Capillary Structure + Types of Capillaries

Tissue Perfusion

Blood flow to a tissue through a capillary bed; Occurs in most tissues except cartilage, the sclera and cornea of the eye, and epithelial tissue

Capillary Walls

Capillary walls are about 0.2 μm in thickness with only endothelium rolled into a tube and a small amount of basal lamina secreted by the endothelial cells

Capillary Size

Measure about 50 μm in length and about 8 – 10 μm in diameter; Walls consists of one to three endothelial cells joined by tight junctions around the circumference

Pericytes

Contractile fibers around some capillaries that appear to control blood flow

Capillary Exchange

Movement of nutrients, gases, ions, and waste, between the blood in the capillary and the tissue cells; Three mechanisms:

Diffusion and osmosis through gaps and fenestrations

Many capillaries have small gaps between endothelial cells and some have Fenestrations, or pores, within their cells; Water and small solutes, such as amino acids, can move through these pores

Diffusion through the membranes of endothelial cells

Lipid-soluble substances including oxygen, carbon dioxide, and some lipids move easily through the cells

Transcytosis

Larger substances are taken into the cell by endocytosis and then leave the other side of the cell by exocytosis

Importance of Thin Capillary Walls

If capillaries contained thicker cells or multiple tissue layers, diffusion and transcytosis would occur too slowly to meet the body’s needs

Continuous Capillaries

Majority in the body; Located in muscles, skin, and most nervous and connective tissues; Least “leaky”with tight junctions; Most substances diffuse through the endothelial cells or move by transcytosis; Modified continuous capillaries in the brain form the blood brain barrier and have specialized tight junctions

Fenestrated Capillaries

Endothelial cells contain fenestrations; Located in endocrine glands, small intestine, and kidneys; Much leakier than continuous capillaries; Pores allow for faster diffusion

Sinusoidal Capillaries (Sinusoids)

Endothelial cells are a discontinuous sheet with an irregular basal lamina and very large pores; Three to four times larger in diameter than other capillaries; Located in liver, spleen, lymphoid organs, and bone marrow; Transfer large substances such as blood cells and larger proteins