OVERVIEW OF CIRCULATIONDr. Sumaira Iqbal

Learning Objectives

•By the end of lecture student should be to:• Explain types of circulation•Comprehend the principles of circulation• Explain structure and function of various

components of circulation• Explain blood flow dynamics• Summarize vascular circuits

Why circulation is needed????

• To transport nutrients

• To remove wastes

• To transport hormones

• To maintain homeostasis

Circulation– Percentage of Blood Volume

•Pulmonary circulation 16%• Heart 7%• Pulmonary vessels 9%

• Systemic circulation 84%• Arteries 13%• Veins 64%• Arterioles & capillaries 7%

Principles of circulation

1. Rate of blood flow is controlled by the tissue need

• Active tissue require more blood

• Control their own blood flow

• Cardiac output can be increased only 4-7 times

2. Maintenance of cardiac output

• Heart as automaton, amount of blood comes to it is returned

• Cardiac output increase from 5L/min to 25L/min

Principles of circulation

3. Control of arterial pressure

• Blood flow cannot be increased to all organs at a time

• Balance is maintained by less supply to dormant organs

• When blood pressure falls, nervous reflexes elicit:• Increase in force of contraction

• Constriction of arterioles and venules

BLOOD VESSELSARTERIES

• Elastic vessels

• Strong vascular wall

• Transport of blood from heart to tissues under high pressure

• Pressure reservoir

• Systolic and diastolic pressure --- 120/80 mmHg

• Mean arterial pressure --- 93 mmHg

• Leads to arterioles

BLOOD VESSELS

WINDKESSEL EFFECT:

• Interaction between stroke volume, compliance of vessels and resistance of vessels for account of arterial pressure waveform

• Recoiling effect of aorta and elastic arteries convert pulsatile flow in to continuous flow in capillaries

• Aorta and large arteries act as a SECOND PUMP

BLOOD VESSELS

ARTERIOLES

• Major resistance vessels

• Control conduit– release blood into capillaries

• Thick smooth muscle layer --- thick tunica media

• Under nervous control • Vasoconstriction

• Vasodilation

• Under humoral control• NO, angiotensin, endothelin, prostacyclin

BLOOD VESSELS

CAPILLARIES

• Single layered vessels

• Minimum diameter (5-20 micron)

• Site of exchange of nutrients and waste materials

• Maximum cross sectional area

BLOOD VESSELS

VEINS

• Accounts for 64% blood volume

• Compliant vessels

• Capacitance vessels

• Least pressure --- 10 mmHg

Blood Flow Dynamics

BLOOD FLOW

Volume of blood that passes through in a given point of time

Normal blood flow

5000-6000ml/min

• Total circulation 5000-6000ml/min OR 100ml/sec

Pressures In Systemic And Pulmonary Circulation

PRESSURE VALUE (mmHg)

Pulmonary Systemic

Systolic 25 120

Diastolic 8 80

Mean pressure 16 100

Capillary pressure 7 17

Right atrial pressure 2 (left atrial) 0 (right atrial)

Blood Flow Dynamics

• Blood flow is determined by OHM’S LAW

F=ΔP/R (ml/min or L/min)

Where • ΔP= P2-P1

i.e. pressure difference between two ends of vessel• R is the vascular resistance

• Flow is directly proportional to pressure difference and inversely proportional to resistance offered

• Contraction of heart is the main driving force for the flow

Resistance

• Resistance—hindrance caused by friction between the moving molecules and stationary vessel wall.

• Conductance is reciprocal to resistance

• Resistance is said to be 1 PRU if flow is 1ml/sec and pressure difference is 1mmHg

Blood Flow Dynamics

• Resistance --- hindrance depends upon• Radius of vessel• Length of vessel• Viscosity of blood– friction developed between molecules

when they slide over each other

• Inversely related to fourth power of radius

R∝1/r4

if radius doubles, resistance becomes 1/16th

Blood Flow Dynamics

• Control of blood flow

• In severe vasoconstriction TPR becomes 4 PRU

• In vasodilation TPR becomes 0.2 PRU

• Vasodilation• Decreased resistance• Increased blood flow

• Vasoconstriction• Increased resistance• Decreased blood flow

Total Peripheral Resistance

• Resistance in entire circulatory system is known as total peripheral resistance OR Total resistance offered by all the systemic vasculature

• Resistance in pulmonary vasculature is about 1/7th of systemic vessels

• Total pulmonary vascular resistance is 0.14 PRU

Blood Flow Dynamics

POISEUILLE’S LAW

Interrelation between flow(F), radius(R), pressure difference (ΔP), viscosity (πη) and length (L)

F= πΔPr4/8ηL

F ∝ r4

F ∝ 1/η

• Flow is primarily altered by radius of vessel

Blood Flow Dynamics

• VASCULAR CIRCUITS• Parallel vascular circuits

• Series vascular circuits

Blood Flow Dynamics

SERIES VASCULAR CIRCUIT

• Arteries, arterioles, capillaries, venules and veins are arranged in series

• Individual resistance is less than total resistance

Blood Flow Dynamics

PARALLEL VASCULAR CIRCUIT

• Within a vascular segment vessel of one type lie in parallel arrangement

• Total resistance is less than individual resistance

• Like arteries arise from aorta in parallel

• Capillaries arise from arteries in parallel

Parallel Vascular Circuit

BENEFITS• Distribution of blood to various organs

• Every organs receives fresh blood

• Each organ control its blood supply

• Total blood flow is the sum of individual flow

• Amputation/ removal of organ• Increases TPR

• Decreases cardiac output

Blood Flow Dynamics

• Types of blood flow

• Streamline/laminar blood flow

• Turbulent blood flow

Blood Flow Dynamics

STREAMLINE/LAMINAR BLOOD FLOW

• Flow at a steady rate

• Each layer remain at a same distance from vessel wall

• Do not produce sounds

Blood Flow Dynamics

TURBULENT BLOOD FLOW

• Flow in all directions

• Produce eddy currents

• Produce sounds like korotokoff’s sounds, bruit etc

• Reynold’s number• Measure the tendency of turbulence

• Re= v.d.ρ/η

v(flow velocity), d(vessel diameter), ρ(blood density) and η(blood viscosity)

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