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Gastric activity involved in digestion is divided into three phases of digestion known as the cephalic phase, the gastric phase, and the intestinal phase. These phases overlap and all three can occur simultaneously. [1] A fourth phase of acid secretion is known as the basal state which occurs in the times between meals (interdigestive phase).
The first stage, the cephalic phase of digestion, begins with secretions from gastric glands in response to the sight and smell of food. This stage includes the mechanical breakdown of food by chewing , and the chemical breakdown by digestive enzymes , that takes place in the mouth .
Parasympathetic action helps in digestion and absorption of food by increasing the activity of the intestinal musculature, increasing gastric secretion, and relaxing the pyloric sphincter. It is called the “rest and digest” division of the ANS. [24] The parasympathetic nervous system decreases respiration and heart rate and increases digestion.
Occurring between meals, the migrating motor complex is a series of peristaltic wave cycles in distinct phases starting with relaxation, followed by an increasing level of activity to a peak level of peristaltic activity lasting for 5–15 minutes. [6] This cycle repeats every 1.5–2 hours but is interrupted by food ingestion.
Digestion is a form of catabolism that is often divided into two processes based on how food is broken down: mechanical and chemical digestion. The term mechanical digestion refers to the physical breakdown of large pieces of food into smaller pieces which can subsequently be accessed by digestive enzymes.
Exhalation (or expiration) is the flow of the breath out of an organism. In animals, it is the movement of air from the lungs out of the airways , to the external environment during breathing . This happens due to elastic properties of the lungs, as well as the internal intercostal muscles which lower the rib cage and decrease thoracic volume.
The gastric phase: About sixty percent of the total acid for a meal is secreted in this phase. Acid secretion is stimulated by distension of the stomach and by amino acids present in the food. The intestinal phase : The remaining 10% of acid is secreted when chyme enters the small intestine, and is stimulated by small intestine distension and ...
During the first phase, it requires the breakdown of two ATP molecules. [1] During the second phase, chemical energy from the intermediates is transferred into ATP and NADH. [ 2 ] The breakdown of one molecule of glucose results in two molecules of pyruvate, which can be further oxidized to access more energy in later processes.