Peripheral resistance

Question 1

Learning outcomes

Answer 1

• Delineate some of the physical principles governing fluid flow in tubes and apply these to the vascular system
• Understand how blood flow may be measured by a number of techniques
• Describe the mechanism by which working organs signal their need for perfusion to meet metabolic demands

Question 2

Cardiac output- at rest vs exercise

Answer 2

at rest: 5L/min
• Brain
• Sphlancnic *
• Kidneys* 
• Liver*
• Skin
• Muscles

During exercise: 25L/min
• Brain
• Sphlancnic
• Kidneys
• Liver
• Skin* 
• Muscles*

Question 3

Fluid dynamics

Answer 3

• Although blood is part solid, part liquid and in distensible tubes,
• It still roughly obeys the physical laws of fluid dynamics.
• Those relevant today concern;
• Flow from high to low pressure
• Laminar and turbulent flow
• Resistances in series and parallel
• Hagen Poisseuille law

Question 4

Laminar and turbulent flow

Answer 4

• Laminar flow
• Smooth flow in layers with quickest at centre of lumen
• In most blood vessels up until a “critical velocity”
• Turbulent flow
• Disordered, with extensive mixing
• Beyond critical velocity or flow past an obstruction
• Clotting more likely
• Audible, bruits, murmurs and Korotkoff sounds

Question 5

Vascular resistance

Answer 5

• In series
• Different categories of vessels (arteries-arterioles capillaries-venules-veins)
• More resistances in series, the greater total resistance
• In parallel
• Within a vascular bed, similar categories of vessels are arranged in parallel
• More resistances in parallel, lower total resistance

Question 6

Blood flow measurements

Answer 6

• Doppler flowmetry, using the doppler shift of sound pulses bounced off blood cells to estimate their speed
• Fick Principle to estimate flow (see cardiac ouput measurement)
• Venous occlusion plethysmography. Extremity sealed in a watertight container and its venous drainage occluded. Rate of flow into the extremity is then proportional to the volume change observed

Question 7

Local control of flow

Answer 7

• Cardiovascular system cannot perfuse all vascular beds
• So we must send the cardiac output only where its needed
• Working tissue- e.g exercising muscle
• Otherwise shock results
• Baroreceptor reflex (neuronally mediated generalised vasoconstriction)- CV centre in M.oblongata sends info from vasomotor centre and closes down blood vessels in body
• Defence and exercise responses, also generalised vasoconstriction.

Question 8

Hagen-pouiselle law- flow through any tube

Answer 8

• Hagen-Poisseuille law
(P1 - P2) X r4
(pressure difference across tube)
• Flow = ----------------
8 L n (viscosity coefficient of blood)
• So, to increase flow, either increase
• P1 – P2 (cardiac ouput)
• Change n 
• Increase r (2R = 16Flow) or, flow doubles for a 19% increase in radius (vasodilation occurs)

Question 9

Vasodilation

Answer 9

• In metabolically active tissues
• Mainly in arterioles (remember structure) and pre cap sphincters
• Via metabolites
• K+ (de/repol), H+, adenosine, Pi, CO2, lactate- turnover in metabolites in muscle metabolism
• Heat - K.energy
• EDRF- Endothelium derived relaxing factor

• Remember this MUST be against a background of increased cardiac out put to maintain BP in the face of the opening of these vessels

Question 10

The endothelium

Answer 10

• Marshall and Furgchott in the 1980s
• Not just a nice inner blanket for blood vessels, an active endocrine organ
• Endothelin (vasoconstrictor, one of the most powerful
known)
• EDRF (or NO) Endothelium derived relaxing factor
-causes muscle relaxation, more blood vessels open and more blood flows through endothelium
• Endothelial damage implicated in pathophysiology of strokes and CAD amongst others (high blood pressure)

Question 11

Hormones and nerves

Answer 11

• Hormones
• As with BP lecture, local control is also exerted by similar hormones
• Nerves
• Important in skin, sphlancnic, kidneys and erectile tissues
• Dealt with in relevant lectures
• Sympathetic cholinergic vasodilator system to muscle

Question 12

Clinical relevance- shock

Answer 12

• Circulatory crisis resulting in low BP and inadequate
peripheral perfusion
• Hypovolaemic shock, causes, signs, symptoms
• Baroreceptor compensation and cutting off BF to all but the most essential circulations
• Refractory after 3-5 hours
• Long term compensation
• Distributive shock
• TSS (a mixture of both kinds)