Digestion/ Nausea

Digestion is the process of metabolism whereby a biological entity processes a substance, in order to chemically and mechanically convert the substance into nutrients. more...

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Overview

Digestion occurs at the multicellular, cellular, and sub-cellular levels, usually in animals. This process takes place in the digestive system, gastrointestinal tract, or alimentary canal.

Digestion is usually divided into mechanical manipulation and chemical action. In most vertebrates, digestion is a multi-stage process in the digestive system, following ingestion of the raw materials, most often other organisms. The process of ingestion usually involves some type of mechanical manipulation. Digestion is separated into five separate processes: 1) Ingestion: Placing food into the mouth, 2) Mechanical digestion: Mastication, the use of teeth to tear and crush food, and churning of the stomach. 3) Chemical digestion: Addition of chemicals (acid, bile, enzymes, and water) to break down complex molecules into simple structures, 4) Absorption: Movement of nutrients from the digestive system to the circulatory and lymphatic capillaries through osmosis, active transport, and diffusion, 5) Elimination: Removal of undigested materials from the digestive tract through defecation. Underlying the process is muscle movement throughout the system, deglutition and peristalsis.

Human digestion process

In humans, digestion begins in the oral cavity where food is chewed (mastication) with the teeth. The process stimulates exocrine glands in the mouth to release digestive enzymes such as salivary amylase, which aid in the breakdown of carbohydrates. Chewing (mechanical catabolism) also causes the release of saliva, which helps condense food into a bolus that can be easily passed through the esophagus. The esophagus is about 20 centimeters long. Saliva also begins the process of chemical catabolism, hydrolysis. Once food is chewed properly, the food is swallowed. The bolus is pushed down by the movement called peristalsis, which is an involuntary wave-like contraction of smooth muscle tissue, characteristic of the digestive system. The mechanism for swallowing is co-ordinated by the swallowing centre in the medulla oblongata and pons. The reflex is initiated by touch receptors in the pharynx as a bolus of food is pushed to the back of the mouth by the tongue. The uvula is a small flap that hangs from the roof of the mouth. During swallowing it and the soft palate retract upward and to the rear to close the nasopharynx, which prevents the food from entering the nasal passages by triggering closure of the soft palate. When swallowed, the food enters the pharynx, which makes special adaptations to prevent choking or aspiration when food is swallowed. The epiglottis is a cartilage structure that closes temporarily during swallowing, preventing food and liquids from entering the trachea.

Read more at Wikipedia.org