Microcirculation, Venous Blood Flow and Venous Return

Question 1

Is arterial pressure generally fluctuating or constant ? Why ?

Answer 1

Arterial P is generally constant because need to maintain it to ensure we can pump out CO around system in a way that works. if let P get too high, can damage vessels they’re in, too low won’t be able to pump blood effectively so overall within relatively tight range.

Question 2

Does arterial P depend on local blood flow control ? on CO control ?

Answer 2

No, arterial P is independent of either local blood flow control or CO control, since it needs to be maintained in a tight range.

Question 3

What are the main components of interstitium ?

Answer 3

Collagen and proteoglycan filaments which trap interstitial fluid

Question 4

Describe the viscosity of interstitial fluid.

Answer 4

Fluid trapped amongst filaments, forming a tissue gel so only 1% of the water is free.

Question 5

How quickly does diffusion occur in interstitial fluid ?

Answer 5

Diffusion occurs in gel ~95-99% as rapidly in free fluid

Question 6

What processes of movement across the capillary membrane occur (between the capillary and interstitial fluid) ?

Answer 6

1. Diffusion (depends on concentration gradients)

2. Bulk Flow/Filtration (depends on pressure gradient)

Question 7

Distinguish between diffusion and bulk flow/filtration.

Answer 7

DIFFUSION
Net movement of nutrients, oxygen and metabolic end products

BULK FLOW/FILTRATION
Distribution of extracellular fluid

Question 8

Distinguish between crystalloids and colloids.

Answer 8

CRYSTALLOIDS
Low mol. wt. solutes (e.g. Na+, Cl-, K+)
Can leak out of capillaries with fluid movement

COLLOIDS
Larger molecules like plasma proteins (e.g. albumin)
Cannot leak out of capillaries

Question 9

Define and describe the main features of oncotic pressure.

Answer 9

= “the osmotic (the pressure required to stop osmosis through a semipermeable membrane between a solution and solvent) pressure of a colloid in solution “

Plasma Oncotic pressure of about ~28mmHg generated by plasma proteins (Predominately by albumin, lesser extent by globulins)

Interstitial oncotic pressure is much lower (~5-8mmHg)

Overall, pressure difference means fluid flow towards capillary.

Question 10

Is capillary wall a perfect barrier to plasma proteins ? Give an example.

Answer 10

No, permeability for albumin is 1/1000th that of water (so some will leak out)

Question 11

Define and describe the main features of hydrostatic pressure.

Answer 11

= “the pressure exerted by a liquid”

Capillary hydrostatic pressure (~30-40mmHg at arterial end, ~10-15mmHg at venous end)
forces fluid out of the capillaries and in to the interstitium

Interstitial hydrostatic pressure (can generally be assumed to be 0) will usually not contribute any movement of fluid. In come cases may be slightly positive or negative. If positive (e.g. in encapsulated organ), will force fluid to the capillary and draws fluid in if negative (e.g. in head).

IMPORTANT TO NOTE: flow to capillaries fluctuates (not all capillaries being perfused at all times), so changes in hydrostatic pressure will follow changes in capillary perfusion, will will affect fluid flow.

Question 12

Identify the Starling forces. What does each force push towards ?

Answer 12

• hydrostatic pressure in the capillary (Pc) (tries to push fluid out)
• hydrostatic pressure in the interstitial fluid (Pif) (if positive, tries to push fluid to the capillaries and vice versa)
• Plasma colloid osmotic pressure (πc ) (pushes fluid towards capillary)
• Interstitial fluid colloid osmotic pressure (πif )(pushes fluid towards interstitial fluid)

Question 13

Graph the pressure of different Starling Forces over time, as well as the net filtration pressure over time.

Answer 13

Refer to slide 10 of lectures on “Microcirculation, Venous Flow and Venous Return”

Question 14

What is the overall result of all Starling forces ?

Answer 14

The predominant hydrostatic force is the capillary hydrostatic pressure (because Pif is usually 0), and it draws fluid towards the interstitium. However, it decreases at the end as it gets to the venous end.

The predominant oncotic force is the capillary oncotic pressure (πc) which pushes fluid inside the capillary.

Overall, at arterial end, net flow is towards interstitium (dominant force is hydrostatic pressure of capillary). By time get to venous end, net flow is towards capillary (hydrostatic pressure of capillary drops down). Overall, net loss of fluid.

Question 15

How does the body deal with the net loss of fluid from capillaries ? What happens if this does not occur ?

Answer 15

Fluid accumulated is drained via lymphatic vessels, through lymph nodes, back into veinous circulation.
If this does not occur, accumulation of fluid in interstitial space –> swelling of soft tissue compartment –> oedema

Question 16

How much fluid do we lose per day approximately ?

Answer 16

2-3 Liters per day

Question 17

What are the main functions of the lymphatic system with respect to microcirculation ?

Answer 17

Helps control:

• Concentration of proteins in interstitial fluids (by draining proteins)
• Volume of interstitial fluid
• Interstitial fluid pressure

(Also immune response)

Question 18

Is the veinous high or low pressure system ? high or low volume system ?

Answer 18

Low pressure system
– Between 3-18mmHg

High volume system
– Holds ~60% of total blood volume

Question 19

How does venous return impact cardiac output ?

Answer 19

Through the Frank-Starling mechanism
(increase EDV, increased stretch in ventricle so increased force of contraction which increase stroke volume, which increases CO)

Question 20

Do veins have high or low compliance ?

Answer 20

High compliance

Question 21

Identify the main mechanisms which help venous return.

Answer 21

1. Sympathetic Innervation
   (veins can constrict to push blood back towards the heart)
2. Muscle pump
   (limb movement compresses veins, this pushing blood from peripheral veinous pool to central veinous pool)
3. Inspiratory movements
   – Diaphragm descends (↑abdominal pressure, transmitted passively to intra abdominal veins which squeezes blood to more central/thoracic veins)
   – ↓ Pressure in thorax (↓pressure in intra thoracic veins and right atrium leading to movement of blood to R. atrium).
   – ∴↑Pressure difference between peripheral veins and heart
4. Blood volume (e.g. Haemorrhage, fluid challenge)
   – Haemorrhage decreases veinous return
   – Extra colloid fluid inserted in veins, expanding veinous V –> increases veinous return

Question 22

What are the consequences of increase veinous return ?

Answer 22

Higher veinous return –> higher EDVV –> increased stretch in ventricle so increased force of contraction –> increased stroke volume –> increased CO

Question 23

Identify situations where sympathetic innervation of veins is important.

Answer 23

Blood loss (constrict veins) 
Exercice 

Constrict veins to keep blood moving to the heart (causes decreased compliance, increase veinous return, in order to increase cardiac output)

Question 24

Explain the postural effects of standing completely still on veinous return (assuming circulation completely stops).
What if you laid down instead ?

Answer 24

• Hydrostatic pressure ↑ by 1mmHg for each 13.6mm below the surface (fighting against gravity), starting at negative pressures in the head
• By the feet, +90 mmHg
• Mean arterial pressure at level of heart ~100mmHg (so, in feet ~190mmHg)
• This would result in leg oedema (because extra pressure of fluid hydrostatically pushes water out into peripheral tissues).
• Can lead to accumulation of 10-20% of blood volume in legs within 15-30min. Hence, reduced veinous return to the heart, resulting in reduced BP.

Lying down would normalize pressure across the body (so fainting would help in that regard when standing too long)

Question 25

What is the use of vein valves ?

Answer 25

Ensure one way movement so even tiny little muscle movements will get blood forward.

Question 26

How do muscle pump and valves work together ?

Answer 26

• Squeezing of muscles pushes blood forward
• Valves behind where muscle is contracting stops backflow of blood
• Valves in front of muscle open and push forward

Question 27

Explain what is meant orthostatic (postural) hypotension.

Answer 27

• Immediate effect in going from supine to upright
• Around 500 ml of blood from the upper body to legs
• ↓ venous return (accumulation of blood in legs) leading to ↓ cardiac output leading to ↓ blood pressure (hence dizziness, light headedness)

Question 28

How does the body respond to orthostatic (postural) hypotension ?

Answer 28

Reflex vasoconstriction in legs and lower abdomen (takes a few seconds to kick in)

This decreases capacity of veins, and pushes blood back into heart (increased veinous return) to restore CO and get BP back to normal