Describe the functions of the organs of the digestive system in relation to: (a) ingestion – the taking of substances, e.g. food and drink, into the body (b) digestion – the breakdown of food (c) absorption – the movement of nutrients from the intestines into the blood (d) assimilation – uptake and use of nutrients by cells (e) egestion – the removal of undigested food from the body as faeces

The digestive system is responsible for the ingestion, digestion, absorption, assimilation, and egestion of food. Each of these processes is facilitated by specific organs that work together to ensure the body receives the nutrients it needs. Here’s a detailed description of each function in relation to the organs involved:

(a) Ingestion

• Definition: Ingestion is the process of taking in food and drink into the body.
• Organs Involved:
  • Mouth: The process begins in the mouth, where food is chewed (mastication) by teeth, breaking it into smaller pieces to increase surface area. Salivary glands secrete saliva, which moistens and lubricates the food, making it easier to swallow. Saliva contains enzymes like amylase that initiate the digestion of carbohydrates.

(b) Digestion

• Definition: Digestion involves breaking down food into smaller, absorbable components through mechanical and chemical processes.
• Organs Involved:
  • Oesophagus: After swallowing, food is pushed down the oesophagus through peristalsis, a series of muscular contractions.
  • Stomach: In the stomach, food is mixed with gastric juices (containing hydrochloric acid and digestive enzymes) that break down proteins and further liquefy the food into a semi-liquid substance called chyme.
  • Small Intestine: The duodenum (the first part of the small intestine) receives chyme along with bile from the liver and digestive enzymes from the pancreas. These substances aid in the continued breakdown of fats, proteins, and carbohydrates.

(c) Absorption

• Definition: Absorption is the movement of nutrients from the intestines into the bloodstream.
• Organs Involved:
  • Small Intestine: Most nutrient absorption occurs here. The walls of the small intestine are lined with villi and microvilli, which increase surface area for absorption. Nutrients such as amino acids, simple sugars, fatty acids, vitamins, and minerals are absorbed through the intestinal lining into the blood or lymphatic system.

(d) Assimilation

• Definition: Assimilation is the uptake and utilization of nutrients by cells for energy, growth, and repair.
• Organs Involved:
  • While assimilation primarily occurs at the cellular level throughout the body, organs such as the liver play a crucial role in processing absorbed nutrients. The liver converts glucose into glycogen for storage, synthesizes proteins from amino acids, and regulates nutrient distribution to various tissues.

(e) Egestion

• Definition: Egestion is the removal of undigested food from the body as feces.
• Organs Involved:
  • Large Intestine (Colon): The remaining indigestible material enters the large intestine, where water and electrolytes are reabsorbed. The remaining waste is compacted into feces.
  • Rectum: Feces are stored in the rectum until they are expelled from the body.
  • Anus: The anus is the opening through which feces are excreted during defecation.