Digestion notes

MDCAT Biology

Digestion is the mechanical and chemical breakdown of complex food into simple, soluble substances that can be absorbed and used by the body. The human digestive system includes the alimentary canal and associated glands, which work together in ingestion, digestion, absorption, assimilation and egestion.

Human Digestive System and Its Functions

The human digestive system consists of a continuous muscular tube called the alimentary canal and several associated glands. The alimentary canal extends from the mouth to the anus. Its main parts are oral cavity, pharynx, oesophagus, stomach, small intestine, large intestine, rectum and anus.

Digestion changes large, insoluble food molecules into small, soluble molecules. These molecules are absorbed into the blood or lymph and then assimilated by body cells. Undigested material is removed from the body as faeces.

The main processes of nutrition are ingestion, digestion, absorption, assimilation and egestion.
Ingestion is the taking of food into the oral cavity.
Digestion is the mechanical and chemical breakdown of food.
Absorption is the movement of digested food into blood or lymph.
Assimilation is the use of absorbed nutrients by body cells.
Egestion is the removal of undigested food through the anus.
The alimentary canal includes the oral cavity, pharynx, oesophagus, stomach, small intestine, large intestine, rectum and anus.

Oral Cavity, Teeth and Swallowing

Digestion begins in the oral cavity. Teeth cut, tear and grind food. This mechanical process is called mastication. The tongue mixes food with saliva, forms it into a soft bolus and pushes it towards the pharynx.

An adult human normally has 32 permanent teeth. Humans are heterodont because their teeth are of different types and have different shapes and functions. Saliva moistens food and begins the digestion of starch.

Mastication is the mechanical breakdown of food by teeth.
The four types of permanent teeth are incisors, canines, premolars and molars.
Incisors cut food, canines tear food, premolars crush food, and molars grind food.
Humans are heterodont because they possess different types of teeth.
Salivary glands secrete saliva into the oral cavity.
Salivary amylase, also called ptyalin, begins the digestion of starch.
Starch is converted mainly into maltose by salivary amylase.
The optimum pH for salivary amylase is about 6.8.
Sodium bicarbonate in saliva helps stabilize the pH of food in the oral cavity.

Pharynx, Epiglottis and Oesophagus

After swallowing, the bolus passes from the oral cavity into the pharynx and then into the oesophagus. The oesophagus is a muscular tube that carries food to the stomach by rhythmic muscular contractions called peristalsis.

The epiglottis is a flap of cartilage at the entrance of the larynx. During swallowing, it covers the opening of the respiratory passage and directs food towards the oesophagus. The cardiac sphincter is present at the junction of the oesophagus and stomach.

The pharynx is a common passage for food and air.
The epiglottis prevents food from entering the trachea during swallowing.
Food is directed into the oesophagus by the epiglottis.
Peristalsis is the wave-like contraction and relaxation of muscles in the alimentary canal.
The oesophagus carries food from the pharynx to the stomach.
The cardiac sphincter lies at the junction of the oesophagus and stomach.
The cardiac sphincter helps prevent the acidic contents of the stomach from moving back into the oesophagus.

Stomach and Gastric Digestion

The stomach is a muscular, expandable organ. It stores food temporarily and mixes it with gastric juice by muscular contractions. Gastric juice contains hydrochloric acid, pepsinogen and mucus. Hydrochloric acid converts pepsinogen into pepsin and provides an acidic medium for its action.

Pepsin begins the chemical digestion of proteins. It breaks proteins into shorter chains called peptides. The optimum pH for pepsin is about 1.8. The stomach does not normally digest itself because its epithelial lining is protected by a mucus layer.

The stomach stores food and mixes it with gastric juice.
Hydrochloric acid, or HCl, makes the stomach contents strongly acidic.
HCl activates pepsinogen to form pepsin.
Pepsin digests proteins into peptides.
The optimum pH for pepsin is about 1.8.
Mucus secreted by the stomach wall protects the stomach from HCl and pepsin.
The epithelial cells lining the stomach are covered by a mucus layer.
The cardiac sphincter controls the entry of food into the stomach.
The pyloric sphincter controls the movement of partially digested food from the stomach into the small intestine.

Small Intestine and Its Secretions

The small intestine is the main site of chemical digestion and absorption. It consists of the duodenum, jejunum and ileum. The acidic chyme from the stomach enters the duodenum, where it mixes with bile, pancreatic juice and intestinal juice.

Bile is produced by the liver and stored in the gall bladder. It contains bile salts and pigments. Bile salts emulsify fats into small droplets and help provide an alkaline medium for the action of intestinal and pancreatic enzymes. Bile does not contain a digestive enzyme.

The duodenum is the first part of the small intestine.
The jejunum is the middle part of the small intestine.
The ileum is the last part of the small intestine.
Bile aids fat digestion and absorption because it contains bile salts.
Emulsification is the physical breakdown of large fat globules into smaller droplets.
Bile salts increase the surface area available for lipase action.
Bile helps neutralize the acidic chyme entering the duodenum.
The pancreas secretes pancreatic juice into the duodenum.
Intestinal glands secrete intestinal juice.

Digestion of Carbohydrates, Proteins and Fats

Carbohydrate digestion starts in the oral cavity due to the action of salivary amylase. It continues in the small intestine. Pancreatic amylase and intestinal enzymes convert disaccharides and polysaccharides into monosaccharides, especially glucose.

Protein digestion starts in the stomach with pepsin. It continues in the small intestine through pancreatic and intestinal proteases. Erypsin is a group of intestinal enzymes that acts on peptides and produces amino acids. Fat digestion mainly occurs in the small intestine after emulsification by bile salts and action by lipase.

Starch digestion starts in the oral cavity.
Salivary amylase changes starch into maltose.
Maltose is converted into glucose by maltase.
Sucrase converts sucrose into glucose and fructose.
Lactase converts lactose into glucose and galactose.
Pepsin begins protein digestion in the stomach.
Trypsin and other pancreatic proteases continue protein digestion in the small intestine.
Erypsin acts on peptides and produces amino acids.
Protein digestion is completed in the small intestine, especially in the ileum, producing amino acids.
Lipase digests fats into fatty acids and glycerol.
Carbohydrates are finally converted into monosaccharides, proteins into amino acids, and fats into fatty acids and glycerol.

Villi, Absorption and Assimilation

The inner wall of the small intestine has numerous finger-like projections called villi. Each villus has a thin epithelial covering, a network of blood capillaries and a lacteal. The villi greatly increase the surface area for absorption and provide a short distance for the movement of nutrients.

Glucose and amino acids pass into blood capillaries. Fatty acids and glycerol enter the lacteals mainly in the form of tiny fat particles called chylomicrons. The absorbed nutrients are transported to body tissues, where they are used for energy, growth, repair and storage.

The main function of intestinal villi is to provide a large surface area for absorption.
The epithelium of a villus is one cell thick.
Villi contain blood capillaries for the absorption of glucose and amino acids.
Villi contain a lacteal for the absorption of products of fat digestion.
Glucose and amino acids are absorbed into blood capillaries.
Fatty acids and glycerol are absorbed mainly into lacteals.
The small intestine is the main site of absorption and assimilation of digested food.
Absorption takes place mainly through diffusion, facilitated diffusion and active transport.
Assimilation is the use of absorbed nutrients by cells of the body.

Large Intestine, Water Absorption and Egestion

The large intestine consists of the caecum, colon and rectum. It receives material that has not been absorbed in the small intestine. Water and some salts are absorbed in the large intestine, making the remaining material more solid.

Bacteria living in the large intestine act symbiotically with humans. Some intestinal bacteria produce vitamin K. The faeces are stored temporarily in the rectum and removed through the anus by egestion.

The large intestine includes the caecum, colon and rectum.
The large intestine absorbs water and some mineral salts.
Vitamin K is formed in the large intestine by the activity of symbiotic bacteria.
Symbiotic bacteria live in association with the human body and provide useful substances.
Faeces consist mainly of undigested food, bacteria, mucus, dead cells and water.
The rectum stores faeces temporarily.
The anus is the opening through which faeces leave the body.
Egestion is different from excretion because egestion removes undigested food, while excretion removes metabolic wastes.

Digestive Health and Oral Rehydration

Food poisoning may result from eating food or drinking water contaminated with harmful microorganisms or their toxins. It can cause vomiting and diarrhoea. These conditions may lead to excessive loss of water and salts from the body.

Oral rehydration salts, or ORS, are used to replace water and important ions lost during diarrhoea and vomiting. ORS does not directly kill the causative microorganism. Its main purpose is to prevent or treat dehydration.

Food poisoning may cause vomiting, diarrhoea, abdominal pain and dehydration.
ORS is given as a treatment in cases of food poisoning when fluid loss occurs.
ORS contains water, glucose and important salts in suitable proportions.
Glucose helps the absorption of sodium and water from the intestine.
Dehydration occurs when water loss is greater than water intake.
Clean food, safe drinking water and proper hand washing help prevent food poisoning.
Antibiotics should not be used without appropriate medical advice.
Severe dehydration requires prompt medical attention.

Key terms

Digestion: The mechanical and chemical breakdown of complex food into simple, soluble substances.
Alimentary canal: The continuous muscular tube from the oral cavity to the anus.
Mastication: The chewing and mechanical breakdown of food by teeth.
Heterodont: Having different types of teeth with different shapes and functions.
Peristalsis: Wave-like muscular contractions that move food through the alimentary canal.
Epiglottis: A cartilaginous flap that directs food into the oesophagus during swallowing.
Cardiac sphincter: A muscular valve at the junction of the oesophagus and stomach.
Pepsin: A stomach enzyme that begins the digestion of proteins.
Bile salts: Substances in bile that emulsify fats and help their digestion and absorption.
Emulsification: The conversion of large fat globules into smaller droplets.
Maltase: An intestinal enzyme that converts maltose into glucose.
Erypsin: A group of intestinal proteases that converts peptides into amino acids.
Villus: A finger-like projection of the small intestine that increases the surface area for absorption.
Lacteal: A lymph vessel inside a villus that absorbs products of fat digestion.
Absorption: The movement of digested nutrients from the intestine into blood or lymph.
Assimilation: The use of absorbed nutrients by body cells.
Egestion: The removal of undigested food from the body as faeces.
Oral rehydration salts: A mixture of water, glucose and salts used to replace fluids and ions lost during diarrhoea and vomiting.

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