tailieunhanh - Ebook Ganong's review of medical physiology (25th edition): Part 2

(BQ) Part 2 book "Ganong's review of medical physiology" presents the following contents: Gastrointestinal physiology, respiratory physiology, cardiovascular physiology, renal physiology. Invite you to consult. | SECTION IV Gastrointestinal Physiology For unicellular organisms that exist in a sea of nutrients it is possible to satisfy nutritional requirements simply with the activity of membrane transport proteins that permit the uptake of specific molecules into the cytosol. However for multicellular organisms including humans the challenges of delivering nutrients to appropriate sites in the body are significantly greater particularly if the organisms are terrestrial. Further most of the food we eat is in the form of macromolecules and even when these are digested to their component monomers most of the end products are water-soluble and do not readily cross cell membranes a notable exception are the constituents of dietary lipids . Thus the gastrointestinal system has evolved to permit nutrient acquisition and assimilation into the body while prohibiting the uptake of undesirable substances toxins and microbial products as well as microbes themselves . The latter situation is complicated by the fact that the intestine maintains a lifelong relationship with a rich microbial ecosystem residing in its lumen a relationship that is largely mutually beneficial if the microbes are excluded from the systemic compartment. The intestine is a continuous tube that extends from mouth to anus and is formally contiguous with the external environment. A single cell layer of columnar epithelial cells comprises the semipermeable barrier across which controlled uptake of nutrients takes place. Various glandular structures empty into the intestinal lumen at points along its length providing for digestion of food components signaling to distal segments and regulation of the microbiota. There are also important motility functions that move the intestinal contents and resulting waste products along the length of the gut and a rich innervation that regulates motility secretion and nutrient uptake in many cases in a manner that is independent of the central nervous system. There is also a large