Digestion and Absorption

Learning Outcomes:

1. Understand the structure and function of the human digestive system.
2. Identify the major organs and glands involved in digestion.
3. Recognize the process of digestion and absorption of nutrients.
4. Learn about various disorders related to the digestive system.
5. Grasp the importance of essential nutrients in the diet.

Food is an essential need for all living organisms, and its main components—carbohydrates, proteins, and fats—must be digested into simpler forms to be absorbed by the body. The digestive system is responsible for the breakdown of these large molecules into absorbable nutrients through mechanical and biochemical methods.

Digestive System

The digestive system comprises the alimentary canal and associated digestive glands.

Alimentary Canal

The alimentary canal is a continuous tube that runs from the mouth to the anus. The key structures include:

1. Mouth and Buccal Cavity: The mouth is the entry point for food. Inside the buccal cavity, teeth help in chewing and breaking down food, and the tongue assists in mixing it with saliva. Humans have a heterodont dentition, consisting of incisors, canines, premolars, and molars. These teeth form a specific pattern called a dental formula, which for an adult is 2123/2123.
2. Pharynx and Oesophagus: The pharynx serves as a pathway for both food and air. The epiglottis prevents food from entering the respiratory tract. Food passes through the oesophagus by a process called peristalsis, which moves it toward the stomach.
3. Stomach: This is a J-shaped organ located in the upper left part of the abdominal cavity. It has four regions: cardiac, fundic, body, and pyloric. The pyloric sphincter regulates the movement of food into the small intestine.
4. Small Intestine: It has three parts: duodenum, jejunum, and ileum. The duodenum receives digestive juices from the liver and pancreas, while the jejunum and ileum are responsible for the absorption of nutrients.
5. Large Intestine: It consists of the caecum, colon, and rectum. The caecum contains the vermiform appendix, a vestigial structure. The colon is further divided into ascending, transverse, descending, and sigmoid regions. The rectum stores waste, which is expelled through the anus.

Note: The alimentary canal walls consist of four layers: serosa, muscularis, sub-mucosa, and mucosa. The mucosa in the small intestine forms villi, which increase the surface area for nutrient absorption.

Digestive Glands

There are three major digestive glands:

1. Salivary Glands: These glands produce saliva which contains salivary amylase. The three pairs of salivary glands are the parotids, sub-maxillary, and sub-linguals.
2. Liver: The liver is the largest gland in the body and produces bile, which helps in the digestion of fats. Bile is stored in the gallbladder and released into the duodenum.
3. Pancreas: The pancreas is both an endocrine and exocrine gland. It secretes pancreatic juice, which contains enzymes like trypsinogen, amylase, and lipase.

Digestion of Food

Digestion begins in the buccal cavity and continues through the stomach and small intestine. The process involves both mechanical and chemical actions:

1. In the Mouth: Salivary amylase breaks down starch into maltose. The bolus of food is swallowed and moves through the oesophagus to the stomach.
2. In the Stomach: Gastric glands in the stomach secrete gastric juice containing HCl and pepsinogen. Pepsinogen is activated into pepsin in the presence of HCl, which breaks proteins into smaller peptides. The stomach also secretes rennin, which is important for digesting milk in infants.
3. In the Small Intestine: Digestion is completed here. The pancreatic juice contains several enzymes that act on proteins, carbohydrates, and fats. Trypsinogen is converted into trypsin by enterokinase, which then activates other enzymes. Bile emulsifies fats, and intestinal juice (succus entericus) contains enzymes like maltase, lipase, and nucleotidase to complete digestion.
4. Enzymatic Actions:

• Proteins are broken down into dipeptides by trypsin and chymotrypsin.
• Carbohydrates are converted into disaccharides by pancreatic amylase.
• Fats are broken into diglycerides and monoglycerides by lipases.

Table: Major Enzymes and their Actions

Absorption of Digested Products

Absorption is the movement of digested nutrients from the intestine into the blood or lymph. This occurs by three mechanisms: passive diffusion, facilitated transport, and active transport.

1. Passive Diffusion: Simple molecules like glucose and amino acids move from a region of higher concentration in the lumen to a lower concentration in the blood.
2. Facilitated Transport: Carrier proteins assist the transport of substances like glucose and amino acids.
3. Active Transport: This requires energy (in the form of ATP) to move nutrients like Na+ against the concentration gradient.

Fats and Absorption: Fatty acids and glycerol, being insoluble in water, are first absorbed into micelles and then converted into chylomicrons inside the intestinal cells. These are then transported via lymph vessels into the bloodstream.

Sites of Absorption:

• Mouth: Some drugs are absorbed directly into the bloodstream.
• Stomach: Water, alcohol, and some medications are absorbed.
• Small Intestine: The majority of nutrient absorption occurs here, including glucose, amino acids, fatty acids, and glycerol.
• Large Intestine: Absorption of water, minerals, and some vitamins occurs, solidifying the waste for excretion.

Disorders of Digestive System

Several disorders can affect the digestive system:

1. Jaundice: Caused by liver malfunction, leading to a yellowing of the skin and eyes due to the accumulation of bile pigments.
2. Vomiting: A reflex action controlled by the vomit center in the medulla. It leads to the expulsion of stomach contents through the mouth.
3. Diarrhoea: Characterized by frequent, watery stools, it can result in dehydration and poor nutrient absorption.
4. Constipation: Irregular bowel movements lead to the retention of feces, often due to insufficient fiber or hydration.
5. Indigestion: Occurs when food is not properly digested, leading to discomfort and a feeling of fullness. Causes include overeating, stress, and improper enzyme secretion.

Important Note: Indigestion can be caused by conditions like GERD (Gastroesophageal Reflux Disease), where acid from the stomach backflows into the esophagus.

Protein-Energy Malnutrition (PEM)

PEM is common in underdeveloped regions and can lead to two major conditions:

1. Marasmus: A deficiency of both proteins and calories, affecting infants under one year. It leads to severe muscle wasting and stunted growth.
2. Kwashiorkor: Caused by protein deficiency without a lack of calories. It affects older children and results in muscle wasting and swelling (oedema) due to fluid retention.

Summary of Absorption

Absorption happens throughout the digestive tract, with the majority occurring in the small intestine.

Table: Absorption Sites and Substances Absorbed

           | Some drugs                            |

| Stomach | Water, alcohol, simple sugars |
| Small Intestine | Glucose, fatty acids, amino acids |
| Large Intestine | Water, minerals, some vitamins |

Note: Nutrients absorbed in the small intestine are used by tissues for metabolism, growth, and repair in a process called assimilation.

Calorific Value of Food

Food provides energy, which is measured in kilocalories (kcal) or kilojoules (kJ). The gross calorific value is the total energy released from 1g of food, while the physiological calorific value is the actual amount of energy the body can use. Carbohydrates and proteins provide 4 kcal/g, while fats provide 9 kcal/g.

MCQ:
What is the principal organ for nutrient absorption?
Answer: Small intestine