What is animal nutrition?
Animals, unlike green plants cannot manufacture their food from simple substances.They either eat plants, or the flesh of other animals which eats plants.
Animals which eat only plants are called HERBIVORES (L:herba, herb; vorare, to eat), and those which eat flesh are called CARNIVORES (L: carnis, flesh). The carnivores include animals which hunt and kill their prey, examples include cats, birds of prey, and carrion-eaters, examples vultures and hyenas, which find their food already dead. Animals which eat both plants and flesh are called OMNIVORES (L. omnia, all). Man is an example.
The food of most animals is mainly solid, and consists of substances with large, insoluble molecules like starch and proteins. An animal cannot use food in this form. First it had to break down the large, insoluble molecules into smaller soluble molecules, which it can take into its body and rebuild into its own molecules.
Animal nutrition is largely concerned with converting unusable food into usable food materials. The following terms are used to describe the various stages of animal nutrition.
INGESTION : the taking in of food from outside.
DIGESTION : the breakdown of foods with large, insoluble molecules into substances with smaller, soluble molecules.
ABSORPTION : the passage of digested food through the wall of the alimentary canal to the body fluids (or in a protozoan, through the cell wall to the cytoplasm).
EGESTION : the discharge of any undigested and unabsorbed parts of an animal's food. (Note that this is not excretion)
Nutrition in man
In man the above stages take place in the alimentary canal or gut. This is a coiled tube in the head and trunk, with two openings, the mouth and the anus. It is about 7.5 - 9.0 meters long in an adult, or about 4 to 5 times his or her own height. It only fits into the body by being much folded. Food while in the alimentary canal is not yet part of the body. First it has to be digested and absorbed. Undigested food never becomes part of the body and is egested periodically through the anus. Digestion is brought about by enzymes, and takes place in stages.
The alimentary canal
The parts of the gut or alimentary canal are shown in the diagram below. Their secretion and functions are described below and summarized in the table below.
The mouth
The main functions of the mouth cavity, or buccal cavity are ingestion, and mastication or chewing. Chewing breaks the food into smaller pieces and so helps digestion by exposing a larger surface area to enzymes action. Also, the digestion of starch begins in the mouth.
The muscular lips helps to pass the food into the mouth and prevent it falling out while it is being chewed. The tongue is a muscular flap which lies in the floor of the mouth. Its surface is covered by small projections called papillae, with taste buds on them. The sides of the mouth are formed by the cheeks and the roof by palate. The palate consist of a bonyvhard palate in front, and a muscular soft palate behind. The back of the mouth cavity or pharynx is continuous with the nasal cavity, and is connected to each middle ear by an Eustachian tube. At the base of the pharynx a small flap called the epiglottis lies over the opening to the trachea or wind pipe.
Three pairs of salivary glands secretes a liquid called saliva into the mouth cavity. Saliva is somewhat slimy and contains the enzyme ptyalin or salivary amylase. Ptyalin helps to convert cooked starch into the disaccharide maltose. Saliva also mixes with and moistens the food as it is being chewed, so that it is easier to swallow.
The chewed and moistened food is worked into a ball or bolus, and then swallowed. During the swallowing, the soft palate closes the opening between the pharynx and the nasal cavity, and the epiglottis closes over the opening to the trachea, thus preventing food from going into it.
The oesophagus
The oesophagus or gullet is a straight tube which passes through the thorax and diaphragm into the abdomen, where it opens into the stomach.
Its walls are muscular and push each bolus of food towards the stomach by a wave-like muscular contraction. This type of movement is known as peristalsis and occurs throughout the alimentary canal
The stomach
The stomach is a muscular bag in which a meal is temporarily stored, and passed on gradually in convenient amounts to the small intestine for digestion. Also, some digestion and absorption takes place in the stomach.
The part of the stomach nearer the gullet is called the cardiac region. The narrower part nearer the small intestine is the pyloric region. At the entrance to the stomach a circular band of muscle called the CARDIAC SPHINCTER relaxes to allow food to enter. Food remains in the stomach for one to four hours, depending on the amount and type of food eaten. Vegetables generally pass through more slowly than meat.
The ptyalin in saliva continuous to act on food for a time in the cardiac end of the stomach. As the food moves towards the pyloric region, the walls of the stomach start to secrete gastric juice, which because it is acid inactivates the ptyalin.
Gastric juice : is secreted by glands in the stomach wall. It contains hydrochloric acid, the enzyme pepsin, mucus, and in infants, a second enzyme called renin.
Hydrochloric acid : Inactivates ptyalin and kills most of the bacteria in the food.
Pepsin : begins the digestion of proteins to peptides
Renin : curdles milk (converts the liquid protein in the milk to solid curds). Both pepsin and renin are active in acid conditions.
Mucus : helps to protect the stomach wall from its own secretion of hydrochloric acid and pepsin.
The walls of the stomach are muscular, and regular peristaltic movements (about three per minute) churn up the food, mixing it thoroughly with the gastric juice. By the time it is ready to leave the stomach, the food looks like a watery paste. In this state it is called CHYME. The passage from the stomach to the small intestine is normally kept closed by circular band of muscle called the PYLORIC SPHINCTER. The pyloric sphincter only relaxes when food is ready to leave the stomach. It allows a little chyme at a time to pass through to the duodenum.
The small intestine and associated organs
The small intestine is the longest part of the alimentary canal. In an adult it is 6 to 7 meters long. The first 25cm, which is [- shaped and wider than the rest, is called the DEODENUM. The rest of the small intestine, called the ileum, is a long narrow tube arranged in many loops.
Diagram to show successive stages in peristalsis
When food reaches the duodenum, the duodenal wall secretes a hormone called secretin which stimulates the liver and pancreas to secrete digestive juices.
The liver is a large organ lying under the diaphragm and partly covering the stomach. It is divided into two main lobes, each of which is divided into smaller lobes. A gall bladder is embedded in the right lobe of the liver, and is connected to the duodenum by a short bile duct.
The pancreas is a long, narrow organ which lies inside the loop of the duodenum. It consists of a number of small lobes loosely held together. The pancreas secretes pancreatic juice which enters the second part of the duodenum through the pancreatic duct. This duct and the bile duct join before they enter the duodenum.
Bile is a green liquid which contains alkaline salts. Therefore when it mixes with chyme in the duodenum it neutralizes the acid of the gastric juice and stops the action of pepsin.The bile also emulsifies any lipids in the chyme.
The pancreatic juice contains several enzymes which become active when they mix with the alkaline bile in the duodenum. One is an amylase which acts on starch which has not already been hydrolyzed by ptyalin. Three are proteases and converts proteins to peptides. One is a lipase which converts emulsified fats into fatty acids and glycerol.
The glands in the walls of the ileum produce about five enzymes. One is a peptidase which converts peptides into amino acids. The others complete the digestion of carbohydrates and lipids.
Absorption in the small intestine
Water and soluble substances are relatively small molecules, e.g. Simple sugars, common salt and alcohol, are partly absorbed through the stomach wall. However, the ileum is the main organ of absorption. All the products of digestion are absorbed through the wall.
The ileum is well adapted for absorption. It's inner surface is lined with tiny finger-like processes called villi, which are in constant motion slowly expanding and contracting about five times per minute. The villi greatly increase the surface area over which absorption can take place, and are well supplied with blood and lymph capillaries.
Sugars and amino acids pass through the walls of the villi into the blood capillaries, and are carried to the liver in the hepatic portal vein. About one-third of the final products of lipids digestion are fatty acids and glycerol, which pass into the capillaries of the villi. The other two-thirds are emulsified lipids which pass into the lacteals, so called because their contents appear milky. The lacteals are joining to the main lymphatic vessels, which pours their contents directly into the left jugular vein. This means that unlike all other digested substances, lipids enter the general circulation without passing through the liver.
Summary of digestion in man
The large intestine and egestion
The ileum opens at right angles into the large intestine at the caecum, off which lies a small worm-like process called the appendix. In man the appendix serves no useful purpose, and sometimes a collection of hard food particles forms inside it, causing the disease appendicitis.
The large intestine or colon is wider than the ileum. Its inner wall has no villi, but contains many mucus secreting glands. The colon is continuous with the rectum, which in turn opens to the outside of the anus. After passing through the ileum, the contents of the intestine contains a high proportion of water. The walls of the large intestine absorb much of this water, so it is not lost from the body. The semi solid materials which are left are called faeces. They include the following.
Undigested food: mainly the cellulose cell walls and fibrous parts of plants.
Dead bacteria: the normal, healthy alimentary canal contains many millions of bacteria.
Mucus and dead cells from the gut wall: the cells lining the gut have a limited life and are constantly dying off and being renewed.
Surplus bile and other secretions: As faeces pass along the colon, they become more solid as water is absorbed from them. They are also lubricated by the mucus the wall secretes. Finally they pass into the rectum, from which they are egested at the anus.