Week 9 Physiology

What the different Korotkov sounds mean.

Description of sound Clinical implication
Phase I Appearance of clear tapping sounds Correlates with systolic blood pressure
Phase II Sounds become softer and longer No clinical significance
Phase III Sounds become crisper and louder No clinical significance
Phase IV Sounds become muffled and softer Correlates as alternate measure of diastolic blood pressure
Phase V Sounds disappear completely Correlates with diastolic blood pressure


Capillary circulation

Ganong's Review of Medical Physiology, 24th Edition

Ganong’s Review of Medical Physiology, 24th Edition

  • 5microm in diameter at arterial end and 9microm in venule end
  • Minute smooth muscle
  • 5% of blood pool at any one time
  • At human nail bed arterial end capillary pressure 32mmHg, venous end 15mmHg, transit time 1-2 sec



Starling forces & their values. (important).

Ganong's Review of Medical Physiology, 24th Edition

Ganong’s Review of Medical Physiology, 24th Edition

  • The rate of filtration at any point in the capillary is according to the balance of Starling’s forces
  •  hydrostatic pressure gradient
  • Osmotic pressure gradient
  • Fluid moves out at arterial end and moves into the vessel at the venous end

Fluid movement








 Autoregulation (important).

  • Autoregulation is the ability of tissues to maintain a stable tissue perfusion at a wide range of blood pressures
  • Reflex vasoconstriction of stretched vascular smooth muscle to maintain steady blood flow
  • Some metabolites are though to be vasodilators that contribute to this – low flow, these accumulate, vasodilate to increase flow. High flow these metabolites are washed away. (decreased O2, increased CO2, acidosis, high K, lactat, histamine
  • Factors affecting auto-regulation
    • Prostacyclin(PGI2)
      • Prostaglandin that inhibits platelet activation and is a vasodilator
    • Thromboxane A2.
      • Released from platelets
      • Promotes platelet aggregation and potent vasoconstrictor
    • EDRF – endothelium derived relaxing factor
      • Best known is NO (increase cGMP –> reduce Ca2+ –> smooth muscle relaxation)
    • Endothelins:
      • Vasoconstrictors mainly released from endothelium
    • Hormones:
      • Kinins – proteins involved in pain, inflammation, coagulation and BP control eg brady kinin.
      • ANP (will be covered later, but be aware of it at this stage).
      • ADH (Vasopressin)
        • Increased osmolarity sensed in hypothalamus –> ADH release form posterior pituitary –> vasoconstriction (pressin) and  increased aquaporins inserted in apical membrane of renal collecting ducts –> increased water reabsorption –> osmolarity reduces.
        • Other stimulators of ADH release – angiotensin II, CCK, pain, emotion, surgery
        • Inhibitors of ADH release – etOH, ANP
      • NA – wk 4 physiology
      • Angiotensin II. (see wk 9 pharmacology)
    • Nervous regulation: see week 3 physiology
      • innervation of blood vessels.
      • innervation of the heart.
      • the vasomotor centre.



  • Stretch receptors
  • Location – adventitia of carotid sinus, aortic arch, also in LA and RA, pulmonary circulation(cardiopulm receptors)
  • Action – afferent signal via glossopharyngeal nerve to medulla –> glutamate–>increase  inhibitory tone to  vasomotor centre–> increase vagal tone in the balance –> vasodilation, venodilation, bradycardia, drop in BP and decrease CO.
  • be familiar (at least in passing) with the reflexes mentioned in the text:
    • Bainbridge.
    • Bezold-Jarisch.
    • Cushing (probably the most askable).


What is the CVS response to a Valsalva ?

4 steps

  1. Increased intrathoracic pressure
  2. Reduced venous return–> dec CO and SV
  3.  –> when let go –> initial decreased return to LA due to relaxation of pulm vessels
  4. increase venous return–> increse CO–> sensed by Baroreceptors –>Increased vagal tone –> Decrease HR and contractility–> decrease CO


Flow = Pressure/Resistance (in Electricity I = V/R; V=IR)

Laminar flow / turbulent flow.

  • Laminar flow is silent, turbulent flow is noisy
  • Laminar flow occurs in straight tubes upto a critical velocity then becomes turbulent. Fastest flow is in the centre of the tube.
  • Reynold’s number determines the likelihood of turbulent flow
    • Porportional to density, diameter of tube, velocity of flow
    • Inversely proportional to viscosity
    • Reynolds <2000 usually laminar flow, Reynolds>3000 usually turbulent flow


Poiseuille’s Law

  • Relationship between flow in a long tube, viscosity and radius
  • Physiological significance – flow and resistance vary directly and inversely with r^4, a small change in vessel valibre has a big effect on flow and resistance.
  • Flow = pressure difference x r4 x pi/(length of tube x 8 x viscosity)


Laplace’s Law

  • Tension in the wall of a cylinder = transmural pressure x radius /wall thickness
  • Simplified to P = T/r (wall thickness negligible)
  • Explains why a thin wall structure like capillary is not more prone to rupture.
  • Smaller the radius of a blood vessel the less pressure in wall required to balance the distending pressure



  1. Tell me about the control of BP
  2. Baroreceptor response to pressure change
  3. Starling curve and factors effecting CO
  4. Effects of Adrenaline/Norad on CO/HR/BP
  5. Tell me about autoregulation
  6. Tell me about the control of HR
  7. What is the CVS response to a Valsalva ?