The bulbar conjunctival microvasculature contains arterioles, meta-arterioles, venules, capillaries, and communicating vessels. They may be composed of abnormal aggregations of arterioles, capillaries or venules. The arterioles supply capillaries, which in turn empty into venules. Travelling through the arterioles blood moves into the capillaries where gas exchange can occur. The microvessels include terminal arterioles, metarterioles, capillaries, and venules. Its small cells form irregular cords and clusters, separated by capillaries and connective tissue. This process occurs in the pulmonary capillaries adjacent to the alveoli of the lungs. The increased viscosity of blood prevents fluid from entering the bloodstream from outside the capillaries. By lowering the pH levels in the venous capillaries, oxygen unbinds from blood hemoglobin. Histamine causes dilation and increased permeability of capillaries close to the basophil. Flukes migrate into smaller capillaries and bile ducts when feeding within the host. Substances are reabsorbed from the tubule into the peritubular capillaries. Mesangial cells are modified smooth muscle cells that lie between the capillaries. Lymph capillaries are slightly larger than their counterpart capillaries of the vascular system. It contains larger blood vessels that split into the capillaries in the pia mater.