Digestive System

The digestive system breaks down complex food into simpler, soluble substances that the body can absorb. This process involves both mechanical and chemical actions.

The alimentary canal, also known as the digestive tract, is the pathway food follows from ingestion to waste elimination. It includes the mouth, pharynx, esophagus, stomach, small intestine, large intestine, and anus.

Types of Alimentary Tracts

Intracellular Tract: Digestion occurs inside cells, within food vacuoles, as seen in organisms like amoebas.
Extracellular Tract: Food is digested outside cells in a gut cavity, such as in hydras.
Simple Tract: Found in organisms like birds, where digestion is less complex.
Compartmentalized Tract: Specialized for digesting tough substances like cellulose, as seen in cows.
Complex Human Tract: A long, coiled system divided into sections for efficient digestion and absorption, starting from the mouth and ending at the anus.

Digestion in Humans

The human digestive system includes the alimentary canal and accessory glands that aid in digestion.

Alimentary Canal

Mouth: Food is chewed into smaller pieces (mechanical digestion), and saliva starts breaking down carbohydrates (chemical digestion). The food becomes a bolus for easier swallowing.
Pharynx: A passage at the back of the throat that directs food into the esophagus while preventing it from entering the windpipe using the epiglottis.
Esophagus: A tube that pushes food to the stomach through wave-like muscular contractions called peristalsis.
Stomach: A muscular sac that churns food (mechanical digestion) and uses gastric juices to break down proteins. The processed food becomes a semi-liquid called chyme.
Small Intestine: A 6-meter-long tube divided into:
- Duodenum: The first section where bile (from the liver) and pancreatic juices digest fats, proteins, and carbohydrates.
- Ileum: The final section where nutrients are absorbed through villi, tiny finger-like projections that increase surface area.
Large Intestine: Absorbs water and minerals, processes undigested food, and forms feces with the help of beneficial bacteria.
Rectum: Stores feces until they are ready to be expelled.
Anus: The opening through which waste is eliminated.

Accessory Organs

Pancreas: Produces enzymes to break down food and releases insulin to control blood sugar levels.
Liver: Produces bile, which helps digest fats.
Gall Bladder: Stores bile and releases it into the small intestine as needed.

Assimilation

Assimilation is the process where absorbed nutrients are transported to body cells for use. After absorption, nutrients travel to the liver through the hepatic portal vein, where they are processed or stored before being distributed.

How the Liver Processes Nutrients

Glucose: Used for energy or stored as glycogen. This process is regulated by insulin.
Amino Acids: Used to repair cells or make proteins and hormones. Excess amino acids are converted to urea through deamination.
Fats: Used for energy or stored in body tissues.

Digestion in Other Animals

Tapeworms

Tapeworms are parasitic organisms that live in the intestines of mammals. They lack an alimentary canal and absorb nutrients directly from the digested food of their host.

Planaria

Planaria have a simple digestive system with only one opening. They suck food into their pharynx, where partial digestion occurs in the intestine, and nutrients are distributed through branching canals.

Earthworms

Earthworms have a specialized digestive system. Food mixed with soil is drawn into the pharynx, transported through the esophagus, stored in the crop, ground in the gizzard, and finally digested and absorbed in the intestine.

Digestion in Insects

Insects like cockroaches have chewing mouthparts and salivary glands for initial digestion. Food is stored in the crop, ground in the gizzard, and then digested and absorbed in the intestine. Waste is expelled through the rectum.

Digestion in Birds

Birds swallow food whole, storing it in their crop. It then moves to the proventriculus and gizzard, where it is ground with the help of gastric juices and small stones. Final digestion occurs in the small intestine, and waste is eliminated through the cloaca.

Rabbits

Rabbits have a long and coiled digestive tract designed for plant digestion. A large caecum contains bacteria that help break down cellulose, ensuring maximum nutrient absorption.