Post on 24-May-2015
CIRCULATION
Dr. Faraz BokhariAssistant Professor
SKZMDC
Cardiovascular System
• Multicellular organisms need CVS*
General Principles
• Output of the Right and Left Heart Are Interdependent Because Their Chambers Are Connected in Series
• 2-bucket example
General Principles
• Blood Flow to Individual Organs Can Be Controlled Primarily Independently Because Circulations to Individual Organs Are Arranged in parallel
• Liver is an exception – own arterial supply + splanchnic circulation
• Hence tissue need dictates its own blood flow
General Principles
• Lumen diameter of all Arteries & Veins can be actively changed by contraction or relaxation of the circular layers of SM within their walls
• Scores of normal physiological, pathological, and pharmacological agents that can alter vessel lumen
General Principles
• Cardiac output – controlled – sum of all local tissue flows
• Arterial pressure – controlled – independently
HemodynamicsBlood flow, Pressure & Resistance
• Blood flow through a vessel:• ∆P*• Resistance • F= ∆P/R (Ohm’s Law) • Laminar Vs turbulent flow– Eddie currents – more turbulence– Tendency for turbulence – measured by Reynold’s
number (Re) » (Re= v.d.p/ŋ)» Re >2000 – significant turbulence
• Blood pressure is the ‘force exerted by blood against any unit area of vessel wall’
Blood flow, Pressure & Resistance
• Resistance is impediment to blood flow in a vessel
• If ∆P = 1 mm hg • And if, Flow = 1 ml/sec • Then, R = 1PRU
– Total peripheral resistance– Strong sympathetic ++ : R=4 PRU
Vessel Conductance• Diameter – conductance
relationship – 4-fold increase in d caused
256-fold increase in flow
• Hence, conductance of a vessel increases in proportion to the fourth power of diameter
• conductance ∞ diameter4
• Poiseuille’s Law – factors that change resistance of blood vessel (or conductance – F = π ∆P.r4 / 8ŋ.l
Derivations of Poiseuille’s Law • F = ∆P πr4/ 8ŋl or • Q = ∆P πr4 / 8ŋl• ∆P = Q.8ŋl / πr4 where 8ŋl / πr4= R• ∆P = Q.R
• R = ∆P/Q • mm Hg/mL per minute or PRU
• Q = ∆P/R (Ohm’s Law)• Flow is proportional to pressure difference b/w entrance
and exit points of a tube • And inversely proportional to resistance
Vascular Compliance• Vascular compliance
• Increase in V/increase in P • Related to the ease by which a given change in
pressure causes a change in volume• Compliance of a systemic vein is about 24 times that
of its corresponding artery» Since it is about 8 times as distensible, and » Has a volume about 3 times as great » 8 x 3 = 24
• Delayed compliance • Increase in V – increase in P pressure normalizes
(vasodilation)• Vice versa
Vessel Types• Windkessel Vessels
• Elastic reservoir vessels• Large arteries• Highly distensible
– Serve to damp large pressure fluctuations– Pulsatile flow converted to constant blood flow
• Resistance Vessels• Arterioles, metaarterioles and pre-capillary sphincters• Arterioles has extensive ANS innervation
– Alpha receptors – arterioles of skin, splanchnic, renal– Beta receptors – arterioles of skeletal muscle
• Exchange Vessels• Capillaries
• Capacitance Vessels• Veins
• Shunt Vessels• Present in skin and other areas• Temperature regulation
Arterial Pressures
• Systolic pressure • In vascular system is the peak pressure reached
during systole
• Diastolic pressure• Lowest pressure during diastole
• Mean arterial pressure (MAP)• Pulse pressure
Arterial Pressure Pulsations
• Each heart beat–Not only moves the blood in the vessels
forward but also sets up a pressure wave
» This wave travels along the arteries
» It expands the arterial walls as it travels, and the expansion is palpable as the pulse
» Rate at which the wave travels is independent of and much higher than the velocity of blood flow!
Pulse Pressure
• Pulse pressure = systolic P – diastolic P• Depends on:
– Stroke Volume (SV)– Arterial compliance
• Examples of variance in pulses• Weak ("thready") in shock• Strong in exercise / after administration of
histamine• Aortic insufficiency: collapsing, Corrigan, or water-
hammer pulse
Abnormal Aortic Pressure Pulses
Transmission of Arterial Pulsations
• Arterial pressure pulsations• Continuous blood flow Vs pulsatile blood flow• Windkessel effect
Mean arterial pressure (MAP)*
• Average of the arterial pressures measured millisecond by millisecond over a period of time
• Not an arithmetic mean, is closer to diastolic pressure than systolic• Diastolic P + 1/3 Pulse P
• Depends on:• Mean blood volume in arterial system (C.O.)• Arterial compliance (TPR)
• BP = C.O. x TPR» Systolic BP is mainly controlled by CO» Diastolic BP is mainly controlled by TPR (BP ∞ TPR, Increase
TPR – increase BP)
SV, HR & TPR affect MAP, Pulse P
• MAP = CO x TPR• Where CO = SV x HR
• CO influence on MAP is independent!*
• CO influence on Pulse P depends on whether:
• CO has increased due to change in SV or• CO has increased due to change in HR
• Scenario 1:• HR increases, SV decreases
– CO = constant; MAP = constant
• But, Pulse P decreases (since SV has decreased)– Systolic P decreases– Diastolic P increases (decreased runoff due to lower SV)
• Scenario 2:• HR decreases, SV increases (atheletes @ rest)
– CO = constant; MAP = constant
• But, Pulse P increases (since SV increased)– Systolic P increases– Diastolic P decreases (increased runoff due to higher SV)
Overview: MAP
Variations in Arterial BP
• Physiological variations in BP• Diurnal: lowest – early morning; highest –
afternoon• Gender: females tend to have < BP• BP rises with age, BMI and mental stress• BP decreases with sleep and food intake• Exercise: moderate – only systolic increases; severe
– both rise• Posture: standing upright first decreases BP
(decrease VR); SNS activation restores BP