Diffusion and Convection

Question 1

What determines the fluid filtration in the glomerular capillaries?

Answer 1

Starling Forces

Question 2

What maintains glomerular capillary hydrostatic pressure?

Answer 2

Arrangement of afferent and efferent arterioles maintain the glomerular capillary hydrostatic pressure which results in filtration along the length of the capillary

Question 3

What are the main roles of the major vessels in the systemic circulation?

Answer 3

(Elastic) Arteries - Distribution of blood to tissues, Conversion of intermittent to continuous flow.

Arterioles - Resistance

Capillaries - Exchange

Venules & eins - Capacitance

Question 4

What influence does gravity have on systemic circulation?

Answer 4

Gravity influences hydrostatic pressure. (P)

Question 5

What is the equation to work out Hydrostatic pressure using gravity?

Answer 5

P = pgh

p= density g= gravity constant h= height

Question 6

For every meter below the level of the heart (0mmHg) how much does the pressure change?

Answer 6

Pressure increases by 74mmHg for every meter below the level of the heart

Question 7

Whilst lying down, why does gravity have no effect on pressure?

Answer 7

Since everything in the body (eg. head, heart limbs) are all at the same level.

Question 8

What happens to pressure above the level of the heart?

Answer 8

The blood pressure decreases from about 95 mmHg to roughly 53mmHg

Question 9

What happens to pressure below the level of the heart?

Answer 9

Blood pressure increases from 95mmHg to roughly 185mmHg

Question 10

What is the pressure drop through the elastic arteries?

Answer 10

5mmHg

Question 11

What are the driving pressures

Answer 11

Pressure between the arteriole and venous pressure remains the same as gravity has the same effect on venous pressure.

(Pa - Pv)

Question 12

Why do we get fluid filtration across the entire length of the capillary down at the feet?

Answer 12

At the level of the feet:
MAP = 185mmHg

There is a pressure of 60mmHg used to overcome arteriolar resistance so blood enters capillaries with a pressure of Pc = 125mmHg.

Pc falls by 15mmHg to 110mmHg as blood passes through them to the venous end of the capillaries.

The interstitial oncotic pressure is constant at 25mmHg

Therefore we have a greater force favouring filtration along the entire length of the capillary since:

At the Arteriolar end net filtration is 125 - 25 =100mmHg favouring fluid filtration out of the capillary.
At venous end net filtration is 110 - 25 = 85mmHg favouring fluid filtration out of the capillary.

Question 13

What are the types of flow in the cardiovascular and respiratory systems?

Answer 13

Convection
Diffusion
Laminar
Turbulent
Filtration

Question 14

What is convection (bulk flow)?

Answer 14

Bulk flow of fluid (blood or gas) from an area of high pressure to an area of low pressure

Question 15

Where does convection occur in the cardiorespiratory systems?

Answer 15

Large airways (conducting zone) and blood vessels

Question 16

What is diffusion?

Answer 16

Movement of molecules from an area of higher concentration to an area of lower concentration
eg. O2 and CO2 between alveoli and pulmonary capillaries.

Question 17

Where does diffusion occur in the cardiorespiratory system?

Answer 17

Respiratory zone between alveoli and pulmonary capillaries and between systemic capillaries to tissues

Question 18

Why does convection occur within the trachea (respiratory system)?

Answer 18

Small cross sectional area
Higher velocity
Relies on pressure gradient
Therefore transport via convection.

Question 19

Why does diffusion occur within the bronchioles (respiratory system)?

Answer 19

Large cross sectional area
lower velocity
Relies on partial pressure gradient
alveoli appear in walls of bronchioles
Therefore transport via diffusion

Question 20

Why does convection occur within the Aorta (cardiovascular system)?

Answer 20

Small cross sectional area
Higher velocity
Relies on pressure gradient
therefore transport via convection

Question 21

Why does diffusion occur within the capillaries (cardiovascular system)?

Answer 21

Large cross sectional area
Low velocity
relies on concentration gradient or partial pressure gradient
Therefore transport via diffusion

Question 22

What factors determine diffusion?

Answer 22

Surface area
Diffusibility
Partial pressure gradient
Thickness of diffusion barrier
Partial pressure gradient

Question 23

How is the respiratory system adapted for diffusion?

Answer 23

Large surface area packed into a relatively small volume

Thickness of diffusion barrier is very small (0.2 - 0.6 um)

Question 24

What is important about CO2 being more diffusible than O2?

Answer 24

CO2 is 20x more diffusible than O2 which can therefore lead to problems seen with O2

Question 25

What is crucial to drive diffusion?

Answer 25

Partial pressure gradient

Question 26

Where does equilibration occur within the pulmonary capillaries

Answer 26

About 1/3 rd of the way along the capillary (0.25s) sees a 13kPa of O2

Question 27

Why does the fact that it only takes up to 1/3rd of the capillary to reach equilibration important?

Answer 27

Allows us to have a large diffusion reserve.

Question 28

Why is having a large diffusion reserve important?

Answer 28

During exercise blood flow is increased and therefore having a large diffusion reserve, this allows us to still achieve full equilibration (but decreased diffusion reserve)

Question 29

What effect does a thickened diffusion barrier have on the diffusion reserve?

Answer 29

The thickened barrier means that the exchange of gases takes longer and therefore there is a decreased diffusion reserve. Hence equilibration is not achieved until much later along the capillaries.
This means that in exercise (or when there is increased blood flow) means that full equilibration is not achieved

Question 30

What happens when there is a decreased partial pressure gradient?

Answer 30

Eg. at altitude
Less O2 available and therefore lower inspired Pi O2 and therefore this leads to a decreased partial pressure gradient.
This means that diffusion takes longer and means that equilibration of O2 occurs but at a lower PO2.

Question 31

Why is having a higher number of capillaries in the cardiovascular system important?

Answer 31

Minimises the diffusion distance by having a large number of capillaries.
Higher metabolic tissues will have more capillaries.

Question 32

How is a concentration/partial pressure gradient maintained in the cardiovascular system?

Answer 32

Can be maintained by having adequate blood flow.

As long as new fresh blood flows between vessels and capillaries ensures that the concentration gradient is maintained