Unit 8: Nutrition in Human - Part 4 by Ms Hira Asif

Antagonistic Muscles

  • Definition: Antagonistic muscles are pairs of muscles that work against each other; when one muscle contracts, the other relaxes and vice versa.
  • Types:
    • Circular Muscles: Located in the inner wall of the gut.
    • Longitudinal Muscles: Located in the outer wall of the gut.

Peristalsis

  • Definition: Peristalsis refers to the rhythmic, wave-like contractions of the gut walls caused by the action of two antagonistic muscles (circular and longitudinal muscle fibers).
  • Functions:
    • Moves food along the gut.
    • Mixes food with digestive enzymes.

Mechanism of Peristalsis

  1. Contraction of Circular Muscles:

    • Action: Constricts the lumen (the inner opening of the organ) making it narrower and longer.
    • Effect: The diameter of the gut decreases, squeezing food forward from its original position.

  2. Contraction of Longitudinal Muscles:

    • Action: Dilates/expands the lumen making it wider and shorter.
    • Effect: The diameter of the gut increases, allowing food to enter the created lumen.

The Stomach

  • Sphincter Muscle: At the end of the esophagus, there is a sphincter muscle that opens and closes to:

    • Allow food into the stomach.
    • Prevent food from returning to the esophagus.

  • Stomach Lining: Known as mucosa, characterized by many folds forming millions of gastric glands.

Components of Gastric Juice

  • Mucous:

    • Produced by goblet or mucous cells of the mucosa.
    • Function: Prevents the stomach from digesting itself.

  • Pepsinogen:

    • Converted into pepsin by acid in the stomach.
    • Function: Converts protein into smaller peptides.
    • Description: Pepsin is a protease, an enzyme that digests protein.

  • Hydrochloric Acid (HCl):

    • Produced by parietal cells.
    • pH Level: Ranges from 1 to 2, which is highly acidic.

Functions of Hydrochloric Acid (HCl)

  1. Stops the action of salivary amylase by denaturing it.
  2. Activates the inactive enzymes pepsinogen and prorennin into their active forms (pepsin and rennin, respectively).
  3. Provides a slightly acidic medium suitable for gastric enzymes’ activities.
  4. Kills certain potentially harmful microorganisms in food.

Action of Pepsin and Rennin

  • Pepsin: Converts proteins into polypeptides.
  • Rennin:
    • Converts caseinogen (soluble protein) into casein (insoluble).
    • Function: This conversion is necessary as milk proteins need to remain long enough in the stomach to be digested by pepsin; otherwise, they would pass through the duodenum without being digested.

Stomach Dynamics

  • Gastric Gland Stimulation: The presence of food in the stomach stimulates gastric glands to secrete gastric juice into the stomach cavity.
  • Peristalsis in Stomach: Churns and breaks up food, mixing it well with gastric juice.
  • Duration of Stay: Food typically remains in the stomach for about three to four hours.
  • Transformation: The partly digested food becomes liquefied, forming a substance called chyme.