1. Elastic Vessels. 2. Resistance Vessels. 3. Exchange Vessels.

4. Capacitance Vessels. 5. Shunt Vessels / Arterio-venous

Anastomosis.

ELASTIC VESSELS: EXAMPLE: AORTA & LARGE ARTERIES HISTOLOGY: More elastic tissue & loose

collagen fibers. When blood is ejected from Lt. vent. aorta, Aorta & large arteries expand to accommodate

blood by ELASTIC RECOIL it generates & maintains diastolic B.P.

SIGNIFICANCE: if large vessels are not elastic, there would’ve been no blood flow in diastole.

Save an umbrella for rainy days!

In advanced age: pulse pressure increases, due to loss of elastic tissue in arteries increased pr. will be required to expand these arteries systolic B.P will increase & there is less elastic recoil, so increase in diastolic pr. will not be that much, as systolic pr result is increase in pulse pressure.

RESISTANCE VESSELS:

EXAMPLES:

ARTERIOLES, METARTERIOLES &

PRE-CAPILLARY SPHINCTERS.

ROLE:

Size of these vessels control blood flow to tissues. Size is controlled by nerves & local metabolic factors like lactic acid & [CO2].

EXCHANGE VESSELS: EXAMPLE: CAPILLARIES HISTOLOGY: Single layer of endothelial cells

supported by a basement membrane. Through capillary wall, there is exchange between

blood & tissues. Pre-capillary sphincter is there at the beginning of

capillary. SIGNIFICANCE: Capillary itself cannot constrict or

dilate to control blood flow, but sphincter does. At rest 20-25% capillaries are patent, others are

closed. During exercise, more are patent. (OXYGEN DEMAND THEORY).

CAPACITANCE VESSELS: EXAMPLE: VEINS & VENULES SIGNIFICANCE: Veins accommodate 2/3 of

blood volume (64%), without increase in pressure.

Veins are supplied by sympathetic nerve fibers.

When veins are constricted venous return increases. (PATAY NAHIN JAB RAH TO CURH JATAY HAIN NALAY, RUKTI HAY MERI TABA TO HOTI HAY RAWAN OR)

When veins are dilated increased amount of blood is pooled up in veins.

SHUNT VESSELS / ARTERIO-VENOUS ANASTOMOSIS: These vessels pass from metarterioles to

venules & bypass the capillary network. SIGNIFICANCE: Here is rapid flow of

blood. Involved in temperature regulation. Normally blood flow through skin is slow, for temp. regulation, there must be quick flow of blood for heat loss.

LOCATION: These are present in skin of exposed areas of body: (palm of hand, external ear & fingers).

NERVE SUPPLY: These are supplied by sympathetic nerve fibers.

2 TYPES OF BLOOD FLOW: STREAM-LINE / LAMINAR FLOW & TURBULENT FLOW LAMINAR BLOOD FLOW: Blood flows in

layers or laminae. A thin layer of blood in contact with vessel

wall does not move. Next layer moves with a slow velocity &

further next with higher velocity. At centre of vessel, maximum velocity. Unidirectional & without noise or sound.

TURBULENT FLOW: Blood flows in different directions.

Blood mixes within itself. There are eddy currents (BHANWAR) in blood flow.

This type of flow is accompanied by noise or sound.

Normally in all vessels blood flow is streamlined, except ascending aorta & pulmonary trunk, where normally there is some turbulance.

Turbulance can be determined & expressed in terms of REYNOLD’S NUMBER.

: REYNOLD’S NUMBER DEFINITION: It is the unit of turbulance. VALUE: Its value is between 2000-3000. It is directly proportional to the product of velocity,

change in diameter & density. It is inversely proportional to the viscosity. Re = v.d.p n When this no. is more than 3000, blood flow becomes

turbulent. In hyperdynamic circulation, velocity increases

Reynold’s no. increases (hyperthyroidism & severe anemia) HEMIC MURMURS.

Turbulence occurs incase of: high velocity of blood flow, pulsatile nature of flow , sudden change in vessel diameter & large vessel diameter.

Examples of Hemic murmurs: 1. Sound beyond the narrow vessel due to eddy

currents. 2. Recording of B.P. korotkoff sounds are heard.

When brachial artery is occluded & blood flows through partially occluded artery, just beyond the cuff, sounds appear with sudden increase in diameter.

3. In valvular stenosis, just beyond the stenosed valve, diameter increases stenosed murmur.

EXAMPLE OF CLASS ROOM DOOR. When viscosity of blood increases Re

decreases no murmurs. When viscosity of blood decreases Re

increases murmurs.

PRESSURE AT DIFFERENT LEVELS:

Pressure in various portions of circulation: Heart continuous pumping of blood aorta. Mean pressure in aorta is high (100mmHg). Heart pumping is pulsatile arterial pressure alternates between

SYSTOLE (120mmHg) & DIASTOLE (80mmHg). Pressure = 0, where venae-cavae empty into right atrium. Pressure in SYSTEMIC CAPILLARIES: 35mmHg (arteriolar end) &

10mmHg (venous end. Average = 17mmHg. Significance of low average pressure of systemic capillaries:

plasma cannot leak out of minute pores while nutrients can diffuse in easily.

Pressure in pulmonary circulation: In pulmonary arteries, pressure is pulsatile like in aorta, but level is far less: PULMONARY ARTERY SYSTOLIC PRESSURE = 25mmHg & DIASTOLIC PRESSURE = 8mmHg. MEAN PULMONARY ARTERIAL PRESSURE = 16mmHg.

MEAN PULMONARY CAPILLARY PRESSURE = 7mmHg. TOTAL BLOOD FLOW / MINUTE

is the same in Pulmonary & Systemic circulations. Significance of low pressure in pulmonary circulation: IT ALLOWS

EXPOSURE OF BLOOD IN PULMONARY CAPILLARIES TO OXYGEN & OTHER GASES IN PULMONARY ALVEOLI.

BASIC THEORY OF CIRCULATORY FUNCTION:3 principles under all cardiac functions:

1. Rate of blood flow to each tissue of the body is controlled in relation to tissue need.

2. Cardiac output is controlled mainly by sum of all the local tissue flows.

3. Arterial pressure is controlled independently of either local blood flow control or cardiac output control.

PRESSURE, FLOW & RESISTANCE:(INTERRELATIONSHIP)Blood flow through a vessel depends on:

1. PRESSURE DIFFERENCE (pressure gradient).

2. VASCULAR RESISTANCE.

DEFINITION OF BLOOD FLOW: Quantity of blood that passes a given point in the

circulation in a given period of time.UNIT OF BLOOD FLOW: ml/min or L/min or ml/sec.

Overall blood flow in the total circulation of an adult person at rest = 5000ml/min.CARDIAC OUTPUT: The amount of blood pumped into the aorta by the heart each minute.

CALCULATION OF VASCULAR FLOW BY OHM’S LAW: F = Δ P or Δ P = F X R R F = Flow of bloodΔ P = P1 – P2 (Pressure difference)R = Resistance OHM’S LAW states that, blood flow is

directly proportional to pressure difference but inversely proportional to the resistance.

CONDUCTANCE & RESISTANCE CONDUCTANCE is a measure of blood

flow through a vessel for a given pressure difference.

RESISTANCE is the impediment to blood flow in a vessel.

CONDUCTANCE is the exact reciprocal of RESISTANCE:

conductance = 1 resistance

VERY SLIGHT CHANGE IN DIAMETER OF A VESSEL CAN CHANGE ITS CONDUCTANCE TREMENDOUSLY:

Conductance of a vessel increases in proportion to the fourth power of the diameter:

(Conductance) is directly proportional to (diameter)4

POISEUILLE’S LAW: Wide vessel VS Narrow vessel: In a wide vessel, there are concentric rings of blood flowing

at different velocities; fastest column in centre & slowest on periphery.

In a narrow vessel, all the blood is near the wall & rapid central column does not exist.

By integrating the velocities of all the concentric rings of flowing blood & multiplying them by the areas of the rings, one can derive a formula called as POISEUILLE’S LAW:

F = π Δ P r4

8nlF = Rate of blood flow, Δ P = pressure difference, r = radius of

vessel wall, n = viscosity of blood & l = length of vessel.SIGNIFICANCE: Flow is directly proportional to fourth power of

radius. The diameter of a vessel wall (d = 2 x r) is most important factor in determining rate of blood flow through a vessel.

Importance of the vessel diameter ‘fourth power law’ in determining arteriolar resistance: In systemic circulation 2/3 of total

resistance to blood flow is ARTERIOLAR RESISTANCE in small arteries.

An increase in diameter can increase the flow 256x

Fourth power law enables arterioles to totally turn off or cause vast increase in local tissue blood flow under the effect of nervous or chemical signals.

Resistance to blood flow in SERIES & PARALLEL vascular circuits: The arteries, arterioles, capillaries, venules &

veins are collectively arranged in SERIES, so Flow through each blood vessel is the same

& total resistance to blood flow

(Rtotal) is equal to sum of resistances of each vessel:

Rtotal = R1+R2+R3+R4…..

For blood vessels arranged in PARALLEL:

1/ Rtotal = 1/R1 + 1/R2 + 1/R3+ 1/R4……

Many parallel vessels make blood flow

through the circuit easier by providing another pathway or conductance.

Total conductance for blood flow is the sum of conductance of each parallel pathway:

Ctotal = C1 +C2 + C3 + C4….. For example: brain, kidney, muscle, GIT,

skin, and coronary circulations are arranged in parallel, & each tissue contributes to overall conductance of systemic circulation.