BIOLOGY Volume 1 - A Guide to General Biology - 2004

8. HETEROTROPHIC NUTRITION

8.3. The Human Alimentary Canal

8.3.6. The Stomach

In humans, The Stomach is located beneath the Diaphragm on the left side of the Abdominal cavity (Fig. 8.11). It is a muscular sac capable of expanding to accommodate food. In its unstretched state, the stomach walls form folds; when fully distended, the stomach can hold approximately 5 liters of food. The Main Functions of the stomach are listed below.

1. Following food intake, it is temporarily stored in the stomach, from which it is slowly released into other PARTS OF THE digestive tract.

2. The mechanical Digestion of food continues in the stomach. This is facilitated by the fact that, unlike the rest of the digestive tract, which contains only two layers of smooth Muscle, the stomach has three such layers: an outer longitudinal, a middle circular, and an inner oblique layer.

3. The thick mucous membrane contains epithelial Cells that secrete mucus. The mucus forms a barrier between the gastric mucosa and the gastric juice (see below), thereby preventing the stomach walls from digesting themselves.

4. The main part of the stomach is pitted with numerous gastric pits (Figs. 8.19 and 8.20). Long tubular glands that secrete gastric juice open into each pit. The gland cells are subdivided into parietal and chief cells.

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Fig. 8.19. Cross-section of a mammalian stomach wall showing gastric pits.

Fig. 8.20. Diagram of a Cytology/practical/54.html">Longitudinal section of the stomach wall in the region of a gastric gland. Cells secreting the hormone gastrin are also located in the gland wall.

Chief cells (also called zymogenic cells) secrete the inactive Enzymes pepsinogen and prorennin. (Inactive forms of enzymes are called zymogens.)

Parietal, or oxyntic, cells secrete a dilute solution of Hydrochloric acid, which performs several important functions. Due to hydrochloric acid, the pH of gastric juice is 1–2.5, which is ideal for the optimal activity of gastric enzymes. In addition, the acid kills many Bacteria, thus playing a protective role. Hydrochloric acid denatures many Proteins; their tertiary Structure is disrupted, and the molecules unwind, becoming more accessible to digestion. This is especially important for Fibrous proteins, such as Collagen, which are part of animal Connective Tissues. Furthermore, hydrochloric acid loosens the fibers and cellular components of tissues. It participates in The conversion of pepsinogen and prorennin into their active forms, Pepsin and rennin, and in the Hydrolysis of sucrose into glucose and fructose.

5. Pepsin breaks down proteins into shorter Polypeptides. In the presence of Calcium Ions, the action of rennin causes the coagulation of casein, a soluble milk protein, forming its insoluble calcium salt. In the form of this calcium salt, casein is digested by pepsin.

6. The stomach contains endocrine cells that secrete the hormone gastrin. This topic is discussed in Section 8.4.

8.3. Why is it necessary for pepsin to be secreted in an inactive form?

The cardiac sphincter, located at the junction of the Esophagus and the stomach, and the pyloric sphincter, situated between the stomach and the duodenum, prevent the uncontrolled release of food from the stomach. Both sphincters act as Valves, retaining food in the stomach for up to four hours. The regular relaxation of the pyloric sphincter results in the release of small amounts of food into the duodenum.

Due to the contractions of the Muscles in the stomach walls, food is thoroughly mixed with gastric juice, turning into a semi-liquid mass called chyme. From time to time, the ring-shaped pyloric sphincter opens, and a small amount of chyme is pushed out of the stomach into the duodenum.