Function of the CVS

The CVS has an important role in transport of various molecules, via pump and blood vessels. This is important to regulate body tissues :

  1. O2 : Lungs to tissues
  2. CO2 : Tissues to lungs
  3. Hormones : Substance secreted into bloodstream, transport to gland. Determines how much goes where, thus important in regulating
  4. Heat : Dilate blood vessels in skin, take heat to external surface of body, regulate body temperature, can keep heat in core if cold
  5. Nutrients
  6. Excretory products : To kidney, filter

Respiratory and CVS work closely together


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Cardiac output (CO)

  • Volume each ventricle pumps/minute = Cardiac output (CO) = 5L/min at rest
  • Normally, R CO = L CO

There maybe temporary imbalances, which are quickly corrected

Cardiac output depends on:

  1. Heart rate (HR)
  2. Volume pumped/beat = Stroke volume (SV)

Therefore, CO = HR x SV (normally around 70). It is not a constant variable, vary with exercise

Systole and diastole

  • Systole is the contracting phase
  • Diastole is the filling phase, period between beats

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Pressure gradient

  • There is a pressure gradient from arteries to veins
  • Goes from high to low (low pressure = low energy)
  • This means that the circulation has resistance

Venous return

  • Volume that leaves heart/min normally equals volume that returns to heart/min
  • Therefore, venous return = cardiac output (VR = CO)
  • If this is not the case, could lead to problems

Mean arterial blood pressure

  1. Volume of blood pumped out/min (CO)
  2. Resistance of circulation

Systemic MABP = CO x TPR (total peripheral resistance, all vessels downstream of aorta)

Pulmonary MABP = CO x PVR (pulmonary vascular resistance)

TPR = 7 x PVR (i.e. peripheral resistance is greater than pulmonary vascular

CO's are equal as right and left are equal

Systemic MAP = 90mmHg (mm Mercury, supports column of 90 mmHg)

Pulmonary MABP = 12-15 mmHg

Central venous pressure (CVP) = 0-5 mmHg (near 0, in heart failure it is much higher)


  • Low resistance, easier to flow out, therefore, less pressure exerted against artery
  • Therefore, lower pulmonary vascular resistance
  • Increase in pulmonary resistance, increase in size of right ventricle (thicker wall)
  • Blood pressure changes, one or both factors have changed
  • Left ventricle has to do more work than right, as > resistance, therefore much thicker wall

Characteristics of systemic circulation

  • Aorta : Stretch as blood comes out, collagen tough, prevents bursting
  • Arterioles: Lose collagen and elastin, lots of muscle, small thick walls, main resistance vessels, lose most of pressure. Therefore, change resistance by changing contraction of arterioles
  • Veins: Elliptical, not completely filled, "reservoir", can accumulate, store additional volume
  • Capillaries: Tubes, very thin walls

Oscillates between 120 and 80; therefore, mean pressure about 90

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Types of vessel and their function/properties

  1. Elastic arteries: Strength, elastic recoil
  2. Muscular arteries: Distribution
  3. Arterioles: Vascular resistance (variable, determine how much blood goes where)
  4. Capillaries: Exchange (nutrients, metabolites, fluid)
  5. Venules/veins: Capacitance/reservoir function, determine filling of ventricle. Exhibit venous pooling. Reservoir can be used by contraction of smooth muscle as well as skeletal thus the heart can fill more

Venous pooling

Limits filling of ventricle

  • When standing, subject to gravity, leads to distension
  • This means lots of blood in veins below the heart
  • Lose venous pooling when lying down

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Distribution of cardiac output at rest

Proportion of left cardiac output that goes to each tissue at rest, depends on resistance,R

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