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Elmoselhi - Circuitry and Hemo
Cardiovascular Circuitry and Hemodynamics

Additional Physiology Flashcards




What are the functions of the cardiovascular system?

Main functions

1- Delivering O2 and nutrients to the tissues

2- Removing CO2 and waste products from the tissues



1- Delivering hormones from their glands to their sites of action

2- Regulating body temperature

3- Maintaining adequate blood supply to the body in various stress conditions (e.g. exercise)

Explain resting cardiac output and how it relates to pulmonary and systemic circulation

Cardiac output at rest is the volume of blood per minute pumped by the left ventricle - 5 Liters/minute


Because the left ventricle (left pump), systemic system, right ventricle (right pump), and pulmonary system are connected in series (as opposed to parallel) the flow (NOT velocity) of blood is the same everywhere in the body


At steady state the cardiac output is equal to the venous return - the amount per minute pumped by the left ventricle is equal to the amount per minute returned to the right atrium by the vena cava

What is the importance of the pressure gradient for blood flow?

Different levels of pressure are required for blood to flow


Blood will flow from high pressure to low pressure


Left ventricle has the highest pressure because it is pumping to the entire body


Pressure decreases as blood flows from the left ventricle to the arteries to capillaries to veins all the way to the right atrium


Pressure then increases in the right ventricle so the blood can be pumped to the lungs and into the left atrium

What is the sequence of blood flow in the body?

Oxygenated blood enters the left ventricle from the left atrium through the mitral valve


Blood enters the aorta through the aortic valve


Blood is distributed throughout the body by arteries and capillaries -

25% of volume each for renal, gastrointestinal, and skeletal systems.

15% for cerebral,

5% each for coronary and skin (all numbers at rest)


Venules collect deoxygenated blood from capillaries and coalesce into veins


Vena cava returns blood to the heart in the right atrium


Blood enters the right ventricle through the tricuspid valve


Blood enters the pulmonary artery through the pulmonic valve


Gas exchange occurs in the lungs and pulmonary vein returns oxygenated blood to the left atrium

What is the purpose of the chordae tendinae and papillary muscles of the heart?

In tandem the papillary muscles and chordae tendinae prevent backflow in the aortic and mitral valves


For blood to flow to occur properly it is important that no backflow occurs


The muscle contracts and pulls the tendons which pulls the valve open downward.


It prevents the valve from opening upward

Explain the sequence of blood flow in the heart

Vena cava returns deoxygenated blood to the right atrium


Blood enters the right ventricle through the tricuspid valve


Blood enters the pulmonary artery through the pulmonic valve


Pulmonary artery (ONLY artery in the body w/ deoxygenated blood) takes blood to the lungs


Pulmonary vein (ONLY vein in the body w/ oxygenated blood) takes blood to the left atrium


Blood enters the left ventricle through the mitral valve


Blood enters the aorta through the aortic valve is distributed all over the body

What is the function of arteries?
Arteries: transport blood from the heart to the tissues under high pressure
What is the function of arterioles?
Arterioles control blood before entering the capillaries, considered to be the last small branches of conduit (transport) arteries with highest resistance
What is the function of capillaries?
Capillaries: exchange fluid, nutrients, hormones, etc. between the blood and the interstitial spaces
What is the function of venules?
Venules: collect blood from the capillaries before gradually coalescing into larger veins
What is the function of veins?
Veins: transport the blood from the tissues back to the heart under low pressure; serve as a major reservoir of blood
What is compliance?

Compliance = change in volume / change in pressure


Compliance means how easy a vessel can be stretched


a) Not easily stretched -> low compliance ex's - arteries, atherosclerosis or vessels in old age


b) Easily stretched -> high compliance ex - veins

What is capacitance?

Capacitance means capacity to hold blood


Aorta = low capacitance


Veins = high capacitance

What is the distribution of blood volume in different circulatory compartments?

* probably don't have to memorize actual #'s


systemic veins - 60-70%


lungs - 10-12%


systemic arteries - 10-12%


heart - 8-11%


capillaries - 4-5%


Veins are the main reservoir of blood volume because they have a high compliance. They can hold a large blood volume because they can easily stretch

Explain the relationship between transmural pressure and blood volume in an artery and vein

Arterial system

Small change in volume -> large change in pressure Arteries have low compliance so small changes in volume will lead to large changes in pressure


Venous system

Large change in volume -> small change in pressure Veins have high compliance so large (or small) changes in volume will only lead to small changes in pressure The compliance of veins is why the large majority of blood (60-70%) is found in veins

How can the relationship between transmural pressure and blood volume be affected?

Age or Sympathetic stimulation --> increased vascular smooth muscle tone --> increased pressure at each volume in arteries and veins


Curves shift to the right

Explain wall tension

Wall tension is how hard the walls of the a vessel are being stretched when there is pressure inside the vessel


Laplace relationship: Tension is proportional to pressure multiplied by radius


Ex - aorta has the highest wall tension because it has the highest pressure and the biggest radius

How are aneurysms related to wall tension?

If blood is flowing in an area where pressure is the same but the radius increase then the wall tension increases


The area with the increased radius is at risk for dissection (i.e. wall tear) more than the other areas

Explain the relationship among blood flow, resistance and pressure

Blood flow is determined by Ohm's law


Q = delta P / R


Q = blood flow delta


P = the pressure difference between two ends of a vessel (P1 and P2)


R = vascular resistance of the vessel

Explain laminar flow

Fluid molecules flow in layers or lamina


Flow is silent


Velocity: Maximum in the center and minimum in the periphery (due to molecules adherence in the vessel wall)

Explain turbulent flow

Fluid molecules "bounce around" under certain conditions:


a) Vascular aneurysm,


b) Stenosis (narrowing of the vessels) and arterivenous fistula (abnormal connection or passageway between an artery and a vein)


c) Anemia *Noisy *Low velocity

What is Reynold's number (Nr)?

A dimensionless number of the flow, can be determined as follows:


Nr = velocity * diameter * density / viscosity


Nr < 2000 --> laminar flow


Nr > 2000 --> turbulent flow

Explain the relationship between velocity and flow

Velocity: (speed) rate of blood displacement with respect to time (e.g. cm/s)


Flow: volume per unit time (e.g. ml/s) - cardiac output (5 L/min) is flow


The velocity is inversely proportional to the cross-section area of blood vessel


Velocity = flow / cross-section area


Flow is constant so velocity is dependent on cross-sectional area

What are the different cross section areas of blood vessels?

Aorta: 2.5 (smallest) - only one vessel so although it has the largest radius it has the smallest area


Small arteries: 20


Arterioles: 40


Capillaries: 2500 (largest) - individual capillaries have small area but there are millions of capillaries


Venules: 250


Small veins: 80


Venae cavae: 8


*probably don't have to memorize the #'s but understand the differences especially between the aorta and capillaries


Aorta = 33 cm/s vs. Capillaries = 0.3 mm/s under resting conditions

How is blood resistance calculated?

Resistance is calculated as resistance = change in pressure / flow


It can be expressed in mm Hg/ml/min or in Peripheral Resistance Unit (PRU= mm Hg/ml/sec)

What is Poiseuille’s law ?

Resistance is equal to [(blood viscosity * vessel length)/ radius^4)] * (8/pi)


Radius length is the main determinant of the vascular resistance


Radius decreases by half -> resistance increases 16-fold


Radius doubles -> resistance is 1/16 of original value

What is vasoconstriction?
Increased contraction of the circular smooth muscle in the arteriolar wall which leads to increased resistance and decreased flow
What causes vasoconstriction?

increased myogenic activity


increased oxygen


decreased CO2 and other metabolites


increased endothelin


sympathetic stimulation


vasopressin and angiotensin II



What is vasodilation?
Decreased contraction of circular smooth muscle in the arteriolar wall which leads to decreased resistance and increased flow
What causes vasodilation?

decreased myogenic activity


decreased O2


increased C02 and other metabolites


increased NO


decreased sympathetic stimulation


histamine release



How is resistance in a series system calculated?

R total = R1 + R2 + R3 +....


Total resistance is simply calculated by adding all individual resistance together.


Total resistance is always greater than any individual resistances

Explain series resistance in the circulatory system

Adding a resistor in series increases the total resistance of the system


Flow is equal at all points in series system


Vessels are arranged in series around the circulation - arteries, arterioles, capillaries, venules and veins.


Pressure decrease according to the resistance that it has to overcome - i.e. the greatest decrease in pressure occurs in the arterioles because arterioles contribute to the highest resistance

What will happen if central resistance increases?

If only the central resistance (R2) increases:


Flow decreases equally at all points (series system)


Pressure immediately upstream from R2 increases


Pressure immediately downstream from R2 decreases


Thus an increase in resistance leads to an increase in the pressure difference

What will happen if central resistance decreases?

if only the central resistance (R2) decreases:


a) Flow increases equally at all points (series system)


b) Pressure immediately upstream from R2 decreases


c) Pressure immediately downstream from R2 increases decreased resistance leads to decreased difference in pressure remember R = delta P/ Q

Explain parallel resistance

The reciprocal of the total resistance is the sum of the reciprocals of individual resistances


Total resistance is always smaller than any individual resistances


Adding a resistor in parallel decreases the total resistance of the system.


However, increase resistance in an individual resistance will increase total resistance


Flow in individual resistance can be adjusted independently


Vessels are arranged in parallel in various organs e.g. coronary, cerebral, renal, etc.


There is no loss of pressure in parallel resistance arrangement

What controls blood viscosity?

Viscosity = internal "stickiness" of the fluid.


Viscosity of the blood changes with hematocrit (amount of blood cells)


Anemia causes decreased hematocrit which causes decreased viscosity


Polycthemia causes increased hematocrit which causes increased viscosity

What is blood pressure?

In general


Force = Pressure / Area


Blood pressure: the force exerted by the blood against any unit area of the vessel wall

Explain the relationship between mean arterial pressure and systolic and diastolic pressure.

Systolic pressure (SP): is the peak aortic pressure, occurs duringthe ejection of the blood from the left ventricle into aorta


Diastolic pressure (DP): is the minimum aortic pressure


Pulse pressure = Systolic Pressure - Diastolic Pressure


Mean arterial pressure (MAP) = Diastolic Pressure + 1/3 Pulse Pressure = 2/3 Diastolic Pressure + 1/3 Systolic Pressure

How is mean arterial pressure regulated?



MAP = mean arterial pressure (mmHg)


CO = cardiac output (ml/min)


TPR = total peripheral resistance (mmHg/ml/min)

How does pressure change in the systemic circulation?

Aorta & large arteries = 100 mmHg


Capillaries = 17 mmHg


Veins = almost 0 mmHg

How does pressure change in the pulmonary circulation?

Pulmonary arteries = 16 mmHg


Pulmonary capillaries = 7 mmHg

Why is there a large decrease in pressure in the arterioles?
Arterioles have the highest resistance in the systemic circulation
What are major factors that increase arterial systolic pressure?

1. increase stroke volume


2. decrease compliance of the arterial tree


3. decrease heart rate (via increased stroke volume)

What are major factors that decreases arterial diastolic pressure?

1. decrease in total peripheral resistance (TPR)


2. decrease in heart rate


3. decrease in stroke volume

What increases pulse pressure?

Major factors that increases pulse pressure:(systolic increases and diastolic decreases)


1. increase in stroke volume


2. decrease in compliance of the arterial tree


3. decrease TPR

What is damping of pulse pressure and how is it caused?

The progressive diminishment of the pulse pressure in the peripheral circulation



1) the resistance to blood movement in the vessels


2) The compliance of the vessels


Degree of damping is proportional to resistance * compliance

How do pressures in the aorta and arteries compare?

Arteries have a higher systolic pressure but a lower mean arterial pressure


The lower mean arterial pressure is needed for blood to flow from the aorta to the arteries

What are some different conditions that can change abnormal pulse pressure?

Arteriosclerosis - blockage causes increased systolic pressure and increased pulse pressure


Aortic stenosis - narrowing of aorta decreases stroke volume which decreases systolic pressure and thus pulse pressure


Patent ductus arteriosus - connection between aorta and pulmonary artery causes increased systolic pressure and decreased diastolic pressure


Aortic regurgitation causes similar but larger effects

How does arterial pressure change with age?

Systolic and diastolic blood pressure increase with age due to change of the pressure control mechanisms


Two main control mechanisms change with age:


1) Kidney, long term regulation of the blood pressure

2) Arteries, decrease elasticity -> increase in systolic blood pressure

How is central venous pressure regulated?

Central venous pressure is the pressure in the right atrium, normally = 0 mmHg


Regulation: balance between the heart’s ability to pump the blood out of the right atrium and ventricle into the lung and the flow back of the blood from peripheral veins into the right atrium

What factors increase venous return?

1) Increased blood volume


2) Decreased ability of the heart to pump the blood


3) Increased vessel tone -> increased peripheral venous pressure


4) Dilatation of the arterioles -> decreases the peripheral resistance -> increased flow of the blood from arteries to veins

What factors decrease venous return?

1) Decreased blood flow into the heart


2) Increased pumping of the blood by the heart

How does venous pressure in the legs different when standing still as opposed to walking?
In normal condition: during walking or tensing the muscles, the venous pressure in the feet < 25 mmHg, while in standing still position increase up to 90
How does gravitational pressure affect arterial and venous pressure?

The pressure results from the weight of the blood


Venous Pressure (mmHg):


right atrium = 0


veins of the feet = 90


veins inside the skull = -10


Arterial pressure (mmHg):


Heart level = 100


Arteries of the feet = ~ 190


Arteries inside the skull = 90

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