Digestion mechanically and chemically breaks down foods and absorbs them.
Summary
Digestion mechanically and chemically breaks down foods and
absorbs them. The digestive system consists of an alimentary canal and several
accessory organs. Regions of the alimentary canal perform specific functions.
It consists of four major layers: mucosa, submu-cosa, muscular layer, and
serosa. Seven essential steps make up the functions of the digestive system:
inges-tion, propulsion, mechanical processing, digestion, secretion,
absorption, and excretion. The two basic types of motor functions in the
alimentary canal are mixing and propelling movements.
The mouth receives food and begins diges-tion. The tongue
mixes food particles with saliva during chewing. Salivary glands secrete
saliva, which moistens food, helps bind food particles, begins chem-ical
digestion of carbohydrates, makes taste possible, and helps clean the mouth.
Humans have a primary and a secondary set of teeth that form during their lifetimes.
The dentin that makes up a tooth is harder than bone. The pharynx and esophagus
are important passageways, allowing food, liquids, and air to pass.
The stomach receives food, mixes it with gastric juice,
carries on a limited amount of absorption, and moves food into the small
intestine. Pepsin is the most important protein-digesting enzyme produced by
the gastric mucosa. The small intestine extends from the pyloric sphincter to
the large intestine and is the lon-gest portion of the alimentary canal. It
plays the major role in the digestion and absorption of nutrients. The pancreas
produces pancreatic juice with enzymes that can split carbohydrates, fats,
nucleic acids, and pro-teins. The liver metabolizes these substances, storing
some of them, filters the blood, destroys toxins, and secretes bile. The small
intestine receives secretions from the pancreas and liver, completes nutrient
digestion, absorbs the products of digestion, and transports the residues to
the large intestine. The gallbladder is mainly a storage organ for bile, which
is a fat emulsifier. Bilirubin is a waste product of the heme of hemoglobin.
The large intestine reabsorbs water and electrolytes and forms and stores
feces.
Nutrition requires carbohydrates, lipids, and pro-teins in
large amounts. Carbohydrates are organic compounds, and include sugars and
starches. Lipids include fats, fat-like substances, and oils. Proteins are
created from amino acids and include enzymes, plasma proteins, muscle
components, hormones, and antibodies. Vitamins are organic compounds required
for normal metabolism, and include fat-soluble and water-soluble forms.
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