Nutrient transport or storage
 Phagocytosis
 Immune defense
 Blood clotting
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Figure 8.44
Red Blood Cells (Erythrocytes)
 Most abundant cells in blood of vertebrates
 Contain high concentrations of respiratory
pigments (such as hemoglobin)
 Major function is storage and transport of oxygen
 Erythrocytes have evolved independently several
times
 Also found in various worms, molluscs, and
echinoderms
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Hb Saturation
arterial
(fully saturated)
~97%
venous
at rest
mild exercise
heavy exercise
75%
58%
27%
Increased cardiac output (= stroke vol x heart rate)
via:
 stroke volume
 heart rate
In human:
at rest
heavy exercise
trained athlete
CO (L)
SV (ml)
HR (bpm)
5.6
18
5
80
80
100
70
220
50
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Vertebrate Blood
 Separates into three
main components
when centrifuged
 Plasma
 Erythrocytes
 Other blood cells and
clotting cells
 Hematocrit – Fraction
of blood made up of
erythrocytes
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Figure 8.45
Erythrocytes
 Size of erythrocytes varies among vertebrates
 Smaller in mammals than in fish and amphibians
 Generally round or oval in shape
 Mammalian erythrocytes are biconcave disks
 Increased surface area, facilitating oxygen transfer
 Mammalian erythrocytes lack nucleus,
mitochondria, and ribosomes
 Cannot divide
 Have limited lifespan in circulatory system
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
White Blood Cells (Leukocytes)
 Function in immune
response
 All are nucleated
 Found both in blood and
interstitial fluid
 Some able to move across
capillary walls
 Five major types





Neutrophil
Eosinophil
Basophil
Monocyte / macrophage
Lymphocyte
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Figure 8.46
Thrombocytes
 Key role in blood clotting
 In mammals, thrombocytes are anucleated cell
fragments called platelets
 In nonmammals, thrombocytes are spindle-shaped
cells and classified as leukocytes
 Three steps in blood clotting
 Vasoconstriction
 Platelet plug formation
 Platelets stick to break in vessel
 Clot formation through coagulation cascade
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Blood Cell Formation (Hematopoiesis)
 Location of stem cells
 Adult mammals
 Only in red bone marrow
 Fishes
 Kidney
 Amphibians, reptiles, birds
 Spleen, liver, kidney, bone marrow
 Signaling factors regulate hematopoiesis
 For example, erythropoietin is a hormone released by
the kidney in response to low blood oxygen
 Stimulates formation of erythrocytes in red bone marrow
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Blood Cell Formation (Hematopoiesis)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Figure 8.47