Module 6

Flow Calculation

F = ∆P/R

Blood Pressure (BP)

Force exerted by blood against vessel wall
Generated by heart

Highest BP

Cardiac contraction
Increase pressure in left ventricle
Open aortic valve

Pressure Gradient

Pressure decreases away from aorta
Friction between blood and vessel walls
Larger pressure gradient = larger flow rate

Resistance

Friction force opposing blood flow
Increase resistance = increase pressure gradient
Maintain blood flow

Resistance: Blood Viscosity

Thicker blood
More friction
Higher resistance

Resistance: Vessel Length

Longer vessel with constant radius
Higher resistance
Constant in body

Resistance: Vessel Radius

Larger radius
Lower resistance
Most important factor
R = 1/r^4

Poiseuille's Law

Vascular Tree

Systemic circulation blood vessels

Systemic Arteries

Blood enter aorta from left ventricle
Aorta divide into arteries
Small arteries enter organs

Arterioles

Artery branching in organ
Into capillaries

Capillaries

Nutrient and waste exchange

Venules

Capillaries rejoin
Exit organs
Smooth muscle walls
Little tone and resistance
Respond to arteriolar chemical signals

Systemic Veins

Return to heart
Smooth muscle walls
Little tone and resistance

Cardiac Output Distribution

Unequal
Adjustable
Highest: Digestive system
Lowest: Heart muscle
Lungs receive 100% of cardiac output

Blood Vessel Walls

Connective tissue
Smooth muscle cells
Endothelial cells

Blood Vessel: Inner

1 endothelial cell layer
Basement membrane
Connective tissue

Blood Vessel: Middle

Smooth muscle in circular arrangement
Connective tissues

Blood Vessels: Outer

Collagen-rich connective tissues

Blood Vessels: Between Layers

Elastic fibre
Maintain shape and size

Artery Functions

Rapid blood transit (Conductive vessel)
Pressure reservoir

Major Artery Walls

Thick
Elastin fibres in connective tissue layers

Artery Pressure Reservoir: Systole

Left ventricle contract
Eject blood into aorta
Blood into arteries > Blood out of capillaries
Heart pressure stretch artery walls
120 mmHg (Max BP)

Artery Pressure Reservoir: Diastole

Aortic valve closes
Blood into arteries < Blood out of capillaries
Arterial walls passive recoil
Maintain high arterial pressure
Pressure decreases with blood volume
80 mmHg (Min BP)

Sphygomomanometer

Inflatable cuff with pressure gauge and stethoscope
Inflation produce pressure above systolic pressure
Korotkoff sounds

Cuff above 120 mmHg

Exceed BP
No blood flow
No sound

Cuff between 120 and 80 mmHg

First sound: Peak systolic pressure
Intermittent sounds: Turbulent blood flow

Cuff below 80 mmHg

Below BP
Last sound: Min diastolic pressure
No following sounds (laminar flow)

Pulse Pressure

Difference between systolic and diastolic pressures

Pulse Pressure Calculations

systolic pressure - diastolic pressure

Mean Arterial Pressure (MAP)

Driving force for blood flow
Heart 2/3 in diastole and 1/3 in systole

MAP Calculations

diastolic pressure + 1/3 pulse pressure
TPR x CO

Ventricle BP

Large pressure difference

Artery BP

Smaller pressure difference
Pulse pressure

Arteriole BP

Non-pulsatile pressure

Capillary BP

Pressure drop
Non-pulsatile pressure
Efficient nutrient-waste exchange
Small radius lowers pressure and blood flow velocity

Venule and Large Vein BP

Steady pressure drop

Arterioles: Resistance Vessels

Low collagen and elastin in walls
Smaller diameter
Increase resistance
Decrease pressure
Maintain pressure gradient

Total Peripheral Resistance (TPR)

Arteriolar resistance

Shunting

Distribute cardiac output to organs

Shunting: Increase Blood Flow

Increase radius
Decrease resistance
Ex: Digestive system at rest

Shunting: Decrease Blood Flow

Decrease radius
Increase resistance
Ex: Digestive system during exercise

Arterial BP Regulation

Decrease radius
Increase resistance
Increase BP
Control valves

Arteriole Innervation

Sympathetic nervous system
Respond to hormones and chemicals

Vasoconstriction

Contraction
Decrease radius
Increase pressure
Decrease blood flow

Vasodilation

Relaxation
Increase radius
Decrease pressure
Increase blood flow

Vascular Tone

Partial arteriole constriction

Vascular Tone: Intrinsic Control

Local controls
Chemical and physical

Intrinsic Control: Chemical

Metabolic changes
Histamine

Intrinsic Control: Physical

Stress (chemical response)
Stretch (myogenic response)

Vascular Tone: Extrinsic Control

Non-local and metabolic
Neural and hormonal inputs
Regulate BP

Chemical Control: Metabolic Changes

Increase metabolic activity
Vasodilation
Interact with endothelial cells to release chemical messengers

Metabolic Changes: Vasodilation

Decrease O2
Increase CO2
Increase acid
Adenosine
Increase K+
Increase osmolarity
Prostaglandin (from fatty acid)
Nitric oxide

Metabolic Changes: Vasoconstriction

Endothelin

Chemical Control: Histamine

Paracrine molecule
Vasodilation
Increase blood flow
Redness and swelling

Physical Control: Temperature

Heat: Vasodilation (Increase blood flow)
Cold: Vasoconstriction (Decrease blood flow)

Physical Control: Shear Stress

Blood flow friction on endothelial cells
Vasodilation from nitric oxide
Reduce stress

Physical Control: Stretch

Myogenic response to passive stretch from increased pressure
Vasoconstriction
Oppose stretch

Reactive Hyperemia: Increase Blood

Inflate cuff
Pressure reduce blood flow
Vasodilation
Myogenic: Decrease pressure
Chemical: Decrease O2, increase CO2 and acids

Reactive Hyperemia: Decrease Blood

Deflate cuff
Fast blood flow from vasodilation
Restore chemical balances
Increase vascular tone (vasoconstriction)

Extrinsic Control: Neural

Sympathetic innervation
Increase activity = increase arteriolar tone and TPR
Decrease activity = decrease arteriolar tone and TPR
Systemic

Extrinsic Control: Norepinephrine

Release by sympathetic fibres
Bind a1-adrenergic receptors
Increase Ca2+ entry
Vasoconstriction
NOT in brain (Constant blood flow)

Extrinsic Control: Epinephrine

Release from adrenal gland
Bind b-adrenergic receptors
Affect heart rate and contractility
Vasodilation
Increase blood flow during fight or flight

Extrinsic Control: Hormonal

Vasopressin
Angiotensin II
Vasoconstriction
During haemorrhage to maintain BP

Local vs Sympathetic Control

Strong local control override sympathetic activity

Capillary Beds

Branching network in tissues
Material exchange by diffusion
Carrier-mediated transport in brain

Capillary Diffusion: Short Distance

1 layer of endothelial cells
Narrow vessels
RBC squeeze through
Surrounding cells close by

Capillary Diffusion: Large Surface Area

Numerous
Many gas and nutrient exchange sites

Capillary Diffusion: Slow Blood Velocity

More exchange time

Metarteriole

Vessels with some smooth muscle cells
Between arterioles and venules

Precapillary Sphincters

Smooth muscle cells in metarterioles
Control blood flow through capillary bed

Precapillary Sphincters: Relaxed

Blood from metarteriole to capillary bed

Precapillary Sphincters: Contracted

Blood bypass capillary bed

Capillary Pores

Space between 2 endothelial cells
Water and water-soluble molecule passage
Vary in size (none in brain, large in liver)

Capillary Wall Transport

Lipid soluble: Pass
Proteins: Vesicular transport

Bulk Flow

Protein-free plasma movement
Capillary lumen to interstitial space
Transport small water-soluble molecules
Sum of filtration and reabsorption

Bulk Flow: Filtration

Fluid out
Outward driving force

Bulk Flow: Reabsorption

Fluid in
Inward driving force

Bulk Flow Force: Capillary Blood Pressure

PC
Hydrostatic pressure (blood)
Fluid out
In: 37 mmHg
Out: 17 mmHg

Bulk Flow Force: Interstitial Fluid Colloid Osmotic Pressure

πIF
Osmotic pressure (interstitial fluid protein)
Fluid out
Protein leaving capillary into lymphatic system (0 mmHg)

Bulk Flow Force: Interstitial Fluid Hydrostatic Pressure

PIF
Hydrostatic pressure (external pressure)
Fluid in
1 mmHg

Bulk Flow Force: Plasma-Colloid Osmotic Pressure

πP
Oncotic pressure
Osmotic pressure (plasma protein)
Fluid in
No protein exchange across capillary walls (25 mmHg)

Net Exchange Pressure

Outward pressure - Inward Pressure

Net Fluid Exchange: Beginning

Filtration
Fluid into interstitial space

Net Fluid Exchange: End

Reabsorption
Fluid into plasma

Lymphatic System

Absorb excess interstitial fluid

Initial Lymphatics

Start of lymphatic system
In capillary beds
Endothelial cell layer form 1-way valves
Interstitial pressure increase opens valves
Vessel pressure increase closes valves

Lymph Vessels

Empty into venous system at right atrium
Smooth muscle propel lymph

Lymphatic Function: Fluid Return

Correct filtration and reabsorption imbalance
Prevent fluid accumulation in interstitial space

Lymphatic Function: Defense Against Disease

Lymph nodes destroy pathogens
Phagocytes and lymphocytes
Infection cause node swelling

Lymphatic Function: Fat Transport

Large fat molecules cannot absorb into capillaries
Enter lymph vessels

Lymphatic Function: Return Protein

Filtered protein cannot re-enter capillaries
Enter lymph vessels
Prevent accumulation and bulk flow imbalance

Oedema

Tissue swelling from interstitial fluid accumulation

Oedema: Plasma Protein Concentration

Reduce concentration
Reduce plasma-colloid osmotic pressure
More filtration

Oedema: Venous Pressure

Increase pressure
More filtration

Lymphatic Filariasis

Parasite invade lymph vessels
Affect lymph drainage
Oedema in limbs

Vein Functions

Blood volume regulation (capacitance vessel)
Blood reservoir (easy distension)

Venous Capacity

Blood volume accommodated in veins

Venous Capacity: Increase

More blood stored
Decrease circulating blood