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Cardiac output

Cardiac output is the measurement of blood flow from the heart through the ventricles, and is usually measured in liters per minute. Any factor that causes cardiac output to increase, by elevating heart rate or stroke volume or both, will elevate blood pressure and promote blood flow. These factors include sympathetic stimulation, the catecholamines epinephrine and norepinephrine, thyroid hormones, and increased calcium ion levels. Conversely, any factor that decreases cardiac output, by decreasing heart rate or stroke volume or both, will decrease arterial pressure and blood flow. These factors include parasympathetic stimulation, elevated or decreased potassium ion levels, decreased calcium levels, anoxia, and acidosis.


Compliance is the ability of any compartment to expand to accommodate increased content. A metal pipe, for example, is not compliant, whereas a balloon is. The greater the compliance of an artery, the more effectively it is able to expand to accommodate surges in blood flow without increased resistance or blood pressure. Veins are more compliant than arteries and can expand to hold more blood. When vascular disease causes stiffening of arteries, compliance is reduced and resistance to blood flow is increased. The result is more turbulence, higher pressure within the vessel, and reduced blood flow. This increases the work of the heart.

A mathematical approach to factors affecting blood flow

Jean Louis Marie Poiseuille was a French physician and physiologist who devised a mathematical equation describing blood flow and its relationship to known parameters. The same equation also applies to engineering studies of the flow of fluids. Although understanding the math behind the relationships among the factors affecting blood flow is not necessary to understand blood flow, it can help solidify an understanding of their relationships. Please note that even if the equation looks intimidating, breaking it down into its components and following the relationships will make these relationships clearer, even if you are weak in math. Focus on the three critical variables: radius (r), vessel length (λ), and viscosity (η).

Poiseuille’s equation:

Blood flow =  π ΔP r 4 8ηλ MathType@MTEF@5@5@+=feaagyart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLnhiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=xfr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaeOqaiaabYgacaqGVbGaae4BaiaabsgacaqGGaGaaeOzaiaabYgacaqGVbGaae4DaiaabccacaqG9aGaaeiiamaalaaabaGaaeiWdiaabccacaqGuoGaaeiuaiaabccacaqGYbWaaWbaaSqabeaacaqG0aaaaaGcbaGaaeioaiaabE7acaqG7oaaaaaa@4A84@
  • π is the Greek letter pi, used to represent the mathematical constant that is the ratio of a circle’s circumference to its diameter. It may commonly be represented as 3.14, although the actual number extends to infinity.
  • ΔP represents the difference in pressure.
  • r 4 is the radius (one-half of the diameter) of the vessel to the fourth power.
  • η is the Greek letter eta and represents the viscosity of the blood.
  • λ is the Greek letter lambda and represents the length of a blood vessel.

One of several things this equation allows us to do is calculate the resistance in the vascular system. Normally this value is extremely difficult to measure, but it can be calculated from this known relationship:

Blood flow =  ΔP Resistance MathType@MTEF@5@5@+=feaagyart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLnhiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=xfr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaeOqaiaabYgacaqGVbGaae4BaiaabsgacaqGGaGaaeOzaiaabYgacaqGVbGaae4DaiaabccacaqG9aGaaeiiamaalaaabaGaaeiLdiaabcfaaeaacaqGsbGaaeyzaiaabohacaqGPbGaae4CaiaabshacaqGHbGaaeOBaiaabogacaqGLbaaaaaa@4C0B@

If we rearrange this slightly,

Resistance =  ΔP Blood flow MathType@MTEF@5@5@+=feaagyart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLnhiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=xfr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaeOuaiaabwgacaqGZbGaaeyAaiaabohacaqG0bGaaeyyaiaab6gacaqGJbGaaeyzaiaabccacaqG9aGaaeiiamaalaaabaGaaeiLdiaabcfaaeaacaqGcbGaaeiBaiaab+gacaqGVbGaaeizaiaabccacaqGMbGaaeiBaiaab+gacaqG3baaaaaa@4C0B@

Questions & Answers

Examples of glial cells?
Nesh Reply
what is homeostasis
Laura Reply
homeostasis- The ability to maintain relatively stable internal conditions even though the outside world changes continuously. i,e, maintaining normal values in your body such as Adequate blood levels,blood pressure, heart activity and blood pressure.
thank you
Give atleast three reasons to study anatomy and physiology
George Reply
It helps to know about the body structure properly and administer proper care for the patient
what's augmentin
Aphet Reply
augmentin is a type of combination antibiotic.
Can you explain to Heart anatomy in details please?
Handren Reply
definition of anatomy and physiology
Sardar Reply
Anatomy is the study structure of the body while physiology is the study of function of the body
What is a dorsal cavity?
John Reply
Explain in detail mitosis and meiosis
sedeck Reply
What is Sodium Potassium Pump?
Mwamba Reply
The process of moving sodium and potassium ions across the cell membrance is an active transport process involving the hydrolysis of ATP to provide the necessary energy. It involves an enzyme referred to as Na+/K+-ATPase
yes men.
does it mean that there are two definitions of gross anatomy
Esther Reply
what is anatomy?
Hridy Reply
I think it's the study of the internal and external features of the body and its physical relationship between body parts
anatomy is the study of all the structure of body parts of living organisms
to study the structure of human body parts
anatomy is the study of human body
jaundice causes by when bilirubin blockage of bilary system
mahamed Reply
Yeah, when there is a blockage in the bilary system( bile ducts) which lead to obstruction of the bile pigments called bilirubin leading to a characteristically yellowish appearance of the eyes.
there are three main resons why bilirubin levels in the blood may rise: 1. pre-hepatic jaundice,2. hepatic jaundice, 3. post hepatic
hi pls what causes jaundice
Umar Reply
where the love feelings emotions and hate lies in the body?
Loving Reply
what is Regional anatomy
Sharon Reply
to study of the interrelationships to all structures of specific body region

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