This document discusses capillary circulation and trans-capillary exchange. It begins by outlining the objectives and structure of capillaries, including their thin endothelial walls, precapillary sphincters, and connections to arterioles and venules. It then explains capillary functions like nutrient/oxygen exchange, waste drainage, and temperature regulation. Trans-capillary exchange via diffusion, vesicular transport, and filtration is also summarized. Starling's forces that govern fluid movement across capillary membranes are defined. Finally, the document discusses edema, including its definition and causes like increased hydrostatic pressure, decreased oncotic pressure, lymphatic obstruction, and increased permeability.

1. Capillary Circulation &
Trans-Capillary Exchange
Dr. Aamir Magzoub
MB BS, MSc,PhD

2. Objectives
• Capillary structure
• Capillary functions
• Capillary Hemodynamics
• Trans-capillary exchange
• Edema
– Definition
– Causes
– Pathophysiology

3. Capillary structure
• Thin walls made of one layer of flat
endothelial cells.
• There are precapillary smooth muscle
sphincters respond to chemical
substances in the blood or interstitial
fluid (not innervated)
• The final part of an arteriole
(metarteriole) has intermittent
muscles
• True capillaries originates from
metarterioles & form network around.
• The network is connected to a venule
by a capillary channels (thoroughfare
channels)

4. • The diameter of a true capillary is about 5 μm
(arteriolar end) and about 9 μm (venular end).
• Therefore, the red cells have to squeeze through
the capillaries and become convex discs; this
favors exchange of O2
• Also low velocity of blood flow in the
capillaries. Significance?
• More time for exchange at tissue level.
Capillary structure

5. • Special types of “pores” in the capillaries of
certain organs. (fenestrated capillaries - kidney)
• Tight junctions in brain capillaries.
• Exchange of water, nutrients, and other substances
between the blood and interstitial fluid.
Capillary structure

6. Interstitium and Interstitial
Fluid
Collagen fibers:
Tissue strength
Proteoglycans +
water Tissue gel
(holds water)
Free water:
Small amount vesicles
& rivulets – large
amounts in edema

7. Capillary Functions
• Sites of exchange between blood and tissues
(nutrients & Oxygen).
• Drainage of body waste products.
• Temperature regulation.
• Exchange vessels.

8. Capillary Hemodynamics
 Only 5% of the circulating blood is in the capillaries (most
vital part of circulating blood – exchange).
 Peripheral circulation contains about 10 billion capillaries
having a surface area 500 to 700 m2
Active and inactive capillaries:
In resting tissues:
 Only 10% of the capillaries are opened and 90% are
completely closed. After a few seconds, the opened
capillaries become closed spontaneously while a similar
number of the previously closed capillaries are opened
(Alternation phenomenon – tissue activity).

9. Alternation phenomenon can be explained
as follows:
- Gradual  in O2 and ↑ CO2 and H+ around the
closed capillaries  open them.
- Blood flow in this area provides O2 and removes
waste products (CO2 + H+)  abolishing their
vasodilator effect  capillaries are closed again.
In active tissues (e.g. muscular exercise):
The vasodilator metabolites formed in these tissues
dilatation of precapillary sphincters  blood
flow through the closed capillaries (recruitment).

10. Trans-capillary exchange
Diffusion, e.g. O2 &
CO2 (lung capillaries)
Vesicular transport
(Transcytosis -proteins)
Filtration (fluids).
Mediated transport (e.g.
brain capillaries).

11. Exchange of fluids across
capillary membrane
• Movement of fluid across
capillary membrane . i.e. from
plasma to interstitium and vise
versa
• The movement of water drags
along with it dissolved
substance to which the
membrane is permeable
• Fluid movement is governed by
Starling’s forces?

12. Exchange of fluids across capillary
membrane
• Starling’s Forces:
– Capillary hydrostatic pressure (HP)
– Capillary oncotic (osmotic) pressure (OP)
– HP in the interstitial fluid
– OP in the interstitial fluid
 Permeability of the membrane. (Kf)

13. Starling’s forces
• Capillary hydrostatic pressure:
– The pressure of fluids against capillary wall (fluid
volume & blood pressure) – varies between cap. ends
– Promotes ultrafiltration (fluids leave blood)
• Capillary oncotic pressure:
– Osmotic pressure due to plasma proteins namely
Albumin (25 mmHg)
– Promotes reabsorption (fluids return back to blood)
• HP & OP in the interstitium are negligible Why?

14. What are the important determinants of fluid
movement across capillary endothelium?

15. Starling forces (muscle capillary)

16. Normal Capillary fluid Exchange
(muscle capillary)
37 mmHg
25 mmhg
17 mmHg
Arteriole Venule
Capillary
Interstitial Fluid
Filtration Reabsorption

17. Arteriole
Venule
Capillary
Interstitial fluid
37 mmHg 25 mmhg
17 mmHg
100 %
90 %

18. ~( EDEMA )~

19. DEFINITION :
• Accumulation of excess fluid in the
interstitial spaces.

20. Mechanism of edema
(pathophysiology)
1. Increased capillary hydrostatic pressure
2. Decreased capillary oncotic (colloid)
osmotic pressure
3. Lymphatic obstruction (or inadequate flow)
4. Increased capillary permeability

21. 1. Increased hydrostatic (filtration)
pressure:
• Increased capillary hydrostatic pressure
(venous ):
– Heart failure, venous obstruction (DVT)
– Increased total ECF volume (salt & H2O retention)
– Orthostatic edema (prolonged sitting or
standing) – effect of gravity.
• Generalized or localized edema.

22. Mechanism:
1 Increased filtration (hydrostatic) pressure :
37 mmHg
25 mmhg
17 mmHg
Arteriole
Venule
Capillary
* *

23. 2. Decreased oncotic osmotic
pressure
Decreased plasma albumin level due to:
Liver damage (decreased synthesis) - cirrhosis
Renal diseases (Nephrotic syndrome - Loss of
proteins in urine).
Protein malnutrition (Kwashiorkor- decreased
protein intake).
Generalized edema.

24. Mechanism
37 mmHg 17 mmHg
25 mmHg
Capillary
Arteriole Venule
Interstitial Fluid
*
Absorption
Filtration

25. Edema in nephrotic syndrome
(A) & protein malnutrition (B)
A B

26. 3. Blockage of lymph vessels or
inadequate flow
Filariasis (Elephantiasis) – worms
Lymph nodes enlargement or
surgical removal (malignancy)
Localized edema

27. Mechanism
Arteriole
Venule
Capillary
Interstitial fluid
37 mmHg 25 mmhg
17 mmHg
100 %
90 %

28. Lymphedema

29. 4.Increased capillary permeability:
Allergy (serious) ,why?
Due to histamine , substance P & others
Burns (leaky or damaged membrane)
Localized or generalized

30. Pitting & Non-pitting edema

31. Pitting edema
Most types

32. Pitting edema

33. Non- pitting edema
• Does not pit on pressure (fluids unite with
the tissue elements)
• Occurs in:
– Myxedema of severe hypothyroidism.
– Edema due to lymphatic obstruction.

34. Advanced myxedema (non-
pitting edema)

35. Pulmonary Edema

36. Pulmonary Edema
(left ventricular failure)

37. Ascites

38. Edema of the hand

39. Bilateral lower limb edema