USMLE Step 1 Physiology Review 54 13 Cardiovascular Physiology Intro (3 of 3)

USMLE Step 1 Physiology Review 54 13 Cardiovascular Physiology Intro (3 of 3)

 

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Play USMLE Audio MP3 54 13 Cardiovascular Physiology Intro (3 of 3) Below

Begin 54 13 Cardiovascular Physiology Intro (3 of 3) Transcription

Now, let’s talk about cardiac output.

What is the brief definition of cardiac output?

  • Cardiac output is the blood flow generated by each ventricle per minute. Cardiac output is equal to the volume of blood pumped by one ventricle per beat times the number of beats in a minute.

Express this more concisely.

  • Cardiac output equals stroke volume times heart rate.

Normally, what is the average stroke volume for each ventricle?

  • Seven milliliters of blood.

What is the approximate value of a normal heart rate?

  •  Seventy to seventy five beats per minute.

Therefore, at rest what is the normal cardiac output?

  • Five liters per minute.

Is the heart rate under neural control?

  •  Yes.

What effect does cardiac sympathetic efferent activity have on the heart rate (that’s efferent with an “e”)?

  •  It increases the heart rate.

What effect do Parasympathetic or vagal efferent impulses have on the heart rate?

  • Parasympathetic decrease heart rate.

The stroke volume varies with changes in three factors. List the factors. I will provide a cue for each factor.

First, this factor is related to systole.

  • The force of ventricular contraction.

Second, this factor has to do with the arteries.

  • Arterial pressure.

Third, this factor relates to ventricular volume.

  • The volume of blood in the ventricle upon contraction.

So, the stroke volume varies with changes in the force of ventricular contraction, arterial pressure, and the volume of blood in the ventricle upon contraction.

Student Doctor, please pause the tape and summarize the information of cardiac output.

  • Cardiac output is the blood flow generated by each ventricle per minute. Cardiac output equals stroke volume times heart rate. Normally, the average stroke volume for each ventricle is seventy milliliters of blood. The approximate value of a normal heart rate is seventy to seventy-five beats per minute. Therefore, at rest, the normal cardiac output is five liters per minute.  The heart rate is under neural control. Sympathetic efferent activity increases the heart rate. Parasympathetic impulses decrease heart rate.   The stroke volume varies with changes in the force of ventricular contraction, arterial pressure, and the volume of blood in the ventricle upon contraction. 

Now let’s look a bit closer at systemic flow verses pulmonary flow.

Given the fact that the systemic and pulmonary circulations are in series, what can you deduce about the volume of blood pumped by both ventricles of the heart?

  • Both ventricles must pump the same volume of blood during any significant time period.

What is the term for the intrinsic property of cardiac muscle that helps to achieve this balanced output of blood flow?

  • Think of a large black bird whose name is Frank. The further Frank stretches his wings the more forcefully he brings them back to his side. The Frank Starling Mechanism.

Will passive stretching of muscle fibers cause an increase or decrease in contractile strength?

 

  •  An increase.

 

What is the term for this relationship between a passive stretching of muscle fibers and an increase in contractile strength?

  •  The Frank Starling Mechanism.

What is the condition that occurs when the output of one or both ventricles is not enough to supply the needs of the body?

  • Congestive heart failure.

During congestive heart failure which ventricle would limit output?

  • Output would be limited by the ventricle with the lowest output.

Why?

  • Because the healthier ventricle can pump only as much blood as it receives.

The left and right sides of the heart are connected in series, but how about the systemic organs?  Do they receive blood through parallel distribution channels or are they in series like the ventricles of the heart?

  • Systemic organs receive blood through parallel distribution channels.

There are two arterial qualities of the blood supplied to the systemic organs that are the same due to the parallel arrangement of the distribution of the channels. What are the two arterial qualities of the blood to which I’m referring?

  • Arterial composition and arterial pressure.

Student Doctor, please pause the tape and summarize the information on systemic flow verses pulmonary flow.

  • Since the systemic and pulmonary circulations are in series both ventricles must pump the same volume of blood during any significant time period for the cardiovascular system to function properly. The term for the intrinsic property of cardiac muscle that helps to achieve this balanced output of blood flow is the Frank Starling Mechanism. As cardiac muscle experiences passive stretching, contractile strength increases. Congestive heart failure occurs when the output of one or both ventricles is not enough to supply the needs of the body. During congestive heart failure output is limited by the ventricle with the lowest output because the healthier ventricle can pump only as much blood as it receives. The left and right sides of the heart are connected in series, but systemic organs receive blood through parallel distribution channels. The parallel arrangement of the distribution channels supplies the systemic organs with blood that has the same arterial composition and arterial pressure.

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