Subject - General Science
Chapter : The Digestive System

Digestive System

Animals, for the most part, ingest their food as large, complex molecules that must be broken down into smaller molecules (monomers) that can then be distributed throughout the body of every cell. This vital function is accpomplished by a series of specialized organs that comprise the digestive system.

Locations of digestive system in various animals:

• The digestive system uses mechanical and chemical methods to break food down into nutrient molecules that can be absorbed into the blood. Once in the blood, the food molecules are routed to every cell in the animal’s body.

• There are two types of animal body plans as well as two locations for digestion to occur.

• Sac-like plans are found in many invertebrates, who have a single opening for food intake and the discharge of wastes.

• Vertebrates, the animal group humans belong to, use the more efficient tube-within-a-tube plan with food entering through one opening (the mouth) and wastes leaving through another (the anus).

Stages in the Digestive Process

Food for the most part consists of various organic macromolecules such as starch, proteins, and fats. These molecules are polymers made of individual monomer units Breaking these large molecules into smaller components involves:

• movement: propels food through the digestive system

• secretion: release of digestive juices in response to a specific stimulus

• digestion: breakdown of food into molecular components small enough to cross the plasma membrane

• absorption: passage of the molecules into the body’s interior and their passage throughout the body

• elimination: removal of undigested food and wastes

Three processes occur during what we loosely refer to as “digestion”. Digestion proper, which is the mechanical and chemical breakdown of food into particles/molecules small enough to pass into the blood. Absorption is the passage of food monomers into the blood stream. Assimilation is the passage of the food molecules into body cells.

The Human Digestive System

The human digestive system, is a coiled, muscular tube (6-9 meters long when fully extended) stretching from the mouth to the anus. Several specialized compartments occur along this length: mouth, pharynx, esophagus, stomach, small intestine, large intestine, and anus. Accessory digestive organs are connected to the main system by a series of ducts: salivary glands, parts of the pancreas, and the liver and gall bladder (bilary system).

The Mouth and Pharynx

Mechanical breakdown begins in the mouth by chewing (teeth) and actions of the tongue. Chemical breakdown of starch by production of salivary amylase from the salivary glands. This mixture of food and saliva is then pushed into the pharynx and esophagus. The esophagus is a muscular tube whose muscular contractions (peristalsis) propel food to the stomach.

The Stomach

• During a meal, the stomach gradually fills to a capacity of 1 liter, from an empty capacity of 50-100 milliliters.

• Epithelial cells line inner surface of the stomach, and secrete about 2 liters of gastric juices per day.

• Gastric juice contains hydrochloric acid, pepsinogen, and mucus; ingredients important in digestion.

• The stomach secretes hydrochloric acid and pepsin. Hydrochloric acid (HCl) lowers pH of the stomach so pepsin is activated. Pepsin is an enzyme that controls the hydrolysis of proteins into peptides. The stomach also mechanically churns the food. Chyme, the mix of acid and food in the stomach, leaves the stomach and enters the small intestine.

The Small Intestine

• The small intestine, is where final digestion and absorption occur.

• In addition to digestive functions, the liver plays several other roles:
  (1) detoxification of blood;
  (2) synthesis of blood proteins;
  (3) destruction of old erythrocytes and conversion of hemoglobin into a component of bile;
  (4) production of bile;
  (5) storage of glucose as glycogen, and its release when blood sugar levels drop; and
  (6) production of urea from amino groups and ammonia.

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Gall Bladder

The gall bladder stores excess bile for release at a later time. We can live without our gall bladders, in fact many people have had theirs removed. The drawback, however, is a need to be aware of the amount of fats in the food they eat since the stored bile of the gall bladder is no longer available.

The Large Intestine

• The large intestine is made up by the colon, cecum, appendix, and rectum. Material in the large intestine is mostly indigestible residue and liquid.

• Movements are due to involuntary contractions that shuffle contents back and forth and propulsive contractions that move material through the large intestine.

• The large intestine performs three basic functions in vertebrates:
  (1) recovery of water and electrolytes from digested food;
  (2) formation and storage of feces; and
  (3) microbial fermentation

Regulation of Digestion

• The hypothalamus in the brain has two centers controlling hunger. One is the appetite center, the other the satiety center. Gastrin, secretin, and cholecystokinin are hormones that regulate various stages of digestion: The presence of protein in the stomach stimulates secretion of gastrin, which in turn will cause increased stomach acid secretion and mobility of the digestive tract to move food. 
• Food passing into the duodenum causes the production of secretin, which in turn promotes release of alkaline secretions from the pancreas, stops further passage of food into the intestine until the acid is neutralized. Cholecystokinin (CCK) is released from intestinal epithelium in response to fats, and causes the release of bile from the gall bladder and lipase (a fat digesting enzyme) from the pancreas.

Digestion in Animals Facts from NCERT

• Starfish feeds on animals covered by half shells of calcium carbonate. After opening the shell, the starfish pops out its stomach through its mouth to eat the soft animals inside the shell. The stomach the goes back into the body and the food is slowly digested.
• The saliva breakdown the starch into sugar.
• Liver situated in the upper part of the abdomen on the right side. It is the largest gland in the body.
• In the process of digestion carbohydrates get broken down into simple sugars such as glucose. Fats into fatty acid and glycerol. Proteins into amino acid.

Important Facts from Human Machine

• Carbohydrates provide energy to our body moreover fats also gives us energy. If we compare, fats give much more energy than the same amount of Carbohydrates.
• Foods Containing proteins are needed for the growth and repair of our body. Foods containing proteins are often called body building foods.
• Vitamin A keeps our skin and eyes healthy vitamin C helps body to fight against many diseases, vitamin D helps our body to use calcium for bones and teeth.
• Dietary fibres are known as roughage it is mainly provided by plant products in our foods. Whole grains and pulses, potatoes, fresh fruits and vegetable are main sources of roughage. It did not provide any nutrient to our body, but is an essential component of our food and adds to its bulk. This helps our body get rid of undigested food.

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