Biology Class 10 Chapter 1: Digestive System

Ingestion is the taking in of food. Digestion is the breakdown of large food molecules into small soluble molecules with the help of enzymes. Absorption is the passing of the small digested molecules out of the digestive tube into the blood.

Assimilation is the uptake of digested food from the blood into the cells and its use for various purposes (e.g. glucose for ATP). Egestion (defecation) is the removal of the undigested part of food from the body through the anus.

Our food consists of carbohydrates, proteins and fats, which are very large molecules. They cannot pass through the gut wall into the blood or into the cells. Digestion breaks them with enzymes into small soluble molecules (glucose, amino acids, fatty acids and glycerol) that can be absorbed and used by every cell for energy and growth.

Egestion is the removal of undigested food (faeces) that was never absorbed into the cells; it takes place through the anus. Excretion is the removal of metabolic wastes made inside the cells, such as urea and carbon dioxide, through the kidneys, lungs and skin.

Digestion takes place only in the alimentary canal, but every cell of the body needs glucose, amino acids and fats. In a large multicellular animal most cells are far from the gut, so diffusion alone cannot supply them. Therefore the digestive system is helped by the transport system: blood absorbs digested food through the villi and carries it to all cells for assimilation.

Mouth → oral cavity → pharynx → oesophagus → stomach → small intestine (duodenum, jejunum, ileum) → large intestine (colon, rectum, anus).

The digestive glands are the salivary glands, gastric glands, intestinal glands, liver and pancreas.

Incisors bite into food and cut it into smaller pieces; canines tear food; premolars help to chew and grind food; molars chew and grind food.

Sublingual glands lie below either side of the tongue under the floor of the mouth. Submandibular glands lie below the jaw; like the sublingual glands their saliva enters from under the tongue. Parotid glands lie just in front of the ears; their saliva enters the mouth through small ducts near the upper molars.

The stomach is the dilated, roughly J-shaped part of the digestive tube, placed transversely on the left of the abdomen beneath the diaphragm. It has two openings guarded by valves: the cardiac opening from the oesophagus, guarded by the cardiac sphincter (the region around it is the cardiac end), and the pyloric opening into the duodenum, guarded by the pyloric sphincter. Its wall has layers of muscles that churn the food.

The small intestine is small in diameter and has three parts: the duodenum (about 20–25 cm; the bile duct, hepatic duct and pancreatic duct empty into it), the jejunum (about 2.5 m) and the ileum (about 4 m).

The large intestine is a 1.5 m tube, wider than the small intestine, made of the colon, rectum and anus. The colon begins with the caecum (the ileum enters it from the side and the 10 cm appendix hangs from its base) and continues as the ascending colon (right), transverse colon (across), descending colon (left) and sigmoid colon, which joins the rectum. The rectum is a straight 8-inch chamber that stores faeces; the anus is a 2-inch canal with sphincter muscles that control the stool.

The liver is a spongy, wedge-shaped, reddish-brown organ and gland on the right side of the body under the ribs. It has a larger right lobe and a smaller left lobe. The lobes contain many blood vessels and thousands of small lobes called lobules, which connect with bile ducts that carry bile to the duodenum.

The gall bladder, just below the liver, stores the bile made by the liver. When food is being digested, bile flows through the cystic duct and the common bile duct into the small intestine (duodenum).

The pancreas lies behind the stomach and in front of the vertebral column, surrounded by the gall bladder, liver and spleen. Its head is on the right side, tucked beside the curve of the duodenum; its tail extends to the left near the spleen.

Bile is made in the liver, passes through the hepatic ducts, is stored in the gall bladder, flows through the cystic duct and common bile duct into the duodenum, and then moves with the food through the jejunum, ileum and large intestine.

The liver (and also the pancreas) never comes in contact with food. It is still a digestive organ because it secretes bile, which enters the duodenum and emulsifies fats, helping digestion (the pancreas likewise sends pancreatic juice).

Swallowing is the movement of the bolus from the mouth through the pharynx into the oesophagus. Its beginning is voluntary, but once the food reaches the back of the mouth it becomes an automatic (involuntary) action.

(a) The tongue presses upwards and back against the roof of the mouth, forcing the bolus to the back of the mouth. (b) The soft palate closes the nasal cavity at the back. (c) The larynx cartilage around the top of the trachea is pulled upwards so that the glottis lies under the back of the tongue. (d) The epiglottis, a flap of cartilage, prevents the food from entering the glottis, so it goes into the oesophagus. Peristalsis then forces the food down the oesophagus.

Peristalsis is the rhythmic wave of muscular contraction and relaxation in the wall of the alimentary canal that causes food to move through it. Circular muscles just behind the bolus contract and push the food forward into the next region, where the longitudinal muscles contract and the tube becomes wide. This wave travels along the oesophagus and the rest of the gut.

During swallowing the soft palate closes the nasal cavity so food cannot go up into the nose, and the epiglottis (a flap of cartilage) covers the glottis so food cannot enter the trachea. If the epiglottis fails, food enters the windpipe and the person coughs or chokes; if the soft palate fails, food or water comes out through the nose.

A bead that fits tightly in a clear straw represents the bolus; the straw represents the oesophagus. Pinching the straw just above (behind) the bead represents contraction of circular muscles behind the bolus; the bead moves down, just as food is pushed along by the wave of contraction. Repeating the pinch along the straw is like the wave of peristalsis.

Gastric juice contains: mucus, which coats and protects the stomach wall from pepsin; hydrochloric acid, which stops salivary amylase, converts pepsinogen into pepsin, provides the acidic medium and kills many microorganisms; pepsin, which digests proteins into polypeptides and peptides; and rennin (only in infants), which digests milk protein.

(a) HCl stops the action of salivary amylase. (b) It converts the inactive enzyme pepsinogen into active pepsin and provides the acidic medium suitable for the action of pepsin. (c) It kills many microorganisms that enter with food.

Pepsin digests proteins. If it were made in its active form inside the cells of the gastric glands it would digest the proteins of these cells and of the stomach wall. So it is secreted as inactive pepsinogen and converted to pepsin by HCl (or active pepsin) only in the stomach cavity, where mucus protects the wall.

Pepsin works only in an acidic medium. In the small intestine the bile, pancreatic juice and intestinal juice are alkaline and neutralise the acidic chyme, so the medium becomes alkaline and pepsin stops working.

Food is tasted, smelled and felt, and chewed by crushing and grinding with the teeth, cheeks and tongue. Saliva moistens, softens and lubricates the food. Salivary amylase digests starch into maltose, and lipase from the tongue cells breaks triglycerides into diglycerides and free fatty acids. The tongue rolls the food into a slippery ball, the bolus, which is swallowed.

Bile is a greenish-yellow liquid manufactured in the liver and stored in the gall bladder; it is released into the duodenum as soon as food enters it. Bile contains no enzyme. It emulsifies fats, breaking them into small droplets (an emulsion), which provides a large surface area for the action of lipase. Being alkaline it also helps neutralise the acidic chyme.

Pancreatic juice contains pancreatic amylase (polysaccharides/starch → maltose and even glucose), pancreatic lipase (neutral fats/triglycerides → glycerol and fatty acids) and trypsinogen, which is activated to trypsin (proteins → polypeptides).

The lining of the jejunum and ileum secretes erepsin (peptides → amino acids), enterokinase (activates pancreatic trypsinogen into trypsin), lactase (lactose → glucose) and maltase (maltose → glucose). In the ileum digestion is complete.

Without pancreatic juice there would be no pancreatic lipase, so fats would not be digested and would pass out in the faeces; proteins (no trypsin) and starch (no pancreatic amylase) would be only partly digested. The acidic chyme would not be neutralised, so intestinal enzymes could not work and the duodenum could be damaged. The person would become malnourished and lose weight.

The chyme coming from the stomach is strongly acidic because of HCl. The pancreatic and intestinal enzymes can work only in an alkaline medium, so bile, pancreatic juice and intestinal juice are alkaline and neutralise the acidic chyme before digestion continues.

Chewing (mastication) by the teeth, tongue and cheeks is mechanical digestion. It breaks the food into small pieces, increasing the surface area for enzymes to act quickly; it mixes food with saliva so that salivary amylase can start digesting starch and the food is softened and lubricated; and it shapes the food into a bolus that can be swallowed safely.

The stomach wall needs protection against its own gastric juice: the strong hydrochloric acid, which can damage the tissue, and the enzyme pepsin, which would digest the proteins of the stomach wall. If the mucus coating breaks down, the wall is damaged and an ulcer forms.

Salivary glands (salivary amylase), cells of the tongue (lipase), gastric glands of the stomach (pepsinogen, rennin), the pancreas (amylase, lipase, trypsinogen) and the intestinal glands of the jejunum and ileum (erepsin, enterokinase, lactase, maltase). The liver secretes bile, which has no enzyme.

The lining of the small intestine has finger-like villi. Simple sugars, amino acids, mineral salts and vitamins pass through the wall of the villi into their blood capillaries by diffusion and active transport. Glycerol and fatty acids are absorbed by the lacteal and later poured into the blood. The capillaries join into veins that form the hepatic portal vein, which carries the blood to the liver.

From the large intestine water and some vitamins are absorbed into the blood. The semi-solid waste (faeces) passes into the rectum by peristalsis and is expelled through the anus at intervals; this removal of undigested matter is called egestion or defecation.

The liver (1) manufactures bile, (2) stores excess glucose as glycogen, (3) removes amino groups from amino acids (deamination) and converts ammonia into urea, (4) synthesises most plasma proteins, destroys old red blood cells and stores fat-soluble vitamins (A, D, E, K) and iron.

The liver's role in digestion is to manufacture bile, which is stored in the gall bladder and released into the duodenum. Bile emulsifies fats into small droplets so that lipase can digest them, and being alkaline it helps neutralise acidic chyme. The liver also receives all absorbed food through the hepatic portal vein and stores or alters it (e.g. stores glucose as glycogen).

Besides making bile, the liver stores excess glucose as glycogen and converts glycerol and amino acids into glucose; it deaminates amino acids and converts the ammonia into urea; it synthesises most plasma proteins; it destroys old red blood cells; and it stores fat-soluble vitamins A, D, E and K and mineral ions such as iron.

The liver carries out many chemical (metabolic) changes: (1) it regulates blood glucose by storing excess glucose as glycogen and by converting glycerol and amino acids into glucose; (2) it handles protein metabolism by deaminating amino acids, converting the ammonia formed into urea and synthesising plasma proteins. It also makes bile, destroys old red blood cells and stores vitamins and iron, so it is a central metabolic organ.

After a meal the liver removes excess glucose from the blood and stores it as glycogen. Between meals it converts glycogen, and also glycerol and amino acids, into glucose and releases it, so the blood glucose level stays steady.

A villus is a tiny finger-like projection of the internal lining of the small intestine. Millions of villi greatly increase the surface area of the intestine for the absorption of digested food.

Each villus is covered by simple columnar epithelium. Inside it has a network of blood capillaries and a central lymph capillary called the lacteal. Goblet cells in the epithelium secrete mucin to form a protective mucus layer. Between the bases of the villi are intestinal crypts, whose cells provide stem cells for renewal of the epithelium. Below the villus lie an arteriole, a venule and a lymph vessel.

A lacteal is the lymph capillary found in the centre of each villus. It absorbs glycerol and fatty acids (the products of fat digestion) and ultimately pours them into the blood.

(1) Millions of villi give a very large surface area. (2) The wall is only one cell thick (simple columnar epithelium), so digested food diffuses across a short distance. (3) A rich network of blood capillaries carries sugars, amino acids, salts and vitamins away quickly, keeping the concentration gradient steep. (4) The central lacteal absorbs fatty acids and glycerol. (5) Goblet-cell mucus protects the villus and crypt stem cells keep renewing the lining.

The blood capillaries of a villus absorb simple sugars, amino acids, mineral salts and vitamins and carry them through the hepatic portal vein to the liver. The lacteal absorbs fatty acids and glycerol and carries them through the lymph, which is finally poured into the blood.

Goblet cells secrete mucin and create a protective mucus layer over the intestinal lining. The cells of the intestinal crypts (small glands between the bases of villi) provide stem cells that renew the intestinal epithelium.

Symptoms: the stools become watery and the frequency of defecation increases; it may lead to dehydration. Causes: infection of the lower GI tract (e.g. food poisoning from contaminated food, which irritates the intestinal wall and increases peristalsis) and nervous stimulation.

In diarrhoea a lot of water and salts are lost in watery stools, so the patient becomes dehydrated. ORS (oral rehydration solution), or a mixture of salt and sugar in clean water, is given to replace the lost water and salts.

Symptoms: decreased frequency of stools, difficulty and pain in defecation and abdominal pain; the faeces move too slowly through the large intestine, so more water is absorbed and they become hard. Causes: medications, lack of exercise, not enough liquids, not enough fibre (fruit, vegetables, cereals) and a change in routine or eating habits. Treatment: a laxative softens the faeces. Prevention: plenty of water and fibrous food.

An ulcer is an open sore in the wall of the gut caused by gradual disintegration of the tissue; it may be a gastric (stomach) ulcer or a peptic (duodenal) ulcer. The main symptom is stomach pain. Causes: acidity, smoking and bacterial infection. Prevention: stop smoking, avoid spicy food and food containing acids, and avoid stress.

Fibre (from fruit, vegetables and cereals) is not digested; it adds bulk to the faeces and keeps them moving through the large intestine. Without fibre the faeces move too slowly, more water is absorbed from them and they become hard and painful to pass (constipation). A fibre-rich diet with plenty of water keeps the faeces soft and prevents it.

The alimentary canal is a continuous tube about 9 m long from mouth to anus, lined internally by mucous membrane; each region has its own function.

1. Mouth: the external opening into the oral cavity.
2. Oral cavity: bounded by the fixed upper jaw and movable lower jaw; contains teeth and tongue.
3. Pharynx: leads from the mouth and nasal cavity to the oesophagus and trachea; a common passage for food and air.
4. Oesophagus: a muscular tube about 25 cm long from the pharynx to the stomach; no digestion occurs in it.
5. Stomach: a dilated J-shaped bag on the left of the abdomen beneath the diaphragm; the cardiac opening (cardiac sphincter) from the oesophagus and the pyloric opening (pyloric sphincter) into the duodenum; muscular wall churns food.
6. Small intestine: narrow; duodenum (20–25 cm, receives bile, hepatic and pancreatic ducts), jejunum (about 2.5 m), ileum (about 4 m).
7. Large intestine: 1.5 m, wider; colon (caecum with 10 cm appendix, ascending, transverse, descending and sigmoid colon), rectum (8-inch chamber storing faeces) and anus (2-inch canal with sphincter muscles).

Structure: The large intestine is a 1.5 m tube, larger in diameter than the small intestine, running from the ileum to the anus. It has three parts: the colon, the rectum and the anus. The colon starts with the caecum (a pouch into which the ileum opens from the side and from whose base the 10 cm appendix hangs), followed by the ascending colon (right side), transverse colon (across), descending colon (left side) and sigmoid colon, which joins the rectum. The rectum is a straight 8-inch chamber; the anus is a 2-inch canal surrounded by sphincter muscles.

Function: The large intestine absorbs water and some vitamins from the undigested food into the blood, so the waste becomes semi-solid faeces. Peristalsis moves the faeces into the rectum, which stores them temporarily; the anal sphincters control their removal (egestion/defecation). No digestion of food takes place here.

Starch, proteins and fats are all enzymatically broken down in the small intestine. When chyme enters the duodenum it stimulates (a) the gall bladder to release bile, (b) the pancreas to secrete pancreatic juice and (c) the intestinal glands to secrete intestinal juice. All these are alkaline and neutralise the acidic chyme, which the pancreatic and intestinal enzymes need. Bile (made in the liver, stored in the gall bladder) emulsifies fats into small droplets, giving lipase a large surface area. Pancreatic amylase converts starch to maltose and glucose; pancreatic lipase converts triglycerides into glycerol and fatty acids; trypsin (from trypsinogen, activated by enterokinase) converts proteins into polypeptides. The intestinal juice of the jejunum and ileum contains erepsin (peptides → amino acids), lactase (lactose → glucose) and maltase (maltose → glucose). In the ileum digestion is complete.