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Principles of Circulation

Factors that affect cardiac output


I). Stroke Volume:

A). Definition:

volume of blood pumped out of the ventricles per beat.

blood volume




B). the ventricular volume at the end of diastole  and the ventricular volume at the end of systole

Stroke Volume = End Diastolic Volume End Systolic Volume


C). Factors that affect Stroke Volume:

1 ). Preload:


i. Frank-Starling's Law of the Heart:


degree of stretch of the cardiac muscle just before they contract.

a. Further the fibers stretch the more actin-myosin cross bridges can be made

b. The greater the force of the contraction


ii. Increase speed of volume of venous return.


2). Contractility:

This results in better ejection of the blood in the ventricles
 Controlled by extrinsic factors 


  • sympathetic stimulation of the heart
  • hormones
  • K+ and Ca++ channel blockers

3). Afterload:

This is a factor of blood pressure (BP)




II). Heart rate:  




It is affected by:


sympathetic nervous system

hormones: epinephrine releases from the adrenals and thyroxine.


parasympathetic nervous system



Low Ca++ lowers the heart rate

High Na+ blocks Ca++ intake and lowers heart rate

High K+ interferes with depolarization

Low K+ results in a feeble heartbeat.


III). Cardiac Output

A). Definition:


B). heart rate and stroke volume.

Cardiac Output = Heart Rate x Stroke Volume

 factors that effect heart rate and stroke volume effect cardiac output

stroke volume cardiac output

stroke volume cardiac output

heart rate       cardiac output

heart rate       cardiac output until---stroke rate goes down


dramatic increase in heart rate
decrease in stroke volume
in cardiac output


Cardiac Output =(End Diastolic Volume End Systolic Volume) x Heart Rate

IV). Peripheral Resistance

Resistance is a factor of:

A). Blood Viscosity

B). Total Blood Vessel Length

Resistance increases with length, but the length of blood vessels is constant.


C). Vessel Diameter


This effect is called laminar flow

Resistance is proportional to:  1/(radius of the vessel)4


V). Blood Pressure

A). Definition

units:  millimeters of mercury mmHg 

The pressure that its takes to move a column of mercury

The pumping action of the heart determines flow. 

Pressure is created when the flow is resisted.

Blood Pressure


Blood Pressure = Cardiac Output x Resistance


B). Pressure Reading in Different Vessels

1). Arterial Pressure

  • pressure peak is the systolic
  • lower peak diastolic.

Pulse Pressure = Systolic PressureDiastolic Pressure

stroke volume

pulse pressure temporarily

Mean (average) Arterial Pressure (MAP) = (Diastolic Pressure + Pulse Pressure) / 3


2). Capillary Pressure

  • pressure drops from 40 mmHg to 20 mmHg
  • capillary hydrostatic pressure.

Low capillary pressure will still push solute containing fluids through the permeable capillaries.

Capillary hydrostatic pressure is opposed by the interstitial fluid hydrostatic pressure

(which is assumed to be 0 mmHg)

Capillary Hydrostatic Pressure


3). Venous Pressure

Venous blood pressure is very low.

Too low for venous return.

Venous Return


i). Respiratory pump that sucks blood upward.

ii). Muscular pump:  the activity of skeletal muscle pushing on the blood vessels.


C). Maintenance of blood pressure

Changes in blood pressure are controlled by:

i). Neural control of vasoconstriction and dilation

ii). Baroreceptors These regulate cardiac output and resistance.

iii). Adrenal medulla hormones result in vasoconstriction

iv). Atrial Natriuretic Peptide (ANP) decreases blood volume and pressure

v). Antidiuretic Hormone blood volume

vi). Angiotensin II blood volume




Cardiac Output = Stroke Volume x Heart Rate  


Stroke Volume = End Diastolic Volume End Systolic Volume


Cardiac Output = (End Diastolic Volume End Systolic Volume)  x Heart Rate


Blood Pressure = Cardiac Output x Resistance


Cardiac Output =  Blood Pressure / Resistance


VII). Blood Flow

Blood Flow (vascular system) = Cardiac Output

Through the body it is relatively constant but it will vary in the individual organs.

At rest:

The blood flow depends on the blood velocity.

Factors that affect velocity affect blood flow

thus blood flow to local organs can be adjusted by changing the diameter of blood vessels.


VI). Blood Velocity

Blood velocity is inversely related to cross sectional area of the blood vessel.



See if you can understand how all of the factors in the first diagram  interrelate