To be of any value to the body, the food taken in through the mouth must enter the blood stream and be distributed to all the living regions.
Digestion is the process by which insoluble food, consisting of large molecules, is broken down into soluble compounds having smaller molecules. These smaller molecules, in solution, pass through the walls of the intestine and enter the blood stream. Digestion and absorption take place in the alimentary canal, digestion being brought about by by means of active chemical compounds called enzymes.
The alimentary canal is a muscular tube, with an internal glandular lining, running from mouth to anus. Some regions have particular functions and, accordingly, different structures.
Juices are secreted in the alimentary canal from glands in its lining or are poured into it through ducts from glandular organs outside it. As the food passes through the alimentary canal it is broken down in stages until the digestible material is dissolved and absorbed. The indigestible residue is expelled through the anus.
Enzymes
Enzymes are chemical compounds, protein in nature, made in the cells of living organisms. They act as catalysts substances which accelerate the rate of most chemical changes in the organism without altering the end Product. They occur in great numbers and varieties in all protoplasm and without them the chemical reactions would be too slow to maintain life
The vast majority of enzymes are intracellular, that is, they carry out their functions in the protoplasm of the cell in which they are made. Some enzymes, however, are secreted out of the cells in which they are made, to be used elsewhere. These are called extracellular enzymes. Bacteria and fungi secrete such extracellular enzymes into the medium in which they are growing The higher organisms secrete extracellular enzymes into the alimentary tract to act on food taken into it.
These digestive enzymes accelerate the rate at which in soluble compounds are broken down into soluble ones Enzymes which act on starch are called amylases, those acting on proteins are proteinases, and lipase act on fat.
Every enzyme has the following characteristics: (a) it is destroyed by heating, since it is a protein, (b) it acts best within a narrow temperature range, (c) it acts most rapidly in a particular degree of acidity or alkalinity (pH), (d) it acts on only one kind of substance, (e) it always forms the same end product or products, since an enzyme affects only the rate of reaction.
Movement of food through the alimentary canal
Ingestion is the act of taking food into the alimentary canal through the mouth.
Swallowing. In swallowing, the following actions take place: (a) the tongue presses upwards and back against the roof of the mouth, forcing the pellet of food, called a bolus, to the back of the mouth, or pharynx; (b) the soft palate closes the opening between the nasal cavity and the pharynx; (c) the laryngeal cartilage round the top of the trachea, or winds pipe, is pulled upwards by muscles so that the opening of the larynx lies beneath the back of the tongue, and the opening of the trachea is constricted by the contraction of a ring of muscle; and (d) the epiglottis, a flap of cartilage, directs food over the laryngeal orifice. In this way food is able to pass over the trachea without entering it. The beginning of this action is voluntary, but once the bolus of food reaches the pharynx swallowing becomes an automatic or reflex action. The food is forced into and down the oesophagus, or gullet, by peristalsis.
This takes about six seconds with relatively solid food, and then the food is admitted to the stomach. Liquid travels more rapidly down the gullet.
Peristalsis. The walls of the alimentary canal contain circular and longitudinal muscle fibres. The circular muscles, by contracting and relaxing alternately, urge the food in a wave-like motion through the various regions of the alimentary canal.
Egestion. The expulsion from the alimentary canal of the undigested remains of food is called egestion.
Digestion in the mouth
In the mouth the food is chewed and mixed with saliva Che reduces the food to suitable sizes for swallowing and creases the available surface for enzymes to act on.
Saliva is a digestive juice secreted by three pairs of glands the ducts of which lead into the mouth. It is a watery fluid, not particularly acid or alkaline, containing mucus, which helps to lubricate the food and makes the particles adhere to one another. An adult may secrete from 1 to 1-5 litres of saliva per day. One enzyme, salivary amylase, is present in saliva. Salivary amylase acts on cooked starch and begins to break it down into maltose, a soluble sugar.
The longer food is retained in the mouth, the further this starch digestion proceeds and the more finely divided does the food become as a result of chewing. In fact even well chewed food does not remain in the mouth long enough for much digestion of starch to take place, but saliva will continue to act for a time even when food is passed into the stomach.
Digestion in the stomach
This part of the alimentary canal has flexible walls and so can be extended by the accumulation of a relatively large amount of food which is retained by the closure of the pyloric sphincter at the end of the stomach. These characteristics enable food from a particular meal to be stored for some time and released at intervals to the rest of the alimentary canal. If there were no stomach, food would have to be taken every twenty minutes or so.
Very little absorption takes place in the stomach, but its glandular lining produces gastric juice containing the enzyme pepsin, and it may also contain, in young children.
an enzyme called rennin. Pepsin acts on proteins and breaks them down into more soluble compounds called peptides.
Rennin, if present, clots protein in milk. The stomach wall also secretes hydrochloric acid which makes a 0-5 per cent solution in the gastric juice. The acid provides the best degree of acidity (optimum pH) for pepsin to work in, and probably also kills many of the bacteria taken in with the food. The salivary amylase from the mouth cannot digest starch in an acid atmosphere, but it seems likely that it continues to act within the bolus of food until this is broken up and the hydrochloric acid reaches all its contents.
The rhythmic, peristaltic movements of the stomach, about once every twenty seconds, churn up the food and gastric juice to a creamy fluid called chyme. The length of time food is retained in the stomach depends to some extent on its nature.
Water may pass through in a few minutes, a meal of carbon hydrate such as porridge may be retained less than an hour, and a mixed meal containing protein and fat may be in the stomach for one or two hours.
When digestion in the stomach is complete the pyloric sphincter relaxes from time to time, allowing a little chyme to pass through into the first part of the small intestine called the duodenum.
Digestion in the duodenum
An alkaline juice from the pancreas, and bile from the liver, are poured into the duodenum. The pancreas is a cream coloured gland lying below the stomach. Its cells make enzymes which act on carbohydrates, proteins and fats respectively. Three of these enzymes, including trypsin, break down proteins to peptides, and peptides to soluble amino acids. Starch is broken down to maltose and fats are split up into fatty acids and glycerol. Pancreatic juice also contains sodium hydrogen carbonate which partly neutralizes the acid chyme from the stomach, and so creates a suitable environment (pH) for the pancreatic and intestinal enzymes to work in.
Bile is a green, watery fluid made in the liver, stored in the gall bladder and conducted to the duodenum by the bile duct. Its colour is derived largely from breakdown products of the red pigment from decomposing red blood cells. It contains sodium chloride, sodium hydrogen carbonate and organic bile salts.
Bile dilutes the contents of the intestine, and the bile salts reduce the surface tension of fats, so emulsifying them. This results in fats forming a suspension of tiny droplets, the in- creased surface so presented allowing more rapid digestion. Many of the bile salts are reabsorbed in the ileum.
Digestion in the ileum
In the small intestine five or more enzymes are secreted by the glands in its lining. These complete digestion by reducing any unchanged peptides to amino acids, maltose and other sugars to glucose, and unchanged fats to fatty acids and glycerol.
All the digestible material is now reduced to soluble compounds which can pass through the intestinal lining and into the blood stream. The glandular lining of the alimentary canal is continually secreting mucus which helps to lubricate the passage of food between its walls but which also prevents the digestive alimentary canal itself, enzymes would for the digestive Juices from reaching and digesting the alimentary canal itself. The cells which make the protein digesting enzymes would themselves be digested by these chemicals were it not for the fact that the enzymes are made in an inactive form and cannot Work until they reach the cavity of the alimentary canal, Where food between cannot they are activated by the chemicals present. Pepsin, for example, Is made and secreted as an inactive substance, pepsinogen.
When pepsinogen is set free in the stomach the hydrochloric acid present converts it to active pepsin. This pepsin cannot now digest the stomach walls because of their protective coating of mucus.
Absorption in the ileum
Nearly all the absorption of digested food takes place in the ileum, and certain of its characteristics are important adaptations to its absorbing properties:
(a) it is usually fairly long and presents a large absorbing surface to the digested food,
(b) its internal surface is greatly increased by thousands of tiny, finger like projections about 1mm long called villi,
(c) the lining epithelium is very thin and the fluids can pass fairly rapidly through it,
(d) there is a dense network of blood capillaries in each villus.
The small molecules of the digested food, principally amino acids and glucose, pass through the epithelium and the capillary walls and enter the blood plasma. They are then carried away in the capillaries which unite to form veins and eventually join up to form one large vein, the hepatic portal vein. This carries all the blood from the intestine to the liver, which may retain or alter any of the digestion products. The digested food then reaches the general circulation.
Some of the fatty acids, and glycerol from the digestion of fats, enter the blood capillaries of the villi but a large proportion may be recombined in the intestinal lining to form fats once again and then these fats pass into the lacteal. It may be that some of the finely emulsified fat is absorbed directly, i.e. without digestion, as minute droplets which subsequently enter the lacteals. The fluid in the lacteals enters the lymphatic system which forms a network all over the body and eventually empties its contents into the blood stream
The large intestine (colon and rectum)
The material passing into the large intestine consists of water with undigested matter, largely cellulose and vegetable fibres (the roughage), bacteria, mucus and dead cells from the lining of the alimentary canal. The large intestine secretes no enzymes and can absorb very little digested food. It does, however, absorb much of the water from the undigested residues. This semi-solid waste, the faeces, is passed into the rectum by peristalsis and is expelled at intervals through the anus. The residues may spend from 12 to 24 hours in the intestine.
The caecum and appendix
These are relatively small, probably vestigial structures in man. In herbivores like the rabbit and the horse they are much larger, and it is here that most of the cellulose digestion takes place, largely as a result of bacterial activity. *i.e. structures which have become apparently functionless through disuse in the course of evolution.
Utilization of digested food
The products of digestion are carried round the body in the blood plasma. From the blood, most living cells are able to absorb and metabolize glucose, fats and amino acids.
(a) Glucose. During respiration in the protoplasm, glucose is Oxidized to carbon dioxide and water. This reaction releases energy to drive the many chemical processes in the cell, and in specialized cells produces, for example, contraction (muscle cells) and electrical changes (nerve cells).
(b) Fats. Fats are incorporated into cell membranes and other structures in cells. The fats not used for growth and maintenance in this way are oxidized to carbon dioxide and water, releasing energy for the vital processes of the cells. Twice as much energy is obtained from fats as from glucose.
(c) Amino acids are absorbed by cells and reassembled to make proteins. These proteins may form visible structures such as the cell membrane and other components of the protoplasm or the proteins may be enzymes which control and co-ordinate the chemical activity within the cell.
Amino acids not required for building proteins are de-aminated in the liver, that is, their nitrogen is removed and the residue is used in the same way as carbohydrate, namely Oxidized, or converted to glycogen and stored.
Storage of digested food
If the quantity of food taken in exceeds the energy requirements of the body or the demand for structural materials, it is stored in one of the following ways:
(a) Glucose. The concentration of glucose in the blood of a person who has not eaten for eight hours is usually between 90 and 100 mg/100 cm3 blood. After a meal containing carbohydrate, the blood sugar level may rise to 140 mg/100 cm but two hours later, the level returns to about 95 mg. The sugar not required immediately for the energy supply in the cells is converted in the liver and in the muscles to glycogen.
The glycogen molecule is built up by combining many glucose molecules in a long branching chain rather similar to the starch molecule. About 100g of this insoluble glycogen is stored in the liver and about 300 g in the muscles. When the blood sugar level falls below 80 mg/100 cm3, the liver converts its glycogen back to glucose and releases it into the circulation.
The muscle glycogen is not normally returned to the circulation but is used by active muscle as a source of energy in much the same way as glucose. The glycogen in the liver is a "short-term store, sufficient for about only six hours if no other glucose supply is available. Excess glucose not stored as glycogen is converted to fat and stored in the fat cells of the fat depots.
(b) Fats. Certain cells can accumulate drops of fat in their cytoplasm. As these drops increase in size and number, they join together to form one large globule of fat in the middle of the cell, pushing the cytoplasm into a thin layer and the nucleus to one side. Groups of fat cells form adipose tissue beneath the skin and in the connective tissue of most organs.
Unlike glycogen, there is no limit to the amount of fat stored and because of its high energy value it is an important reserve of energy-giving food.
(c) Amino acids. Amino acids are not stored in the body. Those not used in protein formation are deaminated. The protein of the liver and tissues can act as a kind of protein store to maintain the protein level in the blood but absence of protein in the diet soon leads to serious disorders.
The rate of oxidation of glucose and its conversion to glycogen or fat is controlled by hormones. When intake of carbohydrate and fat exceeds the energy requirements of the body, the excess will be stored mainly as fat. Some people never seem to get fat no matter how much they eat, while others start to lay down fat when their intake only marginally exceeds their needs. Putting on weight is unquestionably the result of eating more food than the body needs but it is not clear why in- divisible should differ so much in their reaction. The explanation probably lies in the hormonal balance which, to some extent, is determined by heredity. A slimming diet designed to reduce calorie intake must, nevertheless, always include the essential amino acids, vitamins, mineral salts and certain essential fatty acids.