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)
THANK YOU