Ch 14Human Nutrition — Class 11 Biology, concept-first.

Nutrition is the sum of the processes by which an organism consumes and utilises food substances to obtain energy and the raw materials it needs to grow, repair its tissues and stay alive.

The mouth, also called the oral or buccal cavity, is the first part of the alimentary canal — the point at which food enters the digestive system.

An adult human has 32 teeth housed in the buccal cavity. Human dentition — the study of teeth with respect to their number, arrangement and development — is described by three terms: - Thecodont: each…

The tongue is a muscular, fleshy, roughly triangular organ lying along the floor of the buccal cavity.

The buccal cavity leads into a short pharynx, a passage shared by both food and air. Air passes from the pharynx into the trachea through an opening called the glottis, which is guarded by a flexible,…

The oesophagus is a thin, muscular tube, roughly 25 cm long, that lies behind the trachea. It runs down through the neck and the central part of the rib cage, pierces the diaphragm, and joins the stom…

The stomach is a muscular, sac-like, 'J'-shaped organ located in the upper left part of the abdominal cavity, about 25-30 cm long.

The small intestine is a long, narrow, highly coiled tube — about 6 metres long and 2.5 cm broad in humans — held coiled within the abdominal cavity by sheets of tissue called mesenteries, which also…

The large intestine begins where the ileum opens into it and is about 1.5 metres long — shorter but wider in diameter than the small intestine.

From the oesophagus down to the rectum, the wall of the alimentary canal is built, from the innermost to the outermost layer, of four basic tissue layers — mucosa, submucosa, muscularis and serosa — t…

Besides the alimentary canal itself, the digestive system includes a set of digestive glands that pour their secretions into the canal to help break down food.

Three pairs of salivary glands open into the buccal cavity: the parotid glands, present in front of the ear; the submandibular glands, present below the lower jaw; and the sublingual glands, present b…

The liver is a dark reddish-brown gland lying just below the diaphragm, occupying the right upper part of the abdominal cavity.

The pancreas is a leaf-shaped heterocrine gland — meaning it has both exocrine and endocrine tissue in a single organ — lying in the bend of the duodenum, beneath the stomach.

Having looked at the structure of the alimentary canal and its glands, we now turn to how they work together to digest food.

Both mechanical and chemical digestion begin in the mouth. Mastication (chewing), carried out with the teeth and tongue, crushes and grinds the food — the teeth crush and grind while the tongue manipu…

Both mechanical and chemical digestion continue in the stomach, where food is stored for 4-5 hours. Mechanically, the thick muscular wall of the stomach churns the food, breaking it into finer particl…

In the small intestine, three secretions are mixed with the chyme — intestinal juice, bile juice and pancreatic juice — with the peristaltic movements of the muscularis layer ensuring they mix thoroug…

The mucosa of the large intestine secretes mucus but no digestive enzymes of its own. Some carbohydrates and proteins that escape digestion in the small intestine do enter the large intestine, where t…

Absorption is the passage of the end products of digestion through the mucosal lining of the alimentary canal into the blood and lymph.

Absorption occurs to differing degrees at each level of the alimentary canal: - Mouth: absorption occurs through the mucosa of the mouth and the lower surface of the tongue into blood capillaries — fo…

Different nutrients are absorbed by different mechanisms: - Simple diffusion: part of the glucose and amino acids, and electrolytes such as chloride ions, are absorbed simply by moving down their conc…

Assimilation is the process by which absorbed food material finally reaches the body's tissues and becomes incorporated into the protoplasm of its cells.

The energy content of food can be measured in two related ways. The gross calorific (gross energy) value is the amount of heat released when 1 gram of a food is completely burned (combusted) in a bomb…

Disorders connected with nutrition and digestion can be grouped into two broad kinds: those that arise from what — or how much — a person eats (nutritional disorders), and those that arise from the ma…

Inadequate intake of dietary protein causes Protein Energy Malnutrition (PEM), which may also be accompanied by deficiencies of vitamins and minerals. PEM can lead to two related disorders:

Indigestion is a condition of discomfort in digestion, which can be brought on by overeating, inadequate secretion of digestive enzymes, spicy food, anxiety, improperly digested food, or food poisonin…

Constipation is diagnosed when the frequency of defaecation falls to less than once a week. Straining to defaecate can cause abdominal pain and distension, and, rarely, perforation of the bowel.

Diarrhoea is the passing of loose, watery stools more than three times a day, and can lead to dehydration.

Jaundice shows up as yellowing of the conjunctiva of the eyes and of the skin, together with pale, whitish stools — signs of an abnormality in bilirubin metabolism or excretion.

Vomiting is the forceful expulsion of stomach contents out through the mouth, caused by reverse peristaltic movements of the gastric wall.