Digestion and Absorption: Human Alimentary Canal and Digestive Glands

Digestion is the process of breaking down complex food substances into simpler, absorbable forms that the body can utilize for energy, growth, and repair. This complex process involves both mechanical and chemical breakdown of food, facilitated by the human alimentary canal and associated digestive glands.

The Human Alimentary Canal

The human alimentary canal, also known as the digestive tract, is a continuous, coiled tube extending from the mouth to the anus. It is approximately 8-10 meters long in adults. The primary function of the alimentary canal is to ingest, digest, absorb food, and eliminate indigestible waste.

1. Mouth (Oral Cavity)

The mouth is the initial part of the alimentary canal. It serves for ingestion of food and plays a role in both mechanical and chemical digestion.

• Ingestion: Taking food into the body.
• Mechanical Digestion: This is achieved through mastication (chewing) by the teeth and mixing of food with saliva by the tongue. The teeth cut, tear, and grind food into smaller pieces, increasing the surface area for enzyme action. The tongue manipulates the food, mixing it with saliva and forming a bolus for swallowing.
• Chemical Digestion: Saliva, secreted by salivary glands, contains enzymes and mucus. The enzyme salivary amylase (ptyalin) begins the hydrolysis of starch (a complex carbohydrate) into simpler sugars like maltose. Mucus lubricates the food, making it easier to swallow.

The tongue is a muscular organ that aids in tasting, chewing, swallowing, and speech. It has taste buds that detect different tastes.

2. Pharynx

The pharynx is a part of both the respiratory and digestive systems. After the bolus is formed in the mouth, the tongue pushes it into the pharynx. From here, food passes into the esophagus. The epiglottis, a cartilaginous flap, covers the glottis (opening of the larynx) during swallowing to prevent food from entering the respiratory tract.

3. Esophagus

The esophagus is a muscular tube, about 25 cm long, that connects the pharynx to the stomach. It transports the food bolus from the pharynx to the stomach through a process called peristalsis. Peristalsis involves rhythmic, wave-like muscular contractions of the esophageal wall that propel the food downwards. No significant digestion occurs in the esophagus.

4. Stomach

The stomach is a J-shaped, muscular organ located in the upper abdomen. It acts as a temporary storage site for food and is a major site for both mechanical and chemical digestion. The stomach has three main parts: the cardiac part (near the esophagus), the fundus (dome-shaped region), and the pyloric part (leading to the small intestine).

• Mechanical Digestion: The muscular walls of the stomach churn and mix the food with gastric juices, further breaking it down mechanically.
• Chemical Digestion: The stomach lining secretes gastric juice, which contains hydrochloric acid (HCl) and enzymes like pepsin.
  • Hydrochloric Acid (HCl): Provides an acidic medium (pH 1.5-3.5) necessary for the action of pepsin and also kills most bacteria and other pathogens that enter the stomach with food.
  • Pepsin: A protease that breaks down proteins into smaller polypeptides. Pepsin is secreted in an inactive form called pepsinogen, which is activated by HCl.
  • Mucus: Protects the stomach lining from the corrosive action of HCl and pepsin.
  • Intrinsic Factor: Essential for the absorption of vitamin B12 in the small intestine.

The partially digested food mixture in the stomach is called chyme. The chyme is slowly released into the small intestine through the pyloric sphincter.

5. Small Intestine

The small intestine is the principal site for the completion of digestion and absorption of nutrients. It is a long, coiled tube (about 6.5 meters in adults) divided into three parts: the duodenum, the jejunum, and the ileum.

• Duodenum: The first and shortest part, about 25 cm long. It receives chyme from the stomach and digestive juices from the pancreas, liver, and its own walls.
• Jejunum: The middle section, about 2.5 meters long.
• Ileum: The longest section, about 3.5 meters long. It connects to the large intestine at the ileocecal valve.

The inner lining of the small intestine has numerous folds called plicae circulares, and projections called villi and microvilli. These structures greatly increase the surface area for absorption. Each villus contains a network of capillaries and a lymphatic vessel called a lacteal.

Digestion in the Small Intestine:

• Carbohydrate Digestion: Pancreatic amylase continues the breakdown of starch into disaccharides. Enzymes present in the brush border of the intestinal wall (e.g., maltase, sucrase, lactase) break down disaccharides into monosaccharides (glucose, fructose, galactose).
• Protein Digestion: Pancreatic proteases like trypsin and chymotrypsin (secreted as inactive trypsinogen and chymotrypsinogen) break down polypeptides into smaller peptides. Intestinal peptidases further break down peptides into amino acids.
• Fat Digestion: Bile salts from the liver emulsify fats, breaking large fat globules into smaller ones, increasing the surface area for enzyme action. Pancreatic lipase then breaks down triglycerides into fatty acids and glycerol (or monoglycerides).
• Nucleic Acid Digestion: Pancreatic nucleases break down DNA and RNA into nucleotides. Intestinal nucleotidases and phosphatases break down nucleotides into nucleosides and then into nitrogenous bases, pentose sugars, and phosphates.

Absorption in the Small Intestine:

• Monosaccharides, Amino Acids, Water-Soluble Vitamins, Minerals, and Water: Absorbed into the blood capillaries of the villi and transported to the liver via the portal vein.
• Fatty Acids and Glycerol: Re-esterified into triglycerides within the epithelial cells of the villi, then packaged into chylomicrons, which enter the lacteals (lymphatic vessels) and eventually reach the bloodstream.
• Fat-Soluble Vitamins (A, D, E, K): Absorbed along with fats.

6. Large Intestine

The large intestine is shorter (about 1.5 meters) but wider than the small intestine. It consists of the cecum, colon, rectum, and anal canal.

• Cecum: A small, pouch-like structure at the junction of the small and large intestines. The appendix, a finger-like outgrowth, is attached to the cecum.
• Colon: Divided into ascending, transverse, descending, and sigmoid colon. Its main function is to absorb water and electrolytes from the remaining indigestible food matter and to form feces.
• Rectum: Stores feces before defecation.
• Anal Canal: The terminal part, opening to the exterior through the anus.

The large intestine houses a large population of bacteria, including Escherichia coli, which synthesize certain vitamins (like Vitamin K and some B vitamins). No significant digestive enzymes are secreted here. Absorption is mainly limited to water, electrolytes, and some vitamins.

7. Anus

The anus is the external opening of the alimentary canal, through which undigested and unabsorbed waste material (feces) is eliminated from the body by defecation.

Digestive Glands

Several glands associated with the alimentary canal secrete digestive juices containing enzymes and other substances essential for digestion.

1. Salivary Glands

There are three pairs of major salivary glands:

• Parotid Glands: Located below and in front of the ears. They are the largest.
• Submandibular Glands: Located below the mandible (lower jaw).
• Sublingual Glands: Located below the tongue.

These glands secrete saliva, which contains water, electrolytes, mucus, and enzymes like salivary amylase and lingual lipase. Saliva moistens food, aids in bolus formation, begins carbohydrate digestion (salivary amylase), and has antibacterial properties.

2. Gastric Glands

Located in the lining of the stomach, these glands secrete gastric juice. The gastric glands contain different types of cells:

• Mucus Neck Cells: Secrete alkaline mucus that protects the stomach lining.
• Chief Cells (Zymogenic Cells): Secrete pepsinogen and gastric lipase.
• Parietal Cells (Oxyntic Cells): Secrete hydrochloric acid (HCl) and intrinsic factor.

3. Pancreas

The pancreas is a gland organ in the upper abdomen, situated behind the stomach. It is both an exocrine and endocrine gland. The exocrine part secretes pancreatic juice into the duodenum through the pancreatic duct.

Pancreatic Juice:

• Contains inactive proteases: trypsinogen, chymotrypsinogen, procarboxypeptidase. Trypsinogen is activated by an enzyme called enterokinase (secreted by the intestinal mucosa) into trypsin, which then activates the other proteases. These enzymes digest proteins.
• Contains pancreatic amylase, which digests carbohydrates.
• Contains pancreatic lipase, which digests fats.
• Contains nucleases, which digest nucleic acids.
• Is alkaline due to the presence of bicarbonate ions, which neutralize the acidic chyme entering the duodenum from the stomach, creating an optimal pH for intestinal enzymes.

4. Liver

The liver is the largest gland in the body, located in the upper right quadrant of the abdomen. It produces bile, which aids in fat digestion.

• Bile Production: Bile is a yellowish-green fluid secreted by hepatic cells. It contains bile salts, bile pigments (bilirubin and biliverdin), cholesterol, phospholipids, and electrolytes.
• Bile Function: Bile does not contain digestive enzymes but aids in digestion by:
  • Emulsification of Fats: Bile salts break down large fat globules into smaller ones, increasing the surface area for lipase action.
  • Absorption of Fats and Fat-Soluble Vitamins: Bile salts help in the formation of micelles, which facilitate the absorption of fatty acids, monoglycerides, and fat-soluble vitamins across the intestinal epithelium.
• Bile is stored and concentrated in the gallbladder before being released into the duodenum.

5. Intestinal Glands

The wall of the small intestine contains glands that secrete intestinal juice (succus entericus).

• Brunner's Glands: Located in the submucosa of the duodenum, they secrete an alkaline, mucus-rich fluid that neutralizes stomach acid and lubricates the intestinal wall.
• Crypts of Lieberkühn: Found between the villi, these glands secrete intestinal juice and contain goblet cells that secrete mucus. They also contain Paneth cells that secrete antimicrobial substances and enzymes.

Intestinal Juice:

• Contains enzymes like maltase, sucrase, lactase (disaccharidases), peptidases (aminopeptidases, dipeptidases), and intestinal lipase.
• These enzymes complete the digestion of carbohydrates, proteins, and fats into absorbable units (monosaccharides, amino acids, fatty acids, and glycerol).

Regulation of Digestive Processes

Digestion is a highly regulated process involving both neural and hormonal mechanisms.

1. Neural Regulation

The autonomic nervous system (sympathetic and parasympathetic) controls the motility and secretions of the digestive tract. The parasympathetic nervous system generally stimulates digestion, while the sympathetic system inhibits it.

• Cephalic Phase: Stimulated by the sight, smell, taste, or thought of food. The vagus nerve stimulates gastric secretion and motility even before food enters the stomach.
• Gastric Phase: Initiated by the presence of food in the stomach. Distension of the stomach wall and the presence of partially digested proteins stimulate the secretion of gastric juice and increase stomach motility.
• Intestinal Phase: Triggered by the entry of chyme into the small intestine. This phase involves both stimulation and inhibition of gastric activity and the regulation of intestinal secretions and motility. Hormones play a significant role here.

2. Hormonal Regulation

Several hormones produced by the digestive tract regulate digestive functions:

• Gastrin: Secreted by G cells in the stomach lining. Stimulates the secretion of HCl and pepsinogen, and increases stomach motility.
• Secretin: Released by the duodenum in response to acidic chyme. Stimulates the pancreas to secrete bicarbonate-rich pancreatic juice and inhibits gastric acid secretion.
• Cholecystokinin (CCK): Released by the duodenum and jejunum in response to fats and proteins. Stimulates the gallbladder to contract (releasing bile) and the pancreas to secrete enzyme-rich pancreatic juice. It also slows gastric emptying.
• Gastric Inhibitory Peptide (GIP): Released by the small intestine in response to glucose, fatty acids, and amino acids. Inhibits gastric acid secretion and motility, and stimulates insulin release.
• Vasoactive Intestinal Peptide (VIP): Relaxes intestinal smooth muscle and stimulates intestinal secretions.