 Dear students, in this topic, we shall discuss the capillaries and micro-circulation through them. Capillaries are microscopic blood vessels which are about 1 mm long and about 3 to 10 micrometers in diameter. They have very thin and porous walls. Their walls are only a single layer thick that is made up of endothelial cells and is surrounded by a basement membrane made up of collagen and mucopolysaccharides. Capillaries walls are completely devide of connective tissue and smooth muscles. Dear students, in the tissues, small terminal arterioles sub-divide and form arterioles. These arterioles again sub-divide to form the meta-arterioles. Meta-arterioles subsequently sub-divide and form the network of capillaries which is called the capillary bed of the tissue. These capillary beds, they are also called micro-circulatory beds. Micro-circulatory beds infiltrate each tissue so that any tissue cell is not more than three or four cells away from a capillary. The blood flow in these capillaries is very close to the cells and if every cell is not approaching the capillary, it is 2 or 3 cells away from the capillary so that diffusion can operate in this region. Dear students, now we shall discuss the transfer of materials across the capillary wall. The endothelium of capillary wall is highly permeable. It allows the substances to move with relative ease in and out of the capillaries. The exchange of materials between the blood and tissue cells occurs across the capillary walls by diffusion. These in various tissues differ considerably in permeability. The permeability of all capillaries is not equal. There is variation in this and this variation depends on tissues. The differences in permeability are associated with the structure of endothelium wall of the capillary. The differences in permeability are associated with the structure of endothelium wall of the capillaries. Continuous, penestrated and sinusoidal capillaries. Continuous capillaries have a continuous basement membrane. They are least permeable capillaries. Their endothelium is about 0.2 to 0.4 micrometer thick. They are located in muscles, nervous tissue, lungs, connective tissues and exocrine glands. Prestated capillaries, they are perforated in some regions so they exhibit intermediate permeability. They are found in the renal glomerulus, intestines and endocrine glands. These capillaries are most permeable. In their basement membrane, large parasolular gaps are present. This type of capillaries is found in the liver, bone marrow, spleen, lymph nodes and adenyl cortex.