Human Physiology II · Systems Physiology

Microcirculation and Lymphatics

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In 30 seconds
Why this matters
The college version
Eli explains
Worked example
Key takeaway
Study tools

In 30 seconds

Capillaries are the exchange vessels, built as a thin single layer of endothelial cells that lets oxygen, nutrients, and wastes move between blood and tissues. Exchange happens by (the workhorse), (for large molecules), and (Starling forces). The balance of hydrostatic and oncotic pressures across the capillary wall determines net filtration at the arteriolar end and net reabsorption at the venular end. Any excess filtered fluid and escaped protein is collected by lymphatics and returned to the circulation; when filtration exceeds drainage, forms.

Why this matters

Edema is a common finding, and its mechanism (hydrostatic vs oncotic vs permeability vs lymphatic) guides understanding of its causes; diagnosis and management are outside the scope of these notes and vary by institution and jurisdiction. Lymph node involvement is central to immune surveillance and to how some conditions spread, but patient-specific interpretation requires qualified clinicians.

The college version

1. Capillary Structure

Capillaries are the smallest vessels, thin enough that red blood cells pass single-file, and their walls maximize exchange. Three types differ in leakiness. Continuous capillaries have uninterrupted endothelial cells with tight junctions and are found in muscle, lung, and brain (where they form the tight blood-brain barrier). Fenestrated capillaries have small pores (fenestrations) that permit rapid fluid and small-solute exchange, found in the kidney glomeruli, endocrine glands, and intestine. Sinusoidal capillaries have large gaps and incomplete basement membranes, allowing cells and large proteins through, found in the liver, spleen, and bone marrow.

2. Transcapillary Exchange

Exchange occurs by three mechanisms. Diffusion (Fick's law) moves lipid-soluble gases (O2, CO2) and small solutes down their concentration gradients—the dominant mechanism. Vesicular transport (transcytosis) shuttles larger molecules such as plasma proteins across the endothelium in vesicles. Bulk flow moves water and dissolved solutes together through the capillary wall, driven by pressure differences (Starling forces); it is important for fluid distribution rather than nutrient delivery.

3. Starling Forces, Edema, and Lymphatics

Four pressures determine bulk flow: capillary hydrostatic pressure (Pc, pushes fluid out), interstitial hydrostatic pressure (Pif, usually small and pushes fluid in), capillary oncotic pressure (πc, from plasma proteins, pulls fluid in), and interstitial oncotic pressure (πif, pulls fluid out). Net filtration pressure is:

$NFP = (P c - P if) - (π c - π if)$

Filtration dominates at the arteriolar end (high Pc); reabsorption dominates at the venular end (low Pc). When net filtration exceeds lymphatic drainage—due to raised capillary pressure, lowered plasma protein, increased permeability, or lymphatic blockage—edema forms. Lymphatics are blind-ended vessels that collect excess fluid and protein and return them, via the thoracic duct, to the venous circulation.

How it works

Arterioles deliver blood to capillary beds, where thin walls permit exchange.
Oxygen and nutrients diffuse out; CO2 and wastes diffuse in.
Bulk flow filters fluid at the arteriolar end and reabsorbs most at the venular end.
Excess fluid and protein enter lymphatic capillaries.
Lymphatics transport this fluid through lymph nodes and back to the venous circulation.
If filtration exceeds drainage, edema accumulates.

Common confusions

Do not confuse	With	Difference
Diffusion	Bulk flow	Diffusion moves solutes individually; bulk flow moves water and solutes together by pressure
Filtration	Reabsorption	Filtration moves fluid out of the capillary; reabsorption moves it back in
Hydrostatic pressure	Oncotic pressure	Hydrostatic is physical fluid pressure; oncotic is the osmotic pull of proteins
Edema	Inflammation	Edema is fluid accumulation; inflammation also involves immune cells and mediators

Memory aids

"OUT at the arteriole, IN at the venule"—hydrostatic (water) pressure pushes fluid out where it is high (arteriolar end), while oncotic (protein) pressure pulls fluid back in where hydrostatic is low (venular end).

Quick review

Topic Recap

Capillaries are thin-walled exchange vessels that come in continuous, fenestrated, and sinusoidal forms. Exchange occurs by diffusion, vesicular transport, and bulk flow. The Starling forces (capillary and interstitial hydrostatic and oncotic pressures) determine net filtration at the arteriolar end and reabsorption at the venular end. Lymphatics collect the excess filtered fluid and protein; when filtration exceeds drainage, edema results.

Knowledge Check

Which capillary type would you expect in the brain, and why?
Name the three mechanisms of transcapillary exchange.
Write the net filtration pressure equation and identify which forces push fluid out.
List four general causes of edema.
What two things do lymphatics return to the circulation?

Answers and Rationales

Continuous capillaries—their tight junctions create the blood-brain barrier, restricting free exchange.
Diffusion, vesicular transport, and bulk flow.
NFP = (Pc − Pif) − (πc − πif); Pc (capillary hydrostatic) and πif (interstitial oncotic) push fluid out.
Increased capillary hydrostatic pressure, decreased plasma oncotic pressure, increased capillary permeability, and lymphatic obstruction.
Excess interstitial fluid and plasma proteins.

Eli explains

The same idea, in plain words

Explain it like I’m 10

Think of a capillary as a microscopic, leaky hose running through a wet sponge (the tissue). Small things like oxygen and sugar simply drift through the hose walls toward wherever there is less of them—that is diffusion. Bigger stuff, like proteins, can get carried across in tiny bubbles—that is vesicular transport. And because the hose has slightly higher pressure inside, a little water also gets squeezed out through the walls—that is bulk flow. The clever part is the balance: water squeezes out at one end of the hose and gets sucked back in at the other, while a cleanup crew (the lymphatics) mops up the leftover water and any proteins that leaked out, pouring it all back into the bloodstream. This stops being exact because capillary walls differ between organs (brain capillaries are very tight; liver capillaries are very leaky), and the "filter out, reabsorb in" picture is an average—real capillaries filter along much of their length, with the lymphatics doing most of the cleanup.

Simple Example

When you sprain an ankle and it swells, you are seeing filtration (fluid pushed out of injured, leaky capillaries) outpacing the lymphatics' ability to drain it—fluid accumulates in the tissue and the ankle becomes puffy.

Worked example

Blood enters the capillary with high hydrostatic pressure (about 35 mmHg at the arteriolar end), which forces fluid out into the interstitium—net filtration.
As blood flows along the capillary, hydrostatic pressure falls (to about 15-18 mmHg at the venular end), while plasma oncotic pressure (from albumin) stays roughly constant (about 25-28 mmHg).
The outward push (hydrostatic) now falls below the inward pull (oncotic), so fluid is reabsorbed near the venular end.
Across the whole capillary, slightly more fluid is filtered than reabsorbed; the small remainder (about 2-4 L/day) is collected by lymphatic capillaries.
Lymph moves through one-way-valved vessels, propelled by surrounding muscle contractions and lymphatic smooth muscle, passes through lymph nodes (immune filtering), and drains into the subclavian veins.
If any Starling force shifts to favor filtration, or lymphatics fail, fluid accumulates as edema.

Key takeaways

High yield: Net filtration pressure = (Pc − Pif) − (πc − πif).
High yield: Filtration occurs at the arteriolar end; reabsorption at the venular end.
High yield: Albumin provides the main capillary oncotic (water-holding) pressure.
Diffusion is the main exchange mechanism; bulk flow is small in volume but crucial for fluid balance.
Continuous, fenestrated, and sinusoidal capillaries differ in leakiness by organ.
Edema results from raised Pc, lowered πc, increased permeability, or lymphatic blockage.
Lymphatics return filtered fluid and protein to the circulation.

Keep learning

Ready to build on this? Continue to the next lesson.

Study tools & related lessonsYou’ll learn to · Key vocabulary · Related

You’ll learn to

Compare the structure and locations of continuous, fenestrated, and sinusoidal capillaries.
Explain the three mechanisms of transcapillary exchange: diffusion, vesicular transport, and bulk flow.
Apply the four Starling forces to predict net filtration versus reabsorption.
Describe how edema forms and how the lymphatic system returns filtered fluid and protein.

Key vocabulary

Continuous capillary: Capillary with uninterrupted, tight-junctioned endothelium
Fenestrated capillary: Capillary with small pores (fenestrations)
Sinusoidal capillary: Capillary with large gaps and incomplete basement membrane
Diffusion: Solute movement down a concentration gradient
Vesicular transport: Transcytosis of large molecules in vesicles
Bulk flow: Pressure-driven movement of water and solutes
Capillary hydrostatic pressure (Pc): Blood pressure inside the capillary
Capillary oncotic pressure (πc): Osmotic pull of plasma proteins
Interstitial pressures (Pif, πif): Tissue fluid pressures opposing or aiding exchange
Net filtration pressure (NFP): The algebraic sum of Starling forces
Edema: Excess fluid accumulation in the interstitium
Lymphatic system: Vessel network returning filtered fluid and protein

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