Microcirculation & Lymphatics

The microcirculation is the functional heart of the cardiovascular system — where the entire purpose of circulation, namely exchange of gases, nutrients and waste, actually occurs. This topic unifies Starling forces, oedema physiology and the lymphatic safety valve, and is a non-negotiable prerequisite for portal hypertension, ascites and nephrotic oedema.

Definition & components of the microcirculation

The microcirculation comprises all vessels below ~100 µm in diameter: arterioles, metarterioles, precapillary sphincters, true capillaries, postcapillary venules and the lymphatics that drain the interstitium. It is the site of all transcapillary exchange.

Component	Calibre	Key feature / function
Arteriole	10–100 µm	Major resistance vessel; thick smooth muscle; sets capillary pressure
Metarteriole	~10 µm	Bypass channel (thoroughfare); intermittent smooth muscle
Precapillary sphincter	—	Single smooth-muscle ring guarding entry to true capillary; controls perfused capillary number
True capillary	4–9 µm	Single endothelial layer + basement membrane; exchange occurs here
Postcapillary venule	8–30 µm	Site of inflammatory leucocyte diapedesis and histamine-induced leakage

Blood flow through any tissue is regulated almost entirely at the arteriole and precapillary sphincter level. Vasomotion is the intermittent, rhythmic contraction of metarterioles and precapillary sphincters, driven chiefly by tissue oxygen tension — low O₂ opens sphincters (more capillaries perfused).

High-yield: The single most important local factor regulating precapillary sphincter tone (and therefore the number of perfused capillaries) is local tissue oxygen demand/availability. This is the basis of metabolic autoregulation.

Capillary types (exchange capacity rises down this list)

Type	Structure	Location	Permeability
Continuous	Unbroken endothelium, tight junctions	Skin, muscle, lung, CNS (tightest = blood–brain barrier)	Lowest
Fenestrated	Endothelial pores (~70–100 nm)	Gut mucosa, renal glomerulus, endocrine glands, choroid plexus	Intermediate–high
Discontinuous (sinusoidal)	Large gaps + discontinuous basement membrane	Liver sinusoids, spleen, bone marrow	Highest (proteins/cells pass freely)

High-yield: Hepatic sinusoids are so leaky that interstitial (Disse space) protein concentration nearly equals plasma — interstitial oncotic pressure here is high (~⁠) and capillary oncotic pull is minimal. This is why portal hypertension causes ascites so readily.

Starling forces — the master equation

Fluid movement across the capillary wall is governed by the balance of hydrostatic pressures (which push fluid out/in) and oncotic (colloid osmotic) pressures (which pull fluid in/out), modified by membrane permeability.

Net filtration pressure (NFP) = (P_c − P_if) − σ(π_c − π_if)

Net fluid flux J_v = K_f × [ (P_c − P_if) − σ(π_c − π_if) ]

Where:

• P_c = capillary hydrostatic pressure
• P_if = interstitial fluid hydrostatic pressure
• π_c = capillary (plasma) oncotic pressure — chiefly albumin (~70% of effect; greatest contributor by molar number)
• π_if = interstitial fluid oncotic pressure
• K_f = filtration coefficient (surface area × hydraulic conductivity)
• σ = reflection coefficient (1 = wall totally impermeable to protein; 0 = freely permeable)

Classic Guyton average values (mm Hg)

Force	Arteriolar end	Venular end
Capillary hydrostatic (P_c)	~30–35	~10–15
Interstitial hydrostatic (P_if)	~ −3 (slightly negative)	~ −3
Plasma oncotic (π_c)	~28	~28
Interstitial oncotic (π_if)	~8	~8
Net direction	Outward (filtration)	Inward (reabsorption)

Stepwise logic of fluid exchange:

1. Arteriolar end → high P_c (~30) overcomes oncotic pull → net filtration outward.
2. Along the capillary → P_c falls progressively while π_c stays roughly constant.
3. Venular end → P_c has fallen below oncotic pressure → net reabsorption inward.
4. Filtration slightly exceeds reabsorption overall → the surplus (~2–4 litres/day of net interstitial fluid + filtered protein) → removed by lymphatics → returned to circulation via the thoracic duct.

High-yield: Over the whole body, filtration marginally exceeds reabsorption. The lymphatic system is the obligatory safety valve that returns this surplus fluid AND the small amount of leaked protein. Without lymphatics, oedema is inevitable.

High-yield: Albumin is the dominant determinant of plasma oncotic pressure — not because of mass but because of molar concentration (small size, high number of particles) plus the Gibbs–Donnan effect (its negative charge retains Na⁺, augmenting effective osmotic pull). Plasma colloid osmotic pressure ≈ 25–28 mm Hg.

Modern (revised) Starling principle — the glycocalyx

Classical teaching predicted venular reabsorption; modern data show that most capillaries filter along their entire length and do NOT reabsorb appreciably in steady state. The relevant oncotic gradient is across the endothelial glycocalyx (between plasma and the small subglycocalyx space), not plasma vs. bulk interstitium. The protein-poor subglycocalyx space keeps the effective oncotic gradient higher than expected → continuous low-level filtration → lymphatics clear it. This is the "no reabsorption" / glycocalyx model.

High-yield (newer pattern): Per the revised Starling principle, raising plasma oncotic pressure (e.g. albumin infusion) does NOT cause sustained capillary reabsorption — it merely reduces filtration rate. The "autotransfusion" by reabsorption is transient.

Pathophysiology of oedema

Oedema = excess interstitial fluid. It becomes clinically detectable when interstitial volume rises ~30% (≈ 2.5–3 L extra in an adult) because interstitial fluid pressure must first climb from negative toward positive. Once P_if turns positive, compliance rises sharply and large volumes accumulate easily.

The four/five mechanisms

Mechanism	Starling change	Clinical example
↑ Capillary hydrostatic pressure	↑ P_c	CHF, venous obstruction/DVT, pregnancy, portal hypertension
↓ Plasma oncotic pressure	↓ π_c (hypoalbuminaemia)	Nephrotic syndrome, cirrhosis, kwashiorkor, protein-losing enteropathy
↑ Capillary permeability	↓ σ, ↑ K_f	Inflammation, sepsis, burns, anaphylaxis, ARDS
Lymphatic obstruction	Failure of safety valve	Filariasis, post-mastectomy, malignant nodal blockade, Milroy disease
↑ Interstitial oncotic / Na⁺ & H₂O retention	↑ π_if / total body Na⁺	Myxoedema (hyaluronic acid), renal failure, hyperaldosteronism

High-yield: Pitting oedema = mobile, low-protein transudate (heart failure, hypoalbuminaemia) → finger pressure displaces fluid → leaves a pit. Non-pitting oedema = high protein/mucopolysaccharide or fibrosis → lymphoedema (filariasis), myxoedema (hypo- and pretibial myxoedema of Graves).

Safety factors against oedema (≈17 mm Hg buffer)

Three protective mechanisms must be overcome before oedema appears:

1. Low/negative interstitial fluid pressure → can rise from ~ −3 toward 0 absorbing fluid first (~7 mm Hg of buffer).
2. Increased lymph flow → can increase 10–50× (~7 mm Hg buffer) — the largest single safety factor.
3. Washout of interstitial protein → increased lymph flow carries away interstitial protein, lowering π_if and reducing outward force (~3 mm Hg).

High-yield: Total safety factor against oedema ≈ 17 mm Hg. Capillary pressure must rise far above normal before oedema appears — which is why mild rises in P_c are well tolerated.

Special / frequently-tested capillary beds

• Glomerular capillaries: Filtration occurs along the ENTIRE length — uniquely high P_c (~45–60 mm Hg) sustained because efferent arteriole maintains downstream resistance; net filtration is always outward (no reabsorption phase).
• Pulmonary capillaries: Very low P_c (~7 mm Hg) — keeps alveoli dry; this low pressure is the chief safety factor against pulmonary oedema. Large safety margin (~21 mm Hg) before oedema.
• Hepatic sinusoids: Highly permeable; interstitial (Disse) fluid is nearly as protein-rich as plasma, so π gradient is minimal → portal hypertension → ascites with high protein in early stages.
• Intestinal lacteals: Lymphatic capillaries of villi → absorb chylomicrons (long-chain fats); obstruction → chylous ascites / steatorrhoea; relevant to intestinal lymphangiectasia (protein-losing enteropathy).
• Brain capillaries (BBB): Tightest continuous endothelium → minimal filtration → protects against cerebral oedema.

The lymphatic system

Lymphatic capillaries are blind-ended, single-endothelial-layer tubes with overlapping cells acting as one-way micro-valves; anchoring filaments tether them to surrounding tissue so that interstitial swelling pulls the junctions open, drawing fluid in. There is no basement membrane at the initial lymphatics, allowing entry of large molecules and even cells.

Flow path: initial lymphatics → collecting lymphatics (smooth muscle + valves) → lymph nodes → trunks → right lymphatic duct (right head/neck/arm + right thorax) and thoracic duct (rest of body) → subclavian veins.

Drivers of lymph flow (lymph has no central pump):

1. Intrinsic contraction of lymphangions (segments between valves) — main pump.
2. Skeletal muscle pump + arterial pulsation.
3. Respiratory pump (negative intrathoracic pressure).
4. Rising interstitial fluid pressure — the more interstitial fluid, the higher the lymph flow (up to a plateau).

Functions: (a) return of filtered fluid (~2–4 L/day) and leaked plasma protein to blood — the only route for protein return; (b) lipid absorption via lacteals; (c) immune surveillance (lymph nodes).

High-yield: If lymphatics could not return protein, interstitial oncotic pressure would steadily rise, fluid would accumulate continuously, and death from progressive oedema would occur within ~24 h. Lymphatic protein return is essential to life, not merely a drainage convenience.

Mnemonic for oedema causes — "PILL": Pressure (↑hydrostatic), Inflammation (↑permeability), Low oncotic (↓albumin), Lymphatic obstruction.

Diagnosis & investigation of choice

This is a physiology concept, but clinical correlates have classic investigations:

• Lymphoedema (filariasis): clinical diagnosis; nocturnal peripheral blood smear for microfilariae (Wuchereria bancrofti) is the historical gold standard; circulating filarial antigen (ICT card test) is the modern point-of-care test; lymphoscintigraphy is the investigation of choice to demonstrate lymphatic dysfunction/anatomy.
• Ascites: Serum–Ascites Albumin Gradient (SAAG) — the test. SAAG ≥ 1.1 g/dL = portal hypertension (transudate: cirrhosis, CHF, Budd–Chiari); SAAG < 1.1 g/dL = non-portal (exudate: TB peritonitis, malignancy, pancreatitis).
• Hypoalbuminaemic oedema: serum albumin, urine protein (nephrotic if >3.5 g/day), LFTs.

SAAG	Value	Mechanism	Causes
High	≥ 1.1 g/dL	Portal hypertension (↑P_c)	Cirrhosis, cardiac ascites, Budd–Chiari
Low	< 1.1 g/dL	↑permeability / non-portal	TB, malignancy, nephrotic, pancreatitis

Management / drug-of-choice correlates

• Cardiac/hepatic/renal oedema (Na⁺ & water overload): loop diuretics (furosemide) ± spironolactone (drug of choice combination for cirrhotic ascites: spironolactone + furosemide ~100:40 ratio).
• Hypoalbuminaemia: treat the cause; albumin infusion is adjunctive (note revised-Starling caveat — transient effect) and indicated specifically in large-volume paracentesis (≥5 L) and spontaneous bacterial peritonitis.
• Lymphatic filariasis: Diethylcarbamazine (DEC) ± albendazole; mass drug administration uses DEC + albendazole (and ivermectin where onchocerciasis co-exists). Limb care, compression, elevation for established lymphoedema.
• Inflammatory/anaphylactic capillary leak: adrenaline (anaphylaxis), antihistamines, treat sepsis.

Complications

• Chronic venous hypertension → stasis dermatitis, lipodermatosclerosis, venous ulcer.
• Untreated lymphoedema → recurrent cellulitis, fibrosis, elephantiasis, rarely lymphangiosarcoma (Stewart–Treves syndrome) in chronic post-mastectomy lymphoedema.
• Pulmonary oedema → impaired gas exchange, hypoxaemia, respiratory failure.
• Ascites → SBP, hepatorenal syndrome, respiratory compromise, umbilical hernia.

Key differentials

• Pitting vs non-pitting oedema (see callout above).
• Generalised (anasarca: cardiac, renal, hepatic) vs localised (DVT, lymphatic, inflammatory) oedema.
• Transudate vs exudate: transudate = ↑hydrostatic/↓oncotic (low protein, SAAG high, Light's criteria fail for exudate); exudate = ↑permeability (high protein/LDH, Light's criteria: pleural/serum protein >0.5, pleural/serum LDH >0.6, pleural LDH >⅔ upper normal serum).
• Myxoedema (TSH↑, non-pitting, periorbital) vs nephrotic oedema (periorbital, pitting, proteinuria).

Recently asked / exam angle

• Identify the chief determinant of plasma oncotic pressure → albumin (NEET PG favourite).
• Which capillary bed filters along its entire length? → glomerulus (and, per revised principle, most capillaries) — contrast with the classical arteriolar-filtration/venular-reabsorption model.
• Capillary type matching: liver = sinusoidal/discontinuous; glomerulus = fenestrated; BBB = continuous (tightest).
• Largest safety factor against oedema → increased lymph flow / lymphatic washout of protein.
• SAAG cut-off ≥ 1.1 = portal hypertension — recurring image/clinical vignette.
• Why pulmonary oedema is resisted → low pulmonary capillary hydrostatic pressure (~7 mm Hg).
• Revised Starling principle / glycocalyx → "no net venular reabsorption" is an increasingly tested newer concept.
• Non-pitting oedema causes → lymphatic (filariasis) and myxoedema.
• Vasomotion regulated by → tissue oxygen.

Rapid revision

1. Exchange occurs only at true capillaries; flow controlled by arterioles & precapillary sphincters.
2. Starling: J_v = K_f[(P_c − P_if) − σ(π_c − π_if)] — hydrostatic pushes out, oncotic pulls in.
3. Albumin = main plasma oncotic protein; plasma colloid osmotic pressure ≈ 25–28 mm Hg.
4. Arteriolar end filters (P_c ~30), venular end classically reabsorbs (P_c ~10); net filtration exceeds reabsorption.
5. Lymphatics return the surplus fluid + leaked protein — the only protein-return route; failure → fatal oedema.
6. Total safety factor against oedema ≈ 17 mm Hg; biggest single factor = increased lymph flow.
7. Capillary types: continuous (BBB tightest) < fenestrated (glomerulus, gut) < sinusoidal (liver, leakiest).
8. Glomerulus filters along its whole length; pulmonary capillary pressure is low (~7) to keep alveoli dry.
9. Oedema causes (PILL): ↑Pressure, Inflammation (↑permeability), Low albumin, Lymphatic block.
10. Pitting = low-protein transudate; non-pitting = lymphoedema (filariasis) or myxoedema.
11. SAAG ≥ 1.1 g/dL = portal hypertension; < 1.1 = non-portal (TB, malignancy).
12. Revised Starling (glycocalyx model): most capillaries filter throughout; sustained reabsorption does not occur.