All animals require continued and regular intake of food so that they can meet their bodily nutrient requirements. Briefly, the diversity of the food components includes the following: adequate caloric intake of carbohydrate, fat, and protein to provide the spectrum of substrates required for the generation of the biosynthesis or modification, and repair of tissue components, and also the metabolic energy needed for the generation of mechanical (muscle contraction) and electrical energy (nerve impulse). Also there must be an adequate intake of essential substances not capable of being biosynthesized by the animal in question. These include the essential amino acids, essential fatty acids, and all the vitamins. And finally there must be an adequate intake of macro and trace minerals. Thus, the term “food” is a general label to include the variety of chemical substances, including proteins, carbohydrates, fats, minerals, and vitamins, required to provide nourishment (i.e., maintenance of life).

However, the simple process of swallowing food does not imply that these food substances are actually available to sustain life. It is only after the swallowed and digested food has left the stomach and passed through the intestine and appeared in the blood or lymph that they can be said to be “in the body.” Food substances, such as minerals and free amino acids, can be absorbed with little or no change in their chemical form at the time they were eaten or drunk. Other food components (protein, carbohydrate or fat) must be subjected to extensive physical and chemical modification prior to their entry into the lymph or bloodstream. Thus, proteins, polysaccharides (both are macromolecules), fats (triglycerides), and phospholipids must all be degraded to their constituent components amino acids, mono- and disaccharides, free fatty acids, and glycerol, respectively, before they can be efficiently absorbed.

The overall physical and chemical properties of the various foodstuffs (e.g., water solubility, net charge), as well as the nature of their structural units and bonding (e.g., amide, glycoside, ester, etc.), pose diverse biochemical challenges to be resolved by the digestive system. The system of digestion, then, is the processing of the ingested bulk food into molecular forms capable of participating in the intestinal absorptive process resulting in the successful delivery to the circulatory system.

Table 1 summarizes the steps and processes associated with the digestive system. The digestive process involves the complex integration of voluntary and involuntary muscle contractions, parasympathetic and sympathetic neural actions, release of gastrointestinal and other hormones, and biosynthesis and release of a host of digestive enzymes (e.g., pepsin, trypsin, chymotrypsin, amylase) and digestive reagents (e.g., HCl and HCO−), along with digestive detergents (e.g., bile acids). These substances all collectively work together to process the food and present it to the intestinal absorptive process in an optimal form for absorption. The intestinal absorptive process depends upon the integrated functioning of the mouth, stomach, small and large intestine, and the pancreas, liver, and gallbladder.

As a consequence of evolutionary pressures, a number of hormonal systems have emerged to participate in the regulation of digestion and absorption of key dietary nutrients. These include the gastrointestinal hormones (this chapter), insulin and glucagon (see Chapter 6), and vitamin D3 and its steroid hormone form 1α,25(OH)2D3 (see Chapter 9). In addition, many other hormones are known to exert effects on either the digestion process or the gastrointestinal tract to modulate the absorption of various nutrients.

Many digestive processes including motility, absorption, and ion transport associated with secretion and also the gastrointestinal blood flow are influenced by the nervous system. Some of this control emanates from connections between the digestive system and the central nervous system. However, the digestive system is endowed with its own, local nervous system referred to as the intrinsic or enteric nervous system. The enteric nervous system is large and complex, with as many neurons as the entire spinal cord.

The enteric nervous system (ENS), along with the sympathetic and parasympathetic nervous systems, comprises the autonomic nervous system. The ENS governs the activities of the gastrointestinal system. The functioning of the ENS depends on ~100 million neurons. The principal components of the enteric nervous system are two networks, which are both incorporated in the wall of the digestive tract and extend from esophagus to anus: The myenteric plexus is a network of nerve fibers in the circular and longitudinal layers of the esophagus, stomach, and the entire intestine and governs the motility of the digestive tract. The submucous plexus, as its name implies, is located in the submucosa (see section II.C). Its principal role is in sensing the environment within the intestinal lumen, regulating GI blood flow, and controlling epithelial cell function.

اخر الاخبار

اخبار العتبة العباسية المقدسة

قسم الشؤون الدينية يطلق الدورة التطويرية والفقهية الخامسة لملاكات العتبة العباسية المقدسة

قسم الشؤون الفكرية ينظّم ختمة قرآنية رمضانية لطلبة العلوم الدينية الأفارقة في النجف

شعبة الخطابة تقيم مجلسا للوعظ والإرشاد في الصحن الشريف تزامناً مع حلول شهر رمضان المبارك

قسم التطوير يختتم دورة إعداد المدربين لملاكات مديرية تربية كربلاء

المزيد

الآخبار الصحية

مفاجأة.. تحسين القدرة على التحمل لا يعتمد العضلات فقط

المرتفعات والأكسجين.. دراسة "مبشرة" لمرضى السكري

علماء يبتكرون "آلية" تتوقع موعد ظهور أعراض الزهايمر

ماذا يحدث لجسمك عند التوقف عن تناول الخضراوات والفواكه؟

المزيد

الآخبار التكنلوجية

رصد "القرش النائم" في أحد أقسى الأماكن على وجه الأرض

أسماك أعماق البحر تكشف نظاما بصريا يتجاوز "المألوف"

زوكربيرغ أمام القضاء في قضية إدمان المراهقين

"علي بابا" تكشف عن نموذج جديد للذكاء الاصطناعي

المزيد

مواضيع ذات صلة

General Principles of Gastrointestinal Motility

امتصاص نواتج الهضم الميكروبي

الهضم الميكروبي

أمتصاص الاملاح والماء

امتصاص الفيتامينات

هضم وامتصاص الأحماض النووية

هضم وامتصاص الدهون

هضم وامتصاص البروتين

هضم وامتصاص السكريات

هضم المكونات الغذائية

معدة مواليد المجترات

النفاخ