CorpusHuman Anatomy
CardiovascularCapillary bed

Capillaries

Vasa capillaria

Also known as: capillary, continuous capillary, fenestrated capillary, sinusoid, sinusoidal capillary

Capillaries are the smallest blood vessels, about 5–10 µm wide and about 1 mm long. Their walls are a single layer of endothelium, which lets gases, nutrients, wastes and fluid pass between blood and tissue. There are about 10 billion of them, with a total surface area of about 500–700 m².

Blood Vessel Wall Alveolus & Gas Exchange Intestinal Villus and Absorption Wall of the Gastrointestinal Tract

Overview

Diameter≈5–10 µm
Number≈10 billion
Total exchange surface≈500–700 m²
Blood velocity≈0.3 mm/s
Share of blood volume≈5%

Structure. A capillary wall is a single layer of endothelial cells on a basal lamina. Contractile pericytes are wrapped around it. The lumen is only just wide enough for red cells to pass in single file, and they must deform to fit. Blood flows very slowly, about 0.3 mm/s, so each red cell spends about 1–2 s in a capillary. This is enough time for exchange. Rarely is any cell more than about 20–30 µm from a capillary.

Volume and area. Although capillaries are where exchange takes place, they hold only about 5% of the blood volume at any moment. Their combined cross-sectional area is about 1000 times that of the aorta.

Exchange mechanisms. Lipid-soluble substances such as O₂ and CO₂ diffuse straight through the endothelial cells. Small water-soluble solutes pass through the clefts between cells and through fenestrations. Proteins cross mainly in vesicles (transcytosis). Water moves by bulk flow, driven by Starling forces.

Parts & subdivisions

  1. Continuous capillaries
    The commonest type. They have an unbroken endothelium with intercellular clefts and pinocytotic vesicles. They are found in muscle, skin, lungs, connective tissue and the CNS. In the CNS, tight junctions together with astrocyte end-feet and pericytes form the blood–brain barrier.
  2. Fenestrated capillaries
    The endothelium has pores (fenestrae) about 60–80 nm across, usually spanned by a thin diaphragm. In the renal glomerulus the fenestrae have no diaphragm. They are found in the renal glomeruli and peritubular capillaries, the endocrine glands, the intestinal villi and the choroid plexus, all places of rapid fluid or solute exchange.
  3. Sinusoidal (discontinuous) capillaries
    Wide (up to 30–40 µm), irregular vessels with large gaps between the endothelial cells and an incomplete basal lamina. Whole cells and large proteins can pass through them. They are found in the liver, spleen, bone marrow and some endocrine glands.
  4. Pericytes
    Contractile cells embedded in the basal lamina. They regulate capillary diameter, stabilise the vessel and maintain the blood–brain barrier.

Function

  • Exchange O₂, CO₂, nutrients, hormones and wastes between blood and tissues.
  • Filter and reabsorb fluid, which determines how fluid is shared between plasma and interstitial space.
  • Form specialised barriers and filters (the blood–brain barrier and the glomerular filter).

Clinical significance

Oedema

Oedema develops when fluid filters out of capillaries faster than the lymphatics can remove it. This happens when capillary pressure rises, plasma oncotic pressure falls, capillaries become more permeable or lymph drainage is blocked.

Capillary refill time

Capillary refill time is measured by pressing on a fingertip and timing how long colour takes to return. A time longer than about 2 seconds suggests poor peripheral perfusion, as in shock or dehydration.

Diabetic microangiopathy

In diabetes, the basement membrane of capillaries thickens and pericytes are lost. This underlies diabetic retinopathy, with microaneurysms and new fragile vessels, and diabetic nephropathy.

Test yourself

The capillaries of the liver, spleen and bone marrow are classified as:
  1. Continuous
  2. Fenestrated
  3. Sinusoidal (discontinuous)
  4. Lymphatic

Answer: Sinusoidal (discontinuous)

Sinusoidal capillaries are wide and irregular, with large gaps between endothelial cells and an incomplete basal lamina. Large molecules and whole cells can pass through them. Fenestrated capillaries are found in the kidney, endocrine glands and gut villi. Continuous capillaries are found in muscle, skin, lung and the CNS.

More cardiovascular system questions

Compiled from standard anatomy and physiology references — see the Cardiovascular System sources. For education only; not medical advice.