continuous capillary

**Structure:** Endothelial cells form a continuous layer joined by tight junctions (zonula occludens) and adherens junctions, with an intact, continuous basement membrane. There are no fenestrations. Small intercellular clefts allow limited passage of water, small solutes, and gases. Pericytes surround the vessel.

**Transport:** Passage of small, lipid-soluble molecules (O2, CO2) is by diffusion. Water and small hydrophilic solutes cross through the intercellular clefts. Larger molecules (e.g., insulin, some proteins) cross by pinocytosis/transcytosis in caveolae (vesicular transport).

**Locations:** Skeletal, cardiac, and smooth muscle; skin; subcutaneous fat; lung; connective tissue; exocrine glands; and the CNS (blood-brain barrier), which has the tightest junctions. CNS capillaries also have astrocyte foot processes inducing BBB properties, very few transcytotic vesicles, and specific transporters (GLUT1) and efflux pumps (P-glycoprotein).

**Comparison with other capillary types (high yield):** - **Continuous:** No fenestrations, intact basement membrane. Muscle, skin, lung, CNS. - **Fenestrated:** Pores (about 60-80 nm) covered by a diaphragm, with a continuous basement membrane. Found in the kidney glomerulus (no diaphragm, with a negatively charged basement membrane), intestinal villi, endocrine glands, and choroid plexus. Allow rapid exchange of fluids and solutes. - **Sinusoidal (discontinuous):** Large gaps between endothelial cells and a discontinuous or absent basement membrane. Found in the liver, spleen, and bone marrow. Allow passage of proteins and even cells.

**Clinical correlations:** - Blood-brain barrier: Drugs must be lipid-soluble or use carrier transport to enter the CNS. Disruption (tumor, infection, stroke) causes vasogenic edema, and contrast enhancement on CT/MRI reflects BBB breakdown. - Starling forces govern fluid exchange across continuous capillaries. Increased hydrostatic pressure (heart failure), decreased oncotic pressure (hypoalbuminemia in nephrotic syndrome, cirrhosis, malnutrition), or increased permeability (inflammation, sepsis) causes edema. - Increased permeability in inflammation: histamine, bradykinin, and other mediators cause endothelial cell contraction in postcapillary venules, widening the intercellular gaps. - Areas without BBB (circumventricular organs, e.g., area postrema, OVLT) have fenestrated capillaries. This allows sensing of blood-borne substances (the area postrema acts as the chemoreceptor trigger zone for vomiting). - Glucose transport into the brain uses GLUT1 at the BBB. GLUT1 deficiency syndrome presents with seizures and low CSF glucose.

**Mnemonic:** Continuous = "Closed" tight barrier. Fenestrated = "Filtering/Fluid" (kidney, gut, endocrine). Sinusoidal = "Spaces" (liver, spleen, marrow).