Diffusion and convection

Question 1

Impact of gravity on hydrostatic pressure

Answer 1

• Flow requires driving pressure sufficient to overcome resistance, in cardiovascular system, pressure is generated by work of heart, maintained by recoil of aorta
• Effect of gravity on a column of fluid = hydrostatic pressure will change between different points, below the level of heart causes an increase in hydrostatic pressure
  * Hydrostatic pressure: P= ρgh (ρ = density, g = constant, h = height)
• Pressure at bottom of column > pressure at top, usually measured in mmHg, mercury is 7.5x more dense than water so in a 1m column of blood, exert additional pressure of 73.5mmHg (7.5 x 9.8 x 1)
• Cardiovascular physiology, reference point used to measure effect of gravity on hydrostatic pressure = level of heart, when lying down, gravity will have no effect so blood pressure is determined by work done by heart through contractions
• Decrease in 5mmHg due to energy required to reach end of elastic arteries
• Gravity has same effect on blood in venous circulation as blood in arterial circulation, pressures in venous circulation vary by same amount
• Difference in arterial and venous pressure in recumbent position, difference between 2 pressures = same at level of head as at level of feet
• Difference in venous and arterial pressure is same when standing up, although hydrostatic pressure ↑, it does to same extent of both arterial and venous circulation so there is same driving force for blood flow at feet and at head when standing upright

Question 2

Impact of hydrostatic pressure on capillary filtration

Answer 2

• Important to consider influence of gravity on hydrostatic pressure of capillaries below level of heart and effect this has on filtration of fluid
• Below heart level, due to gravity, capillary hydrostatic pressure will increase
• At feet level, mean arterial pressure is around 185mmHg, approximately 60mmHg of this pressure used to overcome arteriolar resistance so blood enters capillaries with capillary hydrostatic pressure of 125mmHg, capillary hydrostatic pressure will fall by 15mmHg as blood passes through, capillary hydrostatic pressure at venous end = 110mmHg
• Oncotic pressure in plasma will not change under gravity’s influence, only determined by presence of plasma proteins and so will remain at 25mmHg
• Arteriolar end of capillary = net filtration of 100mmHg favouring filtration of fluid out of capillary and at venous = net filtration of 85mmHg favouring filtration of fluid out of capillaries so filtration across entire length of capillary, excess fluid is taken into lymph vessels so, under normal conditions, should not be any build up of tissue fluids
• When standing for long periods, filtration of fluid may exceed capacity of lymph vessels → potential for oedema to form below level of heart

Question 3

Flow in cardiovascular system

Answer 3

• Convection = bulk flow of fluid (blood or gas) from area of higher to lower pressure, e.g large airways (conducting zone) and blood vessels
• Diffusion = movement of molecules from area of higher to lower concentration over short distances, e.g O2 and CO2 between alveoli to pulmonary capillaries
• Convection and diffusion in respiratory system is related to structure of respiratory where structure helps slow gas from high velocities in large airways when transport is occurring by bulk flow
  * Flow = velocity x cross sectional area of the airways
• Cross sectional area of airways ↑, velocity of gas decreases
• Trachea: cross sectional area is small, high velocity of gas transport by convection so relies on pressure gradients
• Branching of bronchioles ↑ cross sectional area, by generation 17, there is low velocity, transport by diffusion, relies on partial pressure gradient alveoli appear
• Convection and diffusion in cardiovascular system dependent on structure of system, structure helps slow blood from high velocities in large elastic arteries, transport by bulk flow (same relationship with flow, velocity and cross sectional area)
• Aorta and large arteries, as cross sectional area is small, high velocity, transport by convection which relies on pressure gradients
• Branching ↑ cross sectional area so the capillaries have a large cross sectional area, blood flow slowed, decreased velocity, transport by diffusion (concentration gradient)

Question 4

Factors influencing rate of diffusion

Answer 4

• Rate of diffusion is proportion to surface area for diffusion, 1/thickness of diffusion barrier, diffusibility of gas/substance, and partial pressure gradient across diffusion barrier (or concentration)
• Fick’s law of diffusion:
  * V ̇=A X 1/T X d x (P_1-P_2)
• Anatomy and organisation of lung (branching structure) allows for a large surface area for gas exchange in a relatively small volume
• Rate of diffusion is inversely proportional to thickness of the thickness barrier, in the lungs diffusion barrier is made up of only alveolar epithelium, basal lamina and capillary endothelium
• Diffusibility of O2 and CO2 is dependent on molecular weight and solubility (CO2 is 20x times diffusible than O2 as it is more soluble in a liquid than in air)
• One variable that can be controlled, in health, is partial pressure gradient in which rate of diffusion is proportional to partial pressure gradient in gas (gradient in lungs is between alveoli and venous blood returning from the body in pulmonary capillaries)
• Surface area, thickness/diffusion barrier, and diffusibility are constant in health, but partial pressure gradient for gases is crucial to drive diffusion

Question 5

What is the diffusion reserve?

Answer 5

• Diffusion occurs so efficiently that by 1/3 of the way along capillary blood would have equilibrated with the gas in the alveoli
• Blood enters pulmonary capillary with low level of O2 as it is used for metabolism in tissues (5kPa), partial pressure in alveoli is 13kPa so by 1/3 of the way in pulmonary artery, there is 13kPa of O2 in blood which returns to heart to be pumped around body
• Equilibration occurs at third of way along capillary means that there is a 0.5 second diffusion reserve so when blood flow ↑, still possible to get full equilibration but this may occur further along capillary and so diffusion reserve is decreased
• ↑ thickness of diffusion barrier, rate of diffusion slows, full equilibration is still reached but again will occur further down the capillary
• During exercise, there is ↑ rate of blood flow so red blood flow transit time will ↓, rate of diffusion will be slowed, full equilibration will not be achieved so ↓ in arterial partial pressure of oxygen
• At altitude, there is a lower partial pressure of O2 so alveolar partial pressure will fall resulting in ↓ partial pressure gradient for diffusion, there will still be full equilibration of blood for oxygen in alveoli but equilibration will be at a lower pO2 resulting in a decreased arterial partial pressure of oxygen at altitude

Question 6

Factors maximising diffusion

Answer 6

• Tissue: O2 + other nutrients move into cells by diffusion and CO2 + other metabolites will move out by diffusion, diffusion distance is minimised by large number of capillaries and highly metabolic tissues will have more capillaries (capillary to fibre ratio)
• Surface area in tissues is maximised by large number of capillaries, e.g in lungs alveolar space is completely covered by capillaries
• Diffusability of substances is related to their molecular structure and whether or not there are transporters/channels for substances (problems with moving oxygen will have an affect much before the effect of problems moving CO2)
• Concertation/ partial pressure gradients determines rate of diffusion, gradient is maintained by ensuring adequate blood flow to tissues
• Cardiovascular system: flow can occur by convection in vessels with small total cross sectional area with high blood flow velocity
• Elastic arteries convert intermittent to continuous flow (aorta in left side of heart and pulmonary artery in right)
• Laminar flow in most blood vessel but there is turbulence at branching points and around valves of heart