Interstitial fluid formation and edema by dr. mudassar ali roomi

U N I T IV

Textbook of Medical Physiology, 11th Edition

Chapter 16:
The Microcirculation and Lymphatic System
By
Dr. Mudassar Ali Roomi (MBBS, M. Phil)

GUYTON & HALL
Copyright © 2006 by Elsevier, Inc.

The Microcirculation
• Important in the transport of nutrients to
tissues.
• Site of waste product removal.
• Over 10 billion capillaries with surface area of
500-700 square meters perform function of
solute and fluid exchange.

Figure 16-1;
Guyton and Hall

Structure of Capillary Wall
• Composed of unicellular layer of
endothelial cells surrounded by a
basement membrane.
• Diameter of capillaries is 4 to 9
microns (um). Figure 16-2; Guyton and Hall

• Solute and water move across
capillary wall via intercellular cleft
(space between cells) or by
plasmalemma vesicles.


Solute and Fluid Exchange
Across Capillaries
• Most important means by
which substances are transferred
between plasma and interstitial
fluid is by diffusion.
• Lipid soluble substances diffuse
directly through cell membrane of
capillaries (i.e. CO2, O2).
• Lipid insoluble substances such as
H2O, Na, Cl, glucose cross capillary
walls via intercellular clefts.
• Concentration differences across
capillary enhances diffusion.

Effect of Molecular Size on Passage
Through Capillary Pores

• The width of capillary
intercellular slit pores is 6 to 7
nanometers.
• The permeability of the capillary
pores for different substances
varies according to their molecular
diameters.
• The capillaries in different tissues
Figure 16-2; Guyton and Hall
have extreme differences in their
permeabilities.
•Intestine, liver, kidney, choroid plexus
have highly permeable capillaries.
•Brain, lungs and skeletal muscles
capillaries are less permeable.


Relative Permeability of Muscle Capillary
Pores to Different-sized Molecules

Substance Molecular Weight Permeability
Water 18 1.00
NaCl 58.5 0.96
Urea 60 0.8
Glucose 180 0.6
Sucrose 342 0.4
Insulin 5000 0.2
Myoglobin 17,600 0.03
Hemoglobin 69,000 0.01
Albumin 69,000 .0001


Interstitium and Interstitial Fluid
• Space between cells is called
interstitium; fluid in this
space is called interstitial fluid.
• Two major types of solid structures in interstitium are
collagen fibers and proteoglycan filaments (coiled
molecules composed of hyaluronic acid).
• Almost all fluid in interstitium is in form of gel (fluid
proteoglycan mixtures); there is very little free fluid
under normal conditions.
•Advantages of tissue gel are as follows:
1. It keep the cells in the tissues held apart so that spaces are
available for the movement of fluid.
2. Tissue gel prevents the movement of fluid from the upper
part of the body to lower part due to gravity.
3. It immobilizes the bacteria so prevents the spread of
infection


Determinants of Net Fluid
Movement across Capillaries (Starling
forces)


• Capillary hydrostatic pressure (Pc)-tends to force
fluid outward through the capillary membrane.
• Interstitial fluid pressure (Pif)- opposes filtration
when value is positive.

Movement across Capillaries


• Plasma colloid osmotic pressure (π c)- opposes filtration
causing osmosis of water inward through the membrane
• Interstitial fluid colloid pressure (π if) promotes filtration
by causing osmosis of fluid outward through the
membrane
NP = (Pc – Pif)- (π p - π if)

Starling Forces (Part I)

• Normal Capillary hydrostatic pressure is
approximately 17 mmHg.
• Interstitial fluid pressure in most tissues is negative
3. Encapsulated organs have positive interstitial
pressures (+5 to +10 mmHg).
• Negative interstitial fluid pressure is caused by
pumping of lymphatic system.
• Colloid osmotic pressure is caused by presence of
large proteins.

Starling Forces (Part II)
• Presence of negative ions on proteins increases the colloid
osmotic effect of proteins–Donnan effect.
• Plasma colloid osmotic = 28mmHg
Plasma protein conc. = 7.3mg/dl
• The reflection coefficient of capillaries quantitates the
amount of protein that is reflected away from the capillary
membrane.
• Reflection coefficient of 1 means all proteins are reflected
and none pass through pores, reflection coefficient of 0
means membrane is permeable to all proteins.

Plasma Proteins and Colloid
Osmotic Pressure
• 75% of the total colloid osmotic pressure of
plasma results from the presence of albumin
and 25% is due to globulins.

gm/dl πp(mmHg)
Albumin 4.5 21.8
Globulins 2.5 6.0
Fibrinogen 0.3 0.2
Total 7.3 28.0


Interstitial Colloid Osmotic
Pressure

• Interstitial protein conc. is approx. 3gm/dl
• The interstitial colloid osmotic pressure is
normally 8mmHg


Movement Across Capillaries


• Filtration rate = net filtration pressure (NFP)
multiplied by the filtration coefficient
• Filtration coefficient (Kf) is a product of surface
area times the hydraulic conductivity of membrane

Net Starting Forces in Capillaries
mmHg
Mean forces tending to move fluid outward:
Mean Capillary pressure 17.3
Negative interstitial free fluid pressure 3.0
Interstitial fluid colloid osmotic pressure 8.0
TOTAL OUTWARD FORCE 28.3
Mean force tending to move fluid inward:
Plasma colloid osmotic pressure 28.0
TOTAL INWARD FORCE 28.0
Summation of mean forces:
Outward 28.3
Inward 28.0
NET OUTWARD FORCE 0.3


Net Starting Forces in Capillaries

Figure opener;
Guyton and Hall

• Net filtration pressure of .3 mmHg which
causes a net filtration rate of 2ml/min for
entire body.

Lymphatic System
• An accessory route by which
fluid and protein can flow from
interstitial spaces to the blood
• Important in preventing edema
• Lymph is derived from interstitial
fluid that flows into the lymphatics
•


FUNCTIONS OF THE LYMPHATIC SYSTEM:

1. It carries excess of interstitial fluid from interstitium into the blood. Rate of lymph
flow is more than 3 liters/day. So this amount is drained by lym phatic system.
2. It drains proteins and electrolytes from Interstitial space into lymphatic system.
Lymphatic system drain 195 grams of blood proteins from interstitium back into the
blood.
3. It provides lymphocytes and antibodies into the circulation
4. Removes bacteria and other microorganisims from the tissues.
5. Lacteals are involved in the absorption and transport of lipids.
6. Many large enzymes which are produced in the tissues get entry into the circulation
through lymphatic system like histaminases and lipase.
7. It maintains the negative interstitial fluid hydrostatic pressure.


Determinants of Lymph Flow:


• The degree of activity of the lymphatic
pump
- smooth muscle filaments in lymph vessel cause
them to contract
- external compression also contributes to lymphatic
pumping

Determinants of Lymph Flow (cont..)
Interstitial fluid hydrostatic pressure Lymph Flow



Determinants of Lymph Flow (cont..)

• Pressure on Lymphatics from outside:
1. Skeletal Muscle Contraction
2. Movements of different parts of body
3. Pressure by objects which come in contact on the
outer surface of the body
4. Pulsations of nearby arteries


FACTORS CONTROLLING THE
FORMATION OF TISSUE FLUID

1. Starlings Forces
2. Capillary permeability
3. Lymphatic obstruction


Causes of Extracellular Edema
I. Increased capillary pressure
A. Excessive kidney retention of salt and water
1. Acute or chronic kidney failure
2. Mineralocorticoid excess
B. High venous pressure and venous constriction
1. Heart failure
2. Venous obstruction
3. Failure of venous pumps
(a) Paralysis of muscles
(b) Immobilization of parts of the body
(c) Failure of venous valves
C. Decreased arteriolar resistance
1. Excessive body heat
2. Insufficiency of sympathetic nervous system
3. Vasodilator drugs


Causes of Extracellular Edema (cont..)

II. Decreased plasma proteins
A. Loss of proteins in urine (nephrotic syndrome)
B. Loss of protein from denuded skin areas
1. Burns
2. Wounds
C. Failure to produce proteins
1. Liver disease (e.g., cirrhosis)
2. Serious protein or caloric malnutrition


Causes of Extracellular Edema (cont…)
III. Increased capillary permeability
A. Immune reactions that cause release of histamine and other immune products
B. Toxins
C. Bacterial infections
D. Vitamin deficiency, especially vitamin C
E. Prolonged ischemia
F. Burns
IV. Blockage of lymph return
A. Cancer
B. Infections (e.g., filaria nematodes)
C. Surgery
D. Congenital absence or abnormality of lymphatic vessels


SAFETY FACTOR to prevent the development
of edema

• The development of edema is not an easy process. First the safety factor has to
be overcome to cause edema.
• Safety factor is equal to 17 mm of Hg.
• It has three components i.e.
1. Negative Interstitial Fluid Hydrostatic pressure: which contributes -3 mm of Hg.
2. Capacity of Lymphatic System: Increased amount of tissue fluid formed by 7 mm of
Hg increase in Capillary pressure can be drained by lymphatic flow
3. Increased Washout of proteins from Interstitium: it contributes 7 mmHg. When
there is increased formation of tissue fluid there is increased amount of fluid along
with proteins that will enter the interstitium and then lymphatics for washing out.
So the interstitial fluid colloid osmotic pressure can decrease from 8 mm of Hg upto
1mm of Hg before edema can start.


Interstitial fluid formation and edema by dr. mudassar ali roomi

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