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