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Flashcards in Module 6: Cardiovascular Deck (237)
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91

- The last small branches of the arterial system
- With extensive development of smooth muscle
- The medium-sized arterioles are the sites of highest resistance in the circulatory system called RESISTANCE VESSEL
- Act as CONTROL CONDUITS through blood is released to the capillaries.

Arterioles

92

(Arterioles)

Various factors that fall into two categories can influence the contractile activity, changing the resistance to blood flow in these vessels

1. Local controls which are important in matching blood flow to the metabolic needs of specific tissues
2. Extrinsic controls important in ABP regulation

93

- innervated by S fibers
- Alpha 1 adrenergic receptors arterioles and skin and splanchnic vascular (activation – vasoconstriction)
- Beta 2 adrenergic receptors in the arteriole of skeletal muscles (activation – vasodilation)
- Vasoconstrictor fibers exhibit tonic activity
*Ex. Sympathectomy – vasodilation
- Vasodilator fibers - no tonic activity

Arterioles

94

- exchange vessels
- thin – walled
- lined with single layer of endothelial cells, surrounded by a basal lamina
- lipid soluble substances (Carbon Dioxide and Oxygen) - cross capillary wall by dissolving in and diffusing across endothelial cells

Capillaries

95

- water soluble substance (ions) – cross capillary wall through water-filled clefts (spaces between endothelial cells) or through largest pores in capillary walls (FENESTRATED Capillaries)
- smallest diameter
- largest total surface area = 2500cm²
- slowest blood flow velocity
- selective perfusion of capillaries depending on metabolic needs of the body
- selective perfusion determined by degree of dilation or constriction of arterioles and pre capillary sphincters

Capillaries

96

- thin walled
- composed of endothelial cell layers, few elastic tissues, smooth muscles and connective tissues
- large capacity to hold blood because of few elastic tissues (CAPACITANCE Vessels)
- contain the largest % of blood in the CVS → 60% to 70%
- volume of blood in the veins called UNSTRESSED VOLUME (the blood volume under low pressure
- Contains one-way valves thus act as venous pumps or muscle pump.
- Venous Pressure in the lower extremities > VP in upper extremities.

Veins

97

DISTENSIBILITY vs COMPLIANCE

- All blood vessels are distensible
- Veins – most distensible
- Arteries – 8x less distensible than veins
- Arterial wall stronger than veins
- A vessel that is highly distensible and has a slight volume may have less compliance than a less distensible vessel that has a large volume


WHY?
- Compliance = Distensibility x Volume
- Thus the compliance of a systemic vein is 24x than that of artery

WHY?
- Because vein is 8x as distensible and has a volume of 3x as great
Compliance → 8 x 3 = 24

98

- Study of physical properties that govern blood flow through the blood vessels and the heart
- Blood Flow – the quantity of blood that passes a given point in the circulation in a given period of time
- Expressed in ml/min or L/min (cm3/min)

Hemodynamics

99

Factors that promote blood flow through the Circulatory System

- Forward motion imparted by the pumping of the heart
- Diastolic recoil of the arterial walls
- The skeletal muscle pump
- The negative intra thoracic pressure during inspiration

100

- the measure of the tendency for turbulence to occur

Re = (v · d · p) / η
v = velocity of blood flow (cm/sec)
d = diameter of vessel (cm)
p = density of blood
η = viscosity of blood (in poise)

Reynold’s Number (Re)

101

Reynold’s Number (Re)

- Reynold’s number above 200 to 400 – turbulent flow
- Turbulent flow – blood flowing in all directions in the vessels, moving in a disorderly pattern forming whorls of blood called eddy currents, often accompanied by audible vibrations (murmur)
- Turbulent flow favored by low blood viscosity, Stenotic valve, Thrombus (which narrow the diameter of the vessel)

102

- Volume of blood which passes through blood vessels per unit time (ml/min)
- Depends on
Pressure Gradient (P = P1 –P2)
P1 → Arterial Blood pressure – directly related to blood flow
P2 → Venous Pressure – inversely related to blood flow
- The force that pushes the blood through the vessel (the driving force for blood flow

BLOOD FLOW

103

Depends on:
a. dimension of vessel (length and radius)
b. physical properties of blood (blood viscosity)

- depends mainly on hematocrit (percentage of volume of blood occupied by the rbc)
- amount of protein in blood (Hyperimmunoglobulin D, E and M)
- resistance of the cell to deformities (hereditary spherocytosis)

BLOOD FLOW

104

- Magnitude of blood flow (Q) directly proportional to size of pressure gradient (P)
Blood Flow (Q) inversely proportional to Resistance
R = P/Q
- This relationship can be used to measure the resistance of the entire systemic vasculature (or the TPR); can be used to measure resistance in a single organ or single blood vessel
TPR – Total Peripheral Resistance
SVR – Systemic Vascular Resistance

- When radius of blood vessel decreases, its resistance increases to the 4th power
Ex. Radius decreased by ½, Resistance increases by 16 fold

POISEUILLE FORMULA

105

- Illustrated by the arrangement of blood vessels within a given organ.
- Within the organ, blood flows from the major artery to smaller arteries then to arterioles, capillaries, venules, veins
- The total resistance of the system arranged in series is equal to the sum of individual resistances expressed as:

R total = R artery + R arterioles + R Capillaries + R venules

Series Resistance

106

__ is the greatest. Therefore the total resistance of a vascular bed is determined in large part by this.

Arteriolar resistance

107

- Illustrated by the distribution of blood flow among the various major arteries branching off the aorta
* Recall: CO flows through aorta and distributed on a % basis among the different organs
-When blood flow is distributed through a set of parallel resistances, - the flow through each organ is a fraction of the total blood
- Adding a resistance to the circuit causes total resistance to decrease, not to increase
Ex. Total Resistance = 2.5

Parallel Resistance

108

__ in a blood vessel. Lateral Pressure decreases as flow velocity of the blood increases.
- Applies to single tube or many tubes arranged in parallel
- When a vessel is narrowed,
>> Velocity of blood flow in a narrowed portion increases
>> The lateral or distending pressure decreases in order to Maintain volume flow of 200 cc / sec and Keep the total energy of the system constant

BERNOUILLI’S PRINCIPLE

109

T = Pr
P = Distending Pressure
T = Wall Tension
R = Radius

- Explains how the narrow lumen and thin wall of capillaries can withstand high pressures without bursting.
- Wall tension opposes the distending force that tends to pull apart a theoretical longitudinal slit in the vessel
- Wall tension acts to prevent rupture of the vessel wall.

LAW OF LAPLACE

110

- force exerted by the blood per unit area of the vessel wall (pressure is exerted equally in all directions).

Blood Pressure

111

- average pressure in any segment of the cardiovascular system during cardiac cycle.

Mean Blood Pressure

112

- The force exerted by the blood against the walls of the arteries by ventricular ejection
- A routine measurement that reflects the status of the CVS

ARTERIAL BLOOD PRESSURE

113

The energy transferred to the arterial system by ventricular ejection generates 2 Pressure Pulses:

1. Systolic Pressure
2. Diastolic Pressure

114

Arterial Blood Pressure

- Systolic pressure (100 – 120 mmHg)
maximal arterial pressure within cardiac cycle

- Diastolic pressure (70 – 80 mmHg)
minimal arterial pressure within cardiac cycle

115

- Maximum pressure attained in the arterial system during ventricular systole / during cardiac ejection
- Reflects the elasticity of the arterial system as it receives blood from the ventricle
*Less elastic aorta --- increase SP

- Governed by the ability of ventricle to contract
*Weak ventricle --- lower SP

- Influenced by the amount of blood in the ventricle
Hemorrhage – less blood volume --- decrease S

SYSTOLIC PRESSURE

116

- Minimum pressure attained in the arterial system during ventricular diastole
- Produced by the recoil of aorta during diastole
- Reflects elasticity of vascular wall
- Is dependent on the duration of diastole
Slow HR --- longer diastole
More time for DP to go down --- lower DP

DIASTOLIC PRESSURE

117

- Also dependent on the PERIPHERAL RUN-OFF which in turn is dependent on the caliber of the arteriole
Constricted arterioles --- poor run-off --- higher DP
Dilated arterioles --- better run-off --- lower DP

DIASTOLIC PRESSURE

118

- The average pressure that push blood through the circulatory system
- The average pressure in the aorta, driving blood into the tissues throughout the cardiac cycle

MEAN ARTERIAL PRESSURE

119

Arithmetic Mean Pressure vs Functional Mean Pressure

ARITHMETIC MEAN PRESSURE:
= (Systolic Pressure + Diastolic Pressure) / 2

FUNCTIONAL MEAN PRESSURE:
= Diastolic Pressure + 1/3 Pulse Pressure
PULSE PRESSURE = SP minus DP
*More accepted because value is closer to DP in actuality

120

Pulse Pressure

Pulse Pressure = Stroke Volume / Arterial Compliance