What are the three structural types of capillaries?

Continuous capillaries with tight junctions and low permeability, fenestrated capillaries with pores in the endothelium, and discontinuous (sinusoidal) capillaries with large gaps that allow cells and large molecules to pass.

What is the endothelial glycocalyx?

A gel-like layer of proteoglycans and glycoproteins lining the luminal surface of endothelial cells; it acts as the primary molecular sieve restricting macromolecule passage and as a sensor of blood flow.

Capillary Structure and Permeability

Capillaries are the thinnest blood vessels, with walls consisting essentially of a single layer of endothelial cells on a basement membrane. This minimal wall, combined with an enormous total surface area, makes the capillary the site where solutes, water, and gases pass between blood and tissue. How readily a substance crosses depends on the structural type of the capillary and on the molecular sieve formed by the endothelial surface.

Definition

Capillary permeability is the property of the capillary wall that determines how readily water and solutes pass between the blood plasma and the surrounding interstitial fluid, set by the structure of the endothelium and its surface layer.

Scope

This topic covers the anatomy of the capillary wall, the three structural classes of capillaries (continuous, fenestrated, and discontinuous/sinusoidal), the pathways by which water and solutes cross the wall, and the concept of permeability — including the central role of the endothelial glycocalyx. It treats fluid filtration forces only briefly, leaving the Starling balance to a sibling topic.

Core questions

What is the structure of the capillary wall, and how do continuous, fenestrated, and discontinuous capillaries differ?
By what pathways do water and different-sized solutes cross the wall?
What is the endothelial glycocalyx, and why is it considered the primary permeability barrier?
How is permeability quantified and conceptualised?

Key concepts

Single endothelial-cell wall on a basement membrane
Continuous, fenestrated, and discontinuous (sinusoidal) capillaries
Diffusive versus convective (filtration) transport
Endothelial glycocalyx as molecular sieve and barrier
Intercellular clefts and tight junctions
Permeability-surface area product

Key theories

Pore theory of capillary permeability: Pappenheimer modelled the capillary wall as containing a population of small pores that allow water and small solutes to pass while restricting larger molecules, relating permeability to an effective pore size estimated from solute transfer data.
Fiber-matrix (glycocalyx) model: Curry and Michel proposed that the molecular sieving properties of the capillary wall reside in a matrix of fibres on the endothelial surface — later identified with the glycocalyx — rather than in discrete cylindrical pores, refining how selective permeability is understood.

Mechanisms

The capillary wall is a single endothelial layer whose properties vary by tissue: continuous endothelium (as in muscle and the central nervous system) has tight intercellular junctions and low permeability; fenestrated endothelium (as in kidney and gut) has pores that raise water and small-solute permeability; and discontinuous or sinusoidal endothelium (as in liver and spleen) has large gaps allowing passage of cells and macromolecules. Lipid-soluble gases such as oxygen and carbon dioxide diffuse directly across the cells, while water-soluble solutes move through intercellular clefts. Pappenheimer interpreted these data in terms of a pore system, and later fibre-matrix models attributed the sieving to the endothelial glycocalyx — a surface layer of proteoglycans and glycoproteins that both restricts macromolecule passage and senses flow.

Clinical relevance

The structural type of a capillary and the integrity of its glycocalyx influence how tissues retain or lose fluid and protein, which is relevant to understanding inflammation, capillary leak, and oedema. This is reference physiology and is not intended to guide diagnosis or treatment.

Evidence & guidelines

Understanding here derives from physiological reviews and structural studies rather than clinical guidelines; Pappenheimer's pore analysis and Michel and Curry's permeability review are foundational, and the glycocalyx literature (Reitsma and colleagues; Curry and Adamson) reflects the current view of the barrier.

History

Early twentieth-century work treated the capillary wall as a porous membrane, and Pappenheimer's 1953 analysis put the pore concept on a quantitative footing. Curry and Michel's 1980 fibre-matrix model reframed the molecular basis of sieving, and subsequent visualisation of the endothelial glycocalyx (reviewed by Reitsma and by Curry and Adamson) identified the surface layer as the principal permeability barrier and a mechanosensor.

Debates

Pores versus glycocalyx as the seat of permeability: Whether selective permeability is best described by discrete pores or by the fibre-matrix structure of the endothelial glycocalyx has shaped microvascular physiology; the glycocalyx model is now favoured but the pore framework remains a useful quantitative description.

Key figures

John Pappenheimer
C. Charles Michel
Fitz-Roy Curry
Hans Vink

Seminal works

pappenheimer-1953
michel-1999
curry-1980

Frequently asked questions

What are the three structural types of capillaries?: Continuous capillaries with tight junctions and low permeability, fenestrated capillaries with pores in the endothelium, and discontinuous (sinusoidal) capillaries with large gaps that allow cells and large molecules to pass.
What is the endothelial glycocalyx?: A gel-like layer of proteoglycans and glycoproteins lining the luminal surface of endothelial cells; it acts as the primary molecular sieve restricting macromolecule passage and as a sensor of blood flow.