Structures of Human

Physical Digestion

- is the breakdown of food by physical means.

- physical digestion starts when we use our knife and fork to break down large substances.

- our teeth take over to grind food into small particles which can be swallowed.

- physical digestion also includes peristalsis which helps move food down the digestive tract and the muscular churning of food within the stomach which helps mix food substances with digestive juices and acid.
 

Chemical Digestion

        Why is chemical digestion needed?

- chewing, churning, and mixing of food with digestive juices can only separate molecules from each other. These actions cannot split up molecules. Many nutrient molecules are too large to pass through cell membranes (i.e. they cannot be absorbed).

- molecules of water, vitamins, and minerals are small enough to be absorbed through the intestinal wall and into the blood stream.

- proteins, carbohydrates, and fat molecules are too large and must be broken down further by chemical means. This is chemical digestion.
 

Chemical Digestion of proteins, carbohydrates, and fats is needed for 3 reasons:
 

                (Most chemical digestion occurs by hydrolysis as seen earlier.)

Dissolving, Digestion, and Lubrication

Process and Processes of Digestion

             Saliva
             Swallowing
             Peristalsis
             Pancreatic Juice
             Bile
             Intestinal Juice
            Absorption

Saliva 1.) Mucous glands in the inner lining of your cheeks produce mucin. 2.) Salivary ducts under the tongue lead to the salivary glands below the mouth. These glands produce water, mucin, and the enzyme amylase. Opposite each second upper molar tooth is the entrance to a duct from the other salivary glands. These two glands are located on each side of your face, in front of the ears. They produce water and amylase. (swelling of these glands is an indication of a viral infection called mumps) - Water helps moisten and soften food and to dissolve any soluble nutrients Swallowing - The initial stages of swallowing are under voluntary control. - Pressure between the tip of the tongue and the hard palate (roof of the mouth) squeezes the bolus to the back of the mouth towards the Pharynx, or throat.

- There is no hard palate in the rear of the mouth. However the tissue forming the roof of the mouth continues. This is called the soft palate. As the tongue presses the bolus up, the soft palate is pushed upward by the bolus. This closes the air passageway from the nasal cavity.

- The uvula, the fold of skin hanging down from the soft palate, helps cover the nasal passageway. (this prevents food from being pushed up the nasal passageway).

- Once the bolus reaches the back of the throat, you lose voluntary control of it. A swallowing reflex has started. The food now enters the esophagus.

Peristalsis
- is the scientific name for the coordinated contractions and relaxation's of the esophagus. Peristalsis also occurs in the intestines.

(this helps explain why someone can drink a glass of water while standing on his/her head)

Pancreatic Juice
  - When the acidic chyme from the stomach inters the small intestine, it stimulates cells in the intestinal lining to secrete two hormones into the blood.

1.  secretin       2. cholecystokinin

These hormones stimulate the pancreas to secrete pancreatic juice and pancreatic enzymes which pass through the pancreatic duct and into the upper part of the small intestine.

- Pancreatic juice contains sodium bicarbonate which neutralizes the acid in the chyme and makes it slightly alkaline (pH 8). Pancreatic enzymes work on all the major food components - proteins, carbohydrates, fats and nucleic acids.

Pancreatic enzymes include: amylase, which hydrolyzes any remaining starch to maltose; protease's (protein splitting enzymes) including trypsin and chymotripsin, which continue the break down of large proteins; and lipase, which breaks down fats.
 

Bile
  - produced in the liver and passes through ducts into the gallbladder, where it is stored. It passes into the upper part of the small intestine through the bile duct. Release of bile is stimulated by the hormone cholecystokinin, which also acts on the pancreas.

- Bile doesn't contain enzymes, it emulsifies fats (causes fats to form small droplets increasing surface area for enzyme action) Bile is alkaline, aiding in the neutralization of chyme.
 

Intestinal Juice
  - Intestinal juice contains enzymes that complete the digestion of carbohydrates, fats, and proteins.
Absorption
  - Simple sugars, amino acids, vitamins, minerals, and other substances are absorbed through the wall of the small intestine into the blood vessels of the circulatory system. Fatty acids and glycerol are absorbed into tiny vessels of the lymphatic system called lacteals. Structural features of the small intestine which promote absorption:

1. the small intestine is very long

2. its lining has many folds

3. the lining is covered with millions of finger like projections called villi.

4. The epithelial cells that make up the lining of the intestine, have brush borders which. In the brush borders, the ends of the cells that face into the intestine have microvilli which further increases the surface area.
 


 


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