The microcirculation: capillary types, vasomotion, diffusion and the interstitium
- Concept
- Exam priority: medium
Capillaries are thin tubes where blood swaps gases, nutrients and water with the tissue. Their wall type and their on–off flow decide what crosses.
Also called: Capillary exchange, Microvascular unit
What it is
The microcirculation is the arteriole–capillary–venule unit of each tissue. Arterioles and precapillary sphincters set how many capillaries are open, the capillary wall decides what can cross, and diffusion does almost all of the exchange. The interstitium is a gel of collagen and proteoglycans that holds fluid in place, with only a small amount of free-flowing fluid.
Key facts
- Permeability to water-soluble molecules falls steeply with size: water and urea cross easily, glucose less, albumin (≈ 69 kDa) almost not at all.
- Encased tissues (skull, renal capsule, fascial compartments, eye) have positive interstitial pressure because a rigid cover limits expansion.
- Most interstitial fluid is trapped in a gel; free fluid is < 1% normally and rises sharply in edema, which is why pitting appears.
How it works, step by step
- Blood enters through an arteriole; metarterioles and precapillary sphincters decide which capillaries in the network carry flow at a given moment.
- When tissue O₂ falls (or adenosine, CO₂ and H⁺ rise), sphincters open longer and more often, so more capillaries are perfused.
- Gases and lipid-soluble solutes diffuse through the whole endothelial surface, so their exchange is limited by blood flow, not by the wall.
- Water and small water-soluble solutes pass through clefts between endothelial cells (≈ 6–7 nm), which make up a tiny fraction of the wall area.
- Large proteins such as albumin barely cross continuous capillaries, which keeps plasma oncotic pressure higher than interstitial.
- Fluid that does leave is held in the interstitial gel; excess is returned by lymphatics, keeping interstitial pressure near zero or slightly negative.
Starling forces along a capillary
Arteriolar NFP +11 mmHgVenular NFP −6 mmHgFiltration 3.6Reabsorption 1.1Net filtration 2.5 a.u. (lymph capacity 7.5)
Normal (classic model): fluid leaves at the arteriolar end, some returns at the venular end and the lymph returns the rest.
Tap a force (Pc, πc, Pif, πif), the lymph or the net pressure value.
- Filtration (fluid out)
- Absorption (fluid in)
- Net filtration pressure
- Normal net pressure
- Lymphatic (returns the excess)
- Edema
Reference values (systemic capillaries, illustrative)
| Parameter | Value |
|---|---|
| Capillary diameter | ≈ 5–9 µm |
| Capillary wall thickness | ≈ 0.5 µm |
| Intercellular cleft width | ≈ 6–7 nm |
| Total capillary surface area | ≈ 500–700 m² |
| Interstitial fluid pressure, loose subcutaneous tissue | ≈ −3 mmHg |
| Interstitial pressure, brain / kidney | ≈ +4 to +6 mmHg |
Approximate values for a healthy adult at rest; each laboratory sets its own ranges.
Capillary types
| Type | Where | What crosses |
|---|---|---|
| Continuous, tight junctions | Brain (blood–brain barrier) | Gases, lipid-soluble substances; others only by carriers |
| Continuous | Muscle, skin, lung, connective tissue | Water and small solutes through clefts; very little protein |
| Fenestrated | Glomerulus, intestinal villi, endocrine glands, choroid plexus | Large volumes of water and small solutes through fenestrae |
| Sinusoidal (discontinuous) | Liver, spleen, bone marrow | Plasma proteins and even cells through wide gaps |
Clinical notes
- Compartment syndrome: swelling inside a closed fascial compartment raises interstitial pressure until it collapses venules and capillaries, causing ischemia.
- Sepsis widens clefts and damages the glycocalyx, so proteins and fluid leak and tissue edema forms despite low intravascular volume.
- Nephrotic syndrome lowers plasma albumin and oncotic pressure, so more fluid filters into the interstitium.
Test yourself
Think of the answer, then tap the question to see it.
Why is O₂ exchange in tissues limited by blood flow rather than by capillary permeability?
O₂ is lipid-soluble and diffuses through the whole endothelial membrane, so it equilibrates quickly; more exchange needs more flow, not a more permeable wall.
Which capillary type lets plasma proteins reach the tissue freely, and where is it found?
Sinusoidal (discontinuous) capillaries with wide gaps, found in the liver, spleen and bone marrow.
What is the main local stimulus that opens precapillary sphincters?
Low tissue oxygen (with rising adenosine, CO₂ and H⁺): sphincters stay open longer, so more capillaries are perfused.
Why is interstitial pressure positive inside a fascial compartment of the leg but negative in the subcutis?
The compartment is wrapped in stiff fascia that resists expansion, so tissue fluid raises pressure; loose subcutis has compliant gel drained by lymphatics, keeping pressure slightly negative.
Connections
In Biochemistry1
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: Acute Compartment Syndrome (StatPearls (NCBI Bookshelf), 2024); Lymphedema (StatPearls (NCBI Bookshelf), 2025); Bacterial Sepsis (StatPearls (NCBI Bookshelf), 2024).
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Representative model, for educational purposes only.