Human Digestive System - SS1 Biology Lesson Note

The human digestive system (alimentary canal) is a complex series of organs and structures that work together to process food and absorb nutrients for the body's functioning. It consists of the following main components:

1.     Mouth: Digestion begins in the mouth, where food is mechanically broken down by chewing and mixed with saliva. Saliva contains enzymes, such as amylase, that initiate the digestion of carbohydrates.

2.     Pharynx and Esophagus: After being chewed and mixed with saliva, the food, now called a bolus, is swallowed. It passes through the pharynx (throat) and enters the esophagus, a muscular tube that carries the food to the stomach through peristaltic contractions.

3.     Stomach: The stomach is a muscular organ that further breaks down food through muscular contractions and mixing with gastric juices. Gastric juices contain hydrochloric acid, pepsin, and other enzymes that begin the digestion of proteins. The stomach also serves as a temporary storage site for food.

4.     Small Intestine: The small intestine is the primary site of nutrient absorption. It consists of three parts: the duodenum, jejunum, and ileum. Here, digestive enzymes from the pancreas and bile from the liver are secreted to help break down carbohydrates, proteins, and fats. The inner lining of the small intestine has numerous finger-like projections called villi, which increase the surface area for absorption.

5.     Pancreas: The pancreas produces digestive enzymes, including amylase, lipase, and proteases, which are released into the small intestine to further break down carbohydrates, fats, and proteins. The pancreas also secretes bicarbonate ions to neutralize the acidic chyme (partially digested food) that enters the small intestine.

6.     Liver and Gallbladder: The liver produces bile, a substance that helps in the digestion and absorption of fats. Bile is stored and concentrated in the gallbladder before being released into the small intestine to emulsify fats and aid in their breakdown.

7.     Large Intestine (Colon): The large intestine absorbs water, electrolytes, and some vitamins produced by gut bacteria. It also helps form and store feces. The remaining undigested food, bacteria, and waste products are moved through the large intestine and eliminated through the rectum and anus during defecation.

Figure 76 image from healthdirect.gov.au