THE HEART AS A PUMP-CARDIOVASCULAR SYSTEM TUTORIAL

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The Right and Left Hearts are Connected in Series, but are Folded Together to Form a Single Unit

  • The right heart pumps blood only to the lungs; its output is low pressure (25 mm Hg)
  • The left heart pumps blood to the rest of the body; its output is high pressure (120 mm Hg)
  • Because the 2 hearts are attached they beat in synchrony
  • The 2 atria receive the incoming blood- they pump extra blood into the ventricles
  • The 2 ventricles produce enough pressure to push blood through the pulmonary and systemic circulations
    This diagram is modified from one in The Sourcebook of Medical Illustration, edited by Peter Cull (Park Ridge, NJ: Parthenon, 1989).
  • The right side of the heart has been colored blue to indicate deoxygenated blood; the red color of the left side indicates oxygenated blood that has come from the lung.
  • There are no valves where the vena cavae and join the right atrium or where the pulmonary veins enter the left atrium. Pressures in the atria are small and valves are not needed.

Pressure Causes Valves to Open and Close in the Heart Cycle

  • The heart has 2 sets of valves:
    • AV valves: between atria and ventricles
      • Flap type
      • Chorda tendinae & papillary muscles keep them from being pushed too far
      • Left heart: bicuspid or mitral (2 flaps)
      • Right heart: tricuspid: (3 flaps)
    • Semilunar valves: where arteries leave heart
      • Blood caught in the 3 cusps pushes them closed
        • Right heart: pulmonary semilunar
        • Left heart: aortic semilunar
    • Leaks in valves -> murmurs
    • Usually there are no valves where veins enter the atria
      • Not needed, low pressure
      • Exception is a valve between the inferior vena cava and right atrium, but this is missing in many adults
  • When the heart contracts pressure builds up, forcing the valves to close
    • Muscles are not required to close the valves
  • Valves in the cardiac cycle:
    Event AV Valves:
    Right: Tricuspid
    Left: Mitral
    Semilunar Valves:
    Right: Pulmonary
    Left: Aortic
    Filling of ventricle Open Closed
    Building up pressure Closed Closed
    Expelling blood Closed Open
  • Opening and closing of valves depends upon the pressures on opposite sides
    • Example: aortic valve
    • Closed during filling and building up of pressure in the left ventricle because pressure in the aorta is higher than pressure in the ventricle
    • When pressure in the left ventricle becomes higher than pressure in the aorta the aortic valve opens and blood is expelled from the heart

Heart Sounds are Produced by the Closing of the Valves

  • Normal heart sounds are produced when valves snap closed: LUB-DUP
  • LUB = closing of AV valves: beginning of systole
  • DUP = closing of semilunar valves: end of systole
  • Abnormal valve sounds:
    • Leakage of valve -> swishing sound (murmur)
    • Narrowing of valve (stenosis) -> high pitched sound

The Cardiac Output is the Product of Heart Rate and Stroke Volume

  • Normally about 5 liters/min
  • The cardiac output per minute (CO) is the product of the size of a single output, the stroke volume (SV), and the heat rate (HR) in beats/minute:
    • CO = HR X SV
    • = 70 beats/min X .07 liters/beat = 5 liters/min
  • If the SV is constant, doubling the HR will double the CO

Heart Rate Can be Increased From About 70 to 200 Beats/Minute

  • Resting heart rate is about 60-80 beats/min (lower in athletes because they have large stroke volumes)
  • The HR can be increased about 3 times in exercise
  • Above about 200 beats/min the heart would not have time to fill properly- therefore nature limits the rate
  • Rate is controlled by the autonomic nervous system

Stroke Volume is Controlled by Sarcomere Length

  • When the venous return of blood to the heart increases the heart beats more forcefully and puts out more blood: Frank- Starling’s law of the heart
  • Can be explained by sarcomere length:
    • Cardiac muscle is like skeletal muscle: there is an optimum length for the sarcomeres
    • At rest heart sarcomeres are too short to give maximum tension
    • Filling heart to a greater volume stretches sarcomeres- they become more efficient and contract more strongly
    • More input -> sarcomeres stretch -> stronger contraction -> more output
  • Mechanism allows SV to increase about 1.5 to 2X.

Blood Pressure is Caused by Cardiac Contraction

  • Blood pressure at the output of the left heart alternates between a high pressure (systole) and a lower pressure (diastole)
  • Systole:
    • When the heart beats (systole) the pressure in the arteries leaving the heart rises to about 120 millimeters of mercury (mm Hg)
  • Diastole:
    • Between beats (diastole) the arterial pressure drops to about 80 mm Hg
    • The diastolic pressure does not drop to 0 because the arterial walls are elastic
    • A force due to wall elasticity pushes on the arterial blood between beats
    • The 80 mm Hg diastolic pressure keeps the blood flowing between beats
  • Blood pressure is reported as systolic pressure over diastolic pressure
    • Example: 120/80
  • Pressure is a force per unit area. In engineering pressure is often given in pounds per square inch (PSI) or in dynes per square centimeter. Why then is blood pressure reported as millimeters of mercury? That seems to be the height of a column, not a pressure at all. Click to see an explanation of mm Hg pressure units.

Systemic Blood Pressure Depends Upon Cardiac Output and Resistance to Flow

  • The more blood pumped into the arteries the higher the pressure
  • Pressure also goes up if there is more resistance to flow- this occurs when large numbers of arterioles constrict
  • The body changes both CO and resistance to adjust blood pressure
  • The higher the blood pressure the more work the heart must do to pump blood

Blood Pressure is Regulated by Reflexes and the Kidney

  • Blood pressure must be closely regulated
    • If too low circulation will be poor;
    • If too high there is danger of arterial damage or hemorrhage
  • Short term regulation (seconds -> minutes): baroreceptor reflex
    • Example:
      • If you are lying down and suddenly stand up the pressure in the aorta will fall as blood flows to the lower limbs.
      • The baroreceptor reflex will cause the heart to speed up and increase its stroke volume.
      • This raises the cardiac output and the blood pressure will go up
    • Components of the reflex:
      • Pressure is measured by sensors in the arch of the aorta and in the carotid sinus (the carotids are the major arteries supplying blood to the brain)
      • The control center is in the medulla of the brain
      • Two nerves control the heart rate:
        • Vagus nerve: slows the heart
        • Accelerator nerve: speeds it up
  • Long term regulation (days -> years) is mainly by the kidney
    • Kidney regulates the salt and water content of the body, and these substances control the blood pressure
      • The more fluid in the blood vessels the higher the pressure
    • Salt:
      • Sodium retention is controlled by the Na pump
      • The hormone aldosterone increases Na pump activity in the kidney
      • The hormones renin and angiotensin control the amount of aldosterone secreted into the blood
    • Water:
      • If Na is retained the blood osmotic pressure rises and this causes water to be retained also- by osmosis in the kidney
      • Water reabsorption in the kidney requires water channels in the kidney tubules
      • The water channels are controlled by the antidiuretic hormone (ADH)
      • If ADH is present at high concentrations there will be much water reabsorbed and the blood pressure will rise
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