Capillary circulation & fluid exchange.pdf

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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 precapillarysmooth 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 networkaround.
•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 O
2
•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 m
2
Active and inactive capillaries:
In resting tissues:
Only10%ofthecapillariesareopenedand90%are
completelyclosed.Afterafewseconds,theopened
capillariesbecomeclosedspontaneouslywhileasimilar
numberofthepreviouslyclosedcapillariesareopened
(Alternationphenomenon–tissueactivity).

9. Alternation phenomenon can be explained
as follows:
-GradualinO
2and↑CO
2andH
+
aroundthe
closedcapillariesopenthem.
-BloodflowinthisareaprovidesO
2andremoves
wasteproducts(CO
2+H
+
)abolishingtheir
vasodilatoreffectcapillariesareclosedagain.
Inactivetissues(e.g.muscularexercise):
Thevasodilatormetabolitesformedinthesetissues
dilatationofprecapillarysphinctersblood
flowthroughtheclosedcapillaries(recruitment).

10. Trans-capillary exchange
Diffusion, e.g. O
2&
CO
2(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 & H
2O retention)
–Orthostatic edema (prolonged sitting or
standing) –effect of gravity.
•Generalized or localized edema.

22. Mechanism:
1\Increased filtration (hydrostatic) pressure :
37mmHg
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
25mmHg
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:
–Myxedemaof 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