Organization of the Intestinal and Colonic Microcirculation Anatomical and Functional Considerations

The microcirculation of the intestine consists of a branching network of arterioles, capillaries, and venules. Arterioles in the intestine are usually less than 500 mm in diameter wrapped by an external muscular coat consisting of two to four smooth muscle cells arranged circumferen-tially. The terminal (precapillary) arterioles have an internal diameter of 15 to 20 mm and are surrounded by only one layer of smooth muscle cells.

The majority of capillaries are derived from terminal arterioles and consist of a tube with an inner diameter of 4

Figure 1 Intestinal immune response to enteric antigens in the absence of regulatory cells. Effector CD4+ T-cells produce Thl-type cytokines in response to T-cell receptor engagement of antigens processed and presented by APCs. These cytokines (such as IFN-g) may affect the gut epithelium directly and/or activate resident Mf to release large amounts of proinflammatory mediators and cytokines as well as reactive metabolites of oxygen (ROS) and NO. The net result is the recruitment of additional leukocytes and subsequent inflammation and injury. APC, antigen presenting cell; Mf, macrophage; NO, nitric oxide; IFN-g, interferon-g. (see color insert)

Figure 1 Intestinal immune response to enteric antigens in the absence of regulatory cells. Effector CD4+ T-cells produce Thl-type cytokines in response to T-cell receptor engagement of antigens processed and presented by APCs. These cytokines (such as IFN-g) may affect the gut epithelium directly and/or activate resident Mf to release large amounts of proinflammatory mediators and cytokines as well as reactive metabolites of oxygen (ROS) and NO. The net result is the recruitment of additional leukocytes and subsequent inflammation and injury. APC, antigen presenting cell; Mf, macrophage; NO, nitric oxide; IFN-g, interferon-g. (see color insert)

to 10 mm and are lined by a single layer of endothelial cells and a thin basement membrane. In the gut, only a minority (e.g., 20-30%) of the capillaries are open to perfusion under normal resting conditions. The capillary network, with its large surface area and an endothelial barrier that is highly permeable to lipid-soluble and small water-soluble molecules, appears well suited for the exchange of gases, nutrients, and water between the bloodstream and tissues.

As with all tissues, capillaries drain into postcapillary venules that are also devoid of smooth muscle cells. These vessels represent the segment of the microvasculature that is most involved in inflammatory responses and contain intercellular endothelial junctions that can open to allow plasma proteins and circulating leukocytes to escape from the blood. The different patterns of microvascular anatomy are illustrated in the small and large intestine [3] (Figure 3). For example, the arteriolar supply from the submucosal arterial plexus passes directly to the villus tip of the small intestine, where this vessel then branches into a fountain-like formation of capillaries (Figure 3B). This dense plexus of sub-epithelial capillaries is drained by a single venule. The capillaries of the colonic mucosa, on the other hand, are arrayed in a honeycomb-like plexus or ring pattern in which each ring of capillaries surrounds the openings to the colonic crypts (Figure 3A).

Endothelial cells and smooth muscle cells represent the major functional elements of the blood vessel wall that allow arterioles, capillaries, and venules to carry out their functions. Although the two cell types are clearly capable of functioning independently, there are processes that enable one cell type to influence the other. Pericytes and mast cells are examples of such auxiliary cells that can exert a profound influence on the function of arterioles, capillaries, and/or venules.

Mast cells are also found closely apposed to the microvasculature, particularly the postcapillary venules. These cells are exquisitely sensitive to activation by a variety of stimuli, including neuropeptides (e.g., substance P), ROS, lipid mediators (PAF, LTB4), and bacterial peptides. Upon activation, mast cells release a number of substances that can influence the function of endothelial cells and vascular smooth muscle in all segments of the microvascu-lature. Mast cell-derived modulators of microvascular function include histamine, adenosine, nitric oxide, cytokines (e.g., tumor necrosis factor, interleukin-1), proteases (e.g., cathepsin G), and oxidants.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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