Digestive Kotkapoora Fdk

Believe it or not, the digestive process starts even before you put food in your mouth. It begins when you smell something irresistible or when you see a favorite food you know will taste good. Just by smelling that homemade apple pie or thinking about how delicious that ice cream sundae is going to taste, you begin to salivate - and the digestive process kicks in, preparing for that first scrumptious bite. Every morsel of food we eat has to be broken down into nutrients that can be absorbed by the body, which is why it takes hours to fully digest food. In humans, protein must be broken down into amino acids, starches into simple sugars, and fats into fatty acids and glycerol. The water in our food and drink is also absorbed into the bloodstream to provide the body with the fluid it needs.

ESOPHAGUS AND STOMACH Foodstuffs entering the stomach have been, to at least some extent, crushed and reduced in size by mastication, and impregnated with saliva. The stomach provides four basic functions that assist in the early stages of digestion and prepare the ingest for further processing in the small intestine: 1. It serves as a short-term storage reservoir, allowing a rather large meal to be consumed quickly and dealt with over an extended period of time. 2. It is in the stomach that substantial enzymatic digestion is initiated, particularly of proteins. 3. Vigorous contractions of gastric smooth muscle mix and grind foodstuffs with gastric secretions, resulting in liquefaction of food, a prerequisite for delivery of the ingest to the small intestine. 4. As food is liquefied in the stomach, it is slowly released into the small intestine for further processing.

LIVER The liver is the largest gland in the body and performs an astonishingly large number of tasks that impact all body systems. One consequence of this complexity is that hepatic disease has widespread effects on virtually all other organ systems. At the risk of losing sight of the forest by focusing on the trees, we will focus on three fundamental roles of the liver: • • •

Vascular functions, including formation of lymph and the hepatic phagocytic system. Metabolic achievements in control of synthesis and utilization of carbohydrates, lipids and proteins. Secretory and excretory functions, particularly with respect to the synthesis of secretion of bile.

The latter is the only one of the three that directly affects digestion - the liver, through its biliary tract, secretes bile acids into the small intestine where they assume a critical role in the digestion and absorption of dietary lipids. However, understanding the vascular and metabolic functions of the liver is critical to appreciating the gland as a whole.

SMALL INTESTINE The small intestine is the portal for absorption of virtually all nutrients into blood. Accomplishing this transport entails breaking down large supramolecular aggregates into small molecules that can be transported across the epithelium. An exception to this statement is seen in herbivores, where large amounts of short chain fatty acids are absorbed at other sites. By the time ingesta reaches the small intestine, foodstuffs have been mechanically broken down and reduced to a liquid by mastication and grinding in the stomach. Once within the small intestine, these macromolecular aggregates are exposed to pancreatic enzymes and bile, which enables digestion to molecules capable or almost capable of being absorbed. The final stages of digestion occur on the surface of the small intestinal epithelium. The net effect of passage through the small intestine is absorption of most of the water and electrolytes (sodium, chloride, potassium) and essentially all dietary organic molecules (including glucose, amino acids and fatty acids). Through these activities, the small intestine not only provides nutrients to the body, but plays a critical role in water and acid-base balance.

LARGE INTESTINNE The large intestine is the last attraction in digestive tube and the location of the terminal phases of digestion. In comparison to other regions of the tube, there are huge differences among species in the relative size and complexity of the large intestine. Nonetheless, in all species it functions in three processes: • • •

Recovery of water and electrolytes from ingesta: By the time ingesta reaches the terminal ileum, roughly 90% of its water has been absorbed, but considerable water and electrolytes like sodium and chloride remain and must be recovered by absorption in the large gut. Formation and storage of feces: As ingesta is moved through the large intestine, it is dehydrated, mixed with bacteria and mucus, and formed into feces. The craftsmanship (for want of a better term) with which this is carried out varies among species. Microbial fermentation: The large intestine of all species teems with microbial life. Those microbes produce enzymes capable of digesting many of molecules that to vertebrates are indigestible, cellulose being a premier example. The extent and benefit of fermentation also varies greatly among species.

RECTUM The last 6 to 8 inches of the large intestine. The rectum stores solid waste until it leaves the body through the anus. The word rectum comes from the Latin rectus meaning straight (which the human rectum is not).