• Discuss the factors which alter the peripheral resistance.

• Discuss the control of peripheral vascular resistance.

–RESISTANCE RESISTANCE

–Resistance is the impediment to Resistance is the impediment to blood flow in a vessel,blood flow in a vessel,

– It cannot be measured by any direct It cannot be measured by any direct means. means.

– It is calculated from measurements of It is calculated from measurements of blood flow and pressure diff b/w two blood flow and pressure diff b/w two points in the vessel .If the pressure diff points in the vessel .If the pressure diff b/w two points is 1 mm Hg and the flow b/w two points is 1 mm Hg and the flow is 1ml/sec, the resistance is 1 peripheral is 1ml/sec, the resistance is 1 peripheral resistance unit or PRU.resistance unit or PRU.

– Occasionally a resistance is expressed in Occasionally a resistance is expressed in basic physical unit called CGS basic physical unit called CGS (centimeters, grams, seconds). This unit (centimeters, grams, seconds). This unit is is

– R = Dyne seconds/centimeterR = Dyne seconds/centimeter 5 5. .

• Resistance to blood flow within a vascular network is determined by

• Size of individual vessels• (length and diameter),• Organization of the vascular network (

series and parallel arrangements)• Physical characteristics of the blood (

viscosity, laminar flow versus turbulent flow), • Extravascular mechanical forces acting upon

the vasculature.• Some pathologial conditions also effect the

peripheral resistance.

• Size of individual vessels

• Changes in vessel diameter are most important for regulating blood flow within an organ, as well as for regulating arterial pressure.

• Changes in vessel diameter (in small arteries and arterioles) enable organs to adjust their own blood flow.

• If an organ needs to adjust its blood flow ( oxygen delivery) cells surrounding these

blood vessels release vasoactive substances constrict or dilate the resistance vessels.

• The ability of an organ to regulate its own blood flow is termed local regulation of blood flow mediated by:

• vasoconstrictor • vasodilator substances• released by:• Tissue surrounding blood vessels (vasoactive metabolites) • Vascular endothelium.• MYOGENIC MECHANISM (intrinsic to the vascular

smooth muscle) involved in local blood flow regulation.

• In organs such as the heart and skeletal muscle, mechanical activity (contraction and relaxation) produces compressive forces that can effectively dec vessel diameters and inc resistance to flow during muscle contraction.

Physical characteristics of the blood

• When the rate of blood flow becomes too great, when it passes by an obstruction in a vessel

• when it makes a sharp turn, or• when it passes over a rough surface, • the flow may then• become turbulent, or disorderly, rather than streamline• .Turbulent flow means that the blood flows crosswise in the

vessel as well as along the vessel,usually forming whorls in the blood called eddy currents.

• When eddy currents are present, the blood flows with greater resistance than when the flow is streamline

b/c eddies add tremendously to the overall friction of flow in the vessel.

• There are extrinsic mechanisms acting on the vasculature to regulate vessel diameter:

• autonomic innervation• circulating vasoactive hormones• AUTONOMIC INNERVATION• In general, sympathetic adrenergic influences acting

through vascular alpha-adrenoceptors cause resistance vessels as well as veins to be partially constricted under basal conditions. This is termed "sympathetic vascular tone.“

• Removal of sympathetic adrenergic influences (e.g, by administration of an alpha-antagonist or by sympathectomy) leads to vasodilation and an increase in organ blood flow.

• CIRCULATING VASOACTIVE HORMONES• Circulating vasoactive hormones have extrinsic

influence on the vasculature such as:• angiotensin II• epinephrine • norepinephrine • vasopressin (antidiuretic hormone, ADH)• atrial natriuretic peptide (ANP)• endothelin. • Both the neural and humoral factors, while

affecting organ blood flow, primarily serve the function of regulating arterial pressure by altering systemic vascular resistance.

• some of the conditions that can decrease the peripheral resistance are:

• 1. Beriberi.• This disease is caused by insufficient quantity

of the vitamin thiamine (vitamin B1) in thediet.• Lack of this vitamin causes diminished ability of

the tissues to use some cellular nutrients, and the local tissue blood flow mechanisms in turn cause marked compensatory peripheral vasodilation.

• Sometimes the total peripheral resistance dec to as little as one-half normal.

• 2. Arteriovenous fistula (shunt).

• Whenever a fistula (also called an AV

• shunt) occurs between a major artery and a major vein, tremendous amounts of blood flow directly from the artery into the vein.

• This greatly dec the total peripheral resistance.

• 3. Hyperthyroidism.

• In hyperthyroidism metabolism of most tissues of the body greatly increased. Oxygen usage inc vasodilator products are released from the tissues.

• Therefore, the total peripheral resistance dec markedly because of the local tissue blood flow control reactions throughout the body;

• 4. Anemia.

• In anemia, two peripheral effects greatly

• dec the total peripheral resistance.

• Reduced viscosity of the blood

• resulting from the dec conc of RBC.

• Diminished delivery of O2 to the tissues,

• which causes local vasodilation.