Download Cell Bio and Physio – Lecture 18: Intro to the Cardiovascular

Cell Bio and Physio – Lecture 18: Intro to the Cardiovascular System
5/3/12



Definitions
o Cardiovascular system = heart and blood vessels
o Circulatory system = heart, blood vessels, blood, and lymphatics
Purpose of Circulatory System
o Primay role: distribution of gases, nutrients, and waste
o Secondary roles:
 Circulation of hormones
 Distribution of heat
 Transport of immune cells, antibodies, and clotting factors
Components
o Heart
 Background
 Weighs less than 1 pound and ~ fist-sized
 Pump Detail
 R heart receives deoxygenated blood from the body and pumps it to the
lungs
 L heart receives oxygenated blood from the lungs and pumps it to the
body
 2 Circuits
 Pulmonary Circuit
o Only way for blood from the R side to get to the L side
 Systemic Circuit
o Provides multiple pathways for blood to get from L to R heart
o Vessels
 Arteries
 Transport blood from the heart to the tissues under high pressure
 Have strong walls w/ significant amts of smooth muscle & elastic tissue
 Arterioles
 Control conduits through which blood is released into the capillaries
 Have strong muscular walls which can completely constrict or dilate
several flod
o 75% of arterioles are empty at any given moment
 Have 1st – 4th order sizes
 Capillaries
 Only vessels allowing exchange of substances b/t blood & interstitial fluid
 Essentially an endothelial tube w/in a basement membrane
 Venules
 Collect blood from the capillaries and gradually merge to form veins
 Have 1st – 4th order sizes
 Veins
 Return blood to the heart
 Low BP
o Have thin, collapsible walls
o Blood
 Blood volume: 5-6 liters (~ 6 quarts) in adult male
Distribution
 Total blood volume is unevenly distributed
 Vast majority of blood is in the systemic circulation (84%)
o Most is in the veins (64%) – blood reservoir
o Relatively little in capillaries (7%)
 Foundations of Circulation
o Premise
 Circulation exists to handle the metabolic needs of working tissue
 Supply nutrients and remove waste
 2 defining aspects
 All tissues need blood
 The amt of blood needed is usu. not static
o Blood Supply
 There is more vasculature than there is blood
 Not all tissues need blood at every moment (efficiency issue)
 Dynamics
 Active tissue may require 20-30 times more blood than at rest
 CV system must be able to quickly adjust and direct the flow of blood to
tissues in need
Major Circulatory Concepts
 Blood Flow Control
o Mainly controlled locally, in relation to tissue need
o Tissue microvasculature monitors extracellular fluid composition (gases, nutrients,
waste) and local blood vessels constrict or dilate accordingly
 Blood Pressure Control
o Regulation of systemic BP is generally independent of control of local blood flow, or
cardiac output
 Autonomic control
o Systemic BP must remain at a certain level (~100 mm Hg) at all times to maintain
adequate blood flow to all tissues
 Cardiac Output Control
o CO is the amt of blood pumped by the ventricles (ml/min)
o CO = HR x stroke volume (SV)
 Stroke volume is the volume of blood pumped out by a ventricle during a single
contraction (~70 ml/min)
o Control
 CO is mainly determined by peripheral factors
 What blood the heart receives (venous return) it pumps back out
 A stretched heart ( venous return) contracts more forcefully and  HR
 Blood Flow
o Usu. measured in ml/min
o Human Blood Flow
 Blood flow to the entire body is equal to cardiac output
 At rest, CO ~ 5 L/min
 Thus, blood flow at the aorta = 5 L/min
o Context
 At rest, the entire blood volume is pumped by the heart in 1 minute
 CO = HR x SV = 75 bpm x 70 ml/min = 5.25 L/min

o Blood flows b/c it’s not compressible like a gas
 It must go somewhere – travels to area of lower pressure – when pressure is
applied
o Factors of flow
 Ohm’s Law: F = P/R
 Pressure
 BP = force per unit area on a vessel wall by the blood
 Pressure head – created by beating heart at the “start” of circulation
o Remains constant over time
 Pressure gradient
o Pressure generated by the heart must be sufficient to overcome
the resistance to blood flow in the pulmonary and systemic
circuits
o BP ranges systemically from ~100 to 2 mm Hg
 Resistance
 Allows pressure to exist in a moving fluid
 Total Peripheral Resistance
o The resistance of the entire circulation
 Varies up to 20-fold
o Resistance in the pulmonary circuit is a/b 1/7th that in the
systemic circuit
o Measured in peripheral resistance units (PRU)
o .
o .
o .
 Poiseuille’s equation and Ohm’s Law explains:
o Resistance is directly proportional to viscosity and length
o Resistance is inversely related to the diameter to the 4th power
 The er the diameter, the er the resistance
 Diameter/Radius Rationale
o The narrower the blood vessel, the greater the proportion of
blood therein that is in contact with the vessel wall
 The outer fluid lamina has a flow velocity of 0
o Thus overall, the narrower the blood vessel the slower the flow of
blood
 Laminar Flow
o Blood tends to flow through long tubes in concentric layers
(lamina)
 Viscosity is a measure of the slipperiness b/t layers
o Outmost layers in contact w/ the wall has a flow velocity of zero
o The more inward layers move more quickly
 Shearing
o A more viscous fluid experiences greater shear (more resistance
to sliding) b/t the fluid layers
 This  resistance (or  velocity) reduces flow at any
given pressure
o Causes of blood viscosity
 Almost entirely related to the hematocrit




Proportion of blood cells
Circuit Resistance
 From the aorta to the vena cava, blood flows through arteries, veins,
and capillaries
o These are arranged in series
 Total resistance = sum of each individual vessel
 Total peripheral resistance = sum of the resistances of all
the arteries, arterioles, capillaries, venules, and veins
 All of the above vessels branch extensively
o Arterioles,capillaries, venules
 Branches are arranged in parallel
 Vessels have a lower total resistance than those
arranged in series
 Conductance
o Each additional vessel provides another pathway, making it easier
for blood flow through the circuit
o Practical Implications
 Most of our tissues contribute to the overall conductance
of the systemic circulation
 Removal of a limb or organ eliminates a parallel circuit
 Increased TPR
Velocity of Blood Flow
 Blood flow = volume through any pt in a given time period (L/min)
 The same volume of blood must flow through each segment of the
circulation each minute
o Principle of continuity/conservation of mass
 Velocity Requirement
o B/c total vessel cross-sectional area changes throughout
circulation, the velocity of blood flow must change in order to
maintain constant flow (~5L/min) in each segment
 Ex: capillary velocity is slower
Turbulent Flow
 Though blood flow is generally laminar, it can become turbulent in
certain conditions
 Reynold’s Number
o The measure of the propensity for turbulence to occur
o Flow velocity is the main modifiable factor
 Causes the parabolic profile of the linear velocity to become blunted
 Practical Meaning
o Turbulence is energetically inefficient as it vastly  resistance
 Heart must produce  pressure head to maintain flow
o Clinical
 Turbulent flow is heard as murmurs – Korotkoff sounds