This document discusses the movement of substances between body fluid compartments through capillaries and the forces that influence this movement. Specifically, it notes that substances diffuse between blood plasma and interstitial fluid through capillary walls. Nutrients, electrolytes, hormones, and gases diffuse from plasma into tissues, while metabolic waste diffuses back into plasma. The key forces that influence fluid movement are capillary hydrostatic pressure, interstitial fluid pressure, plasma oncotic pressure, and interstitial fluid oncotic pressure, known together as the Starling forces. Edema can occur when there is raised capillary pressure, reduced oncotic pressure, endothelial damage, or impaired lymphatic drainage.

1. MOVEMENT OF SUBSTANCES
BETWEEN BODY FLUID
COMPARTMENTS
DR. P. HAMAMBULU

2. Transport across capillaries
• The capillary wall separates plasma from
interstitial fluid.
• Substance exchange between blood plasma
and interstitial fluid is through endothelial
wall of the capillaries
• Various substances diffuse into interstitial fluid
from blood plasma while others diffuse from
interstitial fluid into plasma

3. Transport across capillaries
• Examples substances transported from plasma
into interstitial fluid include nutrients(glucose,
amino acids, fatty acids),
electrolytes(Na⁺,K⁺,Cl⁻,Ca²⁺ e.t.c),hormones
and gases mainly Oxygen
• Examples of substances transported from
interstitial fluid into blood plasma are various
metabolic products e.g. water, urea, CO₂ and
various electrolytes

4. Transport across capillaries
• Capillary wall is a thin membrane made up of
endothelial cells
• Substances pass through the junctions between
endothelial cells and through fenestrations when
they are present.
• Some also pass through the cells by vesicular
transport
• The factors other than vesicular transport that
are responsible for transport across the capillary
wall are diffusion and filtration

5. Transport across capillaries
• Diffusion is the movement of substances from
higher concentration to lower concentration
i.e. down their concentration gradient.
• Filtration is the process by which fluid is
forced through a membrane or other barrier
because of a difference in pressure on the two
sides.

8. Forces producing movements of
substances between compartments
• There are four forces that influence movement of
fluid between plasma and interstitial fluid
(a) Capillary (hydrostatic) pressure (Pc)
(b)Interstitial fluid (hydrostatic) pressure(Pi)
(c)Plasma oncotic pressure(∏c)
(d)Interstitial fluid oncotic pressure(∏i)
 Oncotic pressure is also called colloid osmotic
pressure

9. Forces producing movement of
substances between compartments
• Capillary (hydrostatic) pressure (Pc) is the
pressure of blood fluid against capillary walls
• Capillary pressure tend to force fluid from
capillaries into the interstitial spaces
• Interstitial fluid (hydrostatic) pressure(Pi) is of
interstitial fluid against capillary wall
• Interstitial fluid pressure tend to force fluid
from interstitial spaces into the capillaries

10. Forces producing movement of
substances between compartments
• Plasma oncotic pressure(∏c) is that force
which tends to force water into the plasma by
osmosis due to plasma proteins in the plasma
• Interstitial fluid oncotic pressure(∏i) which is
due to presence of proteins in interstitium
tends to cause osmosis outward through the
capillary membrane into interstitium

11. Forces producing movement of
substances between compartments
• The rate of filtration at any point along a capillary
depends on a balance the above forces sometimes
called the Starling forces i.e. hydrostatic pressure
gradient and osmotic pressure gradient
• The difference between the hydrostatic pressure in the
capillary and the hydrostatic pressure of the interstitial
fluid is called hydrostatic pressure gradient
• The difference between the colloid osmotic pressure of
plasma and colloid osmotic pressure of interstitial fluid
is called osmotic pressure gradient

13. Forces producing movement of
substances between compartments
Fluid movement = k[(Pc– Pi) – (πc– πi)]
Where, k= capillary filtration coefficient
Pc = capillary hydrostatic pressure
Pi = interstitial hydrostatic pressure
πc = capillary colloid osmotic pressure
πi = interstitial colloid osmotic pressure
The capillary filtration coefficient (k) takes into
account, and is proportional to, the permeability
of the capillary wall and the area available for
filtration

15. Forces producing movement of
substances between compartments
• The lymphatic system provides a route for the
transport of fluids and protein away from the
interstitium
• System of fine lymphatic channels throughout
the body passing via lymph nodes to thoracic
duct
• Valves ensure one-way flow

16. Oedema
• Defined as increased volume of interstitial
fluid in a tissue or organ
• May be localised or generalised (systemic)
• Causes of edema include;
(i)Raised capillary pressure
(ii)Reduced oncotic pressure
(iii)Endothelial damage (inflammation)
(iv)Impaired lymphatic drainage

17. (i)Raised Capillary Pressure
 Cardiac failure
 right ventricular failure - systemic oedema
 left ventricular failure - pulmonary
oedema
 congestive cardiac failure - both
 Local venous obstruction
 deep vein thrombosis
 external compression

18. (ii)Reduced oncotic pressure
 Renal disease
 loss of albumin across glomerulus
 Hepatic disease
 inadequate albumin synthesis
 Malnutrition
 inadequate albumin synthesis

19. Lymphatic obstraction
 Renal disease
 loss of albumin across glomerulus
 Hepatic disease
 inadequate albumin synthesis
 Malnutrition
 inadequate albumin synthesis

20. Ascites and generalized edema

21. END!