2.7B. Control of interstitial fluid volume, Starling forces. & 2.8A Lymph flow.

Question 1

I. How is the interstitial fluid volume controlled?

Answer 1

• Fluid moves between the plasma and interstitial fluid compartment by passing through the capillary wall via osmosis, so long as the wall has aqueous pores and there is a pressure difference across the membrane.
• The pressure differences are Starling forces, meaning hydrostatic and osmotic pressures on both sides of the membrane.
• In the capillary blood, proteins are contributor to the osmotic pressure, and in this case, it is called the oncotic pressure.
  +) Filtration = fluid movement out of the capillary and into the interstitium
  +)Absorption = fluid movement into the capillary from the interstitium

Question 2

II. Starling forces
1. What are Starling forces?

Answer 2

• The pressure differences are Starling forces, meaning hydrostatic and osmotic pressures on both sides of the membrane.
• Starling forces combine to drive fluid from the capillaries into the interstitial space

Question 3

II. Starling forces
2A. The role of Hydrostatic pressure (Pc)

Answer 3

Affects filtration by pushing fluid + solute out of the place

Question 4

II. Starling forces
2B. What are the 2 types of Hydrostatic pressure (Pc)

Answer 4

1. Capillary hydrostatic pressure (Pc)
2. Interstitial hydrostatic pressure (Pi)

Question 5

II. Starling forces
2. The role of Capillary Hydrostatic pressure (Pc)

Answer 5

affects filtration by pushing fluid + solute out of the place

Question 6

II. Starling forces
2C. What are the characteristics of Capillary hydrostatic pressure (Pc)

Answer 6

• Capillary hydrostatic pressure (Pc) = forces fluids out of the blood
  -> Arteriolar end (30 – 35mmHg), venular end (10 – 15mmHg)

Question 7

II. Starling forces
2D. What are the characteristics of Interstitial hydrostatic pressure (Pi)

Answer 7

Interstitial hydrostatic pressure (Pi) = forces fluid into the blood
-> Usually negative, but organs with capsule (1mmHg)

Question 8

II. Starling forces
3A. The role of Oncotic (colloid osmotic) pressure

Answer 8

It affects filtration by preventing fluid from leaving

Question 9

II. Starling forces
2. The role of Hydrostatic pressure (Pc)

Answer 9

affects filtration by pushing fluid + solute out of the place

Question 10

II. Starling forces
3B. What are the 2 types of Oncotic (colloid osmotic) pressure

Answer 10

1. Capillary oncotic pressure (πc)
2. Interstitial oncotic pressure (πi)

Question 11

II. Starling forces
3C. What are the characteristics of Capillary oncotic pressure (πc)

Answer 11

reabsorbed into the blood
-> [protein]blood = 25mmHg, presence of plasma proteins

Question 12

II. Starling forces
3D. What are the characteristics of Interstitial oncotic pressure (πi)?

Answer 12

Interstitial oncotic pressure (πi): filtrated back into the interstitium
- A small amount of protein exists in the interstitium and forces some fluid out of capillary walls. This force is the interstitial oncotic pressure
-> [protein]interstitium = 5mmHg

Question 13

III. Effective filtration pressure
1. What is the formula of Effective filtration pressure?

Answer 13

Peff =(Pc –Pi)–σ*(πc –πi)

Question 14

III. Effective filtration pressure
2. What are the characteristics of Effective filtration pressure?

Answer 14

• Formula: Peff =(Pc –Pi)–σ*(πc –πi)
  -> Positive net pressure favors filtration (at arterial end) and negative net pressure favors absorption (at venous end)
• Reflection coefficient (σ): how permeable is the membrane for that particle
• If permeability σ = < 1,0
• Water = 0, because it passes through, but albumin = 1, because it does not

Question 15

IV. What is the Total volume of filtration in microcirculation? How is it collected?

Answer 15

• 20mL/min, but (90%) 18mL/min is reabsorbed = 2mL/min difference
• It remains in the interstitium and will be collected by the lymph vessels

Question 16

VI. Lymph flow
1. What are the functions of lymph flow?

Answer 16

• Lymphatic system is responsible for returning the interstitial fluid and proteins to the vascular component of blood
• To receive the fluid, lymphatic capillaries are situated near vascular capillaries -> These capillaries drain through lymphatic vessels to the thoracic duct or right lymphatic duct and then into the venous system

Question 17

VI. Lymph flow
2. How is lymph flow assisted?

Answer 17

1. Valves ensure of uni-directional flow of the lymph
2. Lymphatic pump
   - Passive: skeletal muscle contraction, arterial pulses, massage
   - Active: movement of unitary smooth muscle
3. Contraction of SM (vasomotion)
4. Respiratory activity: intermittent compression and relaxation

Question 18

VI. Lymph flow
3. What are characteristics of edema?

Answer 18

• Edema is An increase in interstitial fluid volume
• Forms when volume of interstitial fluid exceeds ability of lymphatics to collect the fluid back into the vascular circulation

Question 19

VI. Lymph flow
4. What are the causes of edema?

Answer 19

1. Impaired lymphatic drainage: standing, blockage of lymph
2. ↑filtration : arteriolar dilation, venous construction, ↑ECF volume, venous pressure↑
3. ↓[albumin]
4. Inflammation (histamine release), burn