Digestion: A Brief Overview

A Brief overview of Digestion

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There are two different types of digestion.

Mechanical digestion is the process of breaking down food into smaller components.  Examples of this would be chewing by the mouth, or the churning of the stomach and small intestine. Peristalsis, the wavelike muscular contraction of the entire digestive track would also be considered a type of digestion.

Chemical Digestion-is the breakdown of food at the molecular level. This would be breaking starch into its component simple sugar parts.

In the mouth, both mechanical and chemical digestion occur. Saliva is secreted from the salivary glands after the sight, smell, or presence of food. It contains lubricants, enzymes, and other substance. Teeth serve to breakdown food components, as well as the tongue.

The pharynx is a part of both the digestive and respiratory systems. The epiglottis blocks food from entering the trachea and lungs during swallowing. If food enters the airway, it can be removed by coughing or the Heimlich maneuver.

When food leaves the mouth, it gets a new name and is called the “Bolus” as it travels down to the stomach. As the food enters the stomach and is mixed with the secretions of the stomach the food is considered chyme.

The esophagus connects the pharynx to the stomach. Here, the bolus of food is moved by rhythmic muscle contractions called peristalsis. Food passes through a muscular ring called a spinchter at the end of the esophagus to enter the stomach. The lower esophageal sphincter prevents stomach acid from entering the esophagus

The stomach provides temporary storage for food. The stomach lining produces gastric juice containing water, pepsin (activated into a digestive enzyme), and hydrochloric acid (kills microorganisms, unfolds proteins, activates pepsin from pepsinogen, and inhibits amylase). Mucus protects stomach lining from pepsin and hydrochloric acid. The muscular stomach lining helps mix food. There is a little absorption in the stomach including water, alcohol, aspirin, and acetaminophen. Chyme leaves the stomach to enter the small intestine through a sphincter.

Stomach emptying is regulated by signals from the small intestines and is affected by meal size and composition.

The small intestine is the main site for absorption and digestion. Chyme moved by peristalsis and mixed by rhythimic constrictions called segmentation. At 20 feet long and a large surface area enchanced by folds, villi, and microvilli (collectively called the brush border), the small intestine provides the main area of absorption. The lumen is the name of the area inside the small intestine. Each villi has a capillary for blood, and a lacteal for lymph fluid. Undigested substances pass from the small intestines to the large intestines through a spinchter.

Pancreas secretes pancreatic juice containing bicarbonate (which neutralizes stomach acid), pancreatic amylase (which breaks carbohydrates into sugars), pancreatic protease (which breaks down proteins into amino acids), and pancreatic lipase (which breaks down fats into fatty acids). The brush border secretes digestive enzymes. The liver secretes bile which emulsifies fat so lipases can access molecules. The gall bladder stores bile until fat enters the stomach.

Carbohydrates and proteins digested in the small intestine go directly into the blood circulation, whereas lipid portions go into the lymph which is eventually emptied into the left subclavian vein.

How do substances enter our body? Diffusion is the movement of substances from areas of higher concentration to areas of lower concentration. Simple diffusion is diffusion across membranes. Simple diffusion is diffusion across membranes. Osmosis is the diffusion of water across a membrane. Facilitated diffusion is diffusion through a membrane protein. Active transport is the movement of substances across membranes against concentration gradients; requires energy input (ATP) and a membrane protein.

After the small intestine, chyme enters the large intestine, otherwise known as the colon. The colon is 5 feet in length, and is primarily a storage compartment. In the large intestine, water, some vitamins, and some minerals are absorbed. Bacteria digest some nutrients (such as fiber that could not be digested in the small intestine) and turns these nutrients into vitamin K and some B vitamins. The rectum is the last 8 inches of the large intestine and stores feces. Feces is undigested, unabsorbed food that contains bacteria.

Probiotics are beneficial bacteria contained in some food, and prebiotics are food supplies designed to feed the beneficial bacteria that is in the gut.


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