- DIGESTION IN THE RUMINANT
In the last lecture we considered the digestive process in non-ruminant animals, those with a simple stomach. We will now look at digestion in the ruminant animal; those that have compound stomachs. Examples of ruminants on farms are cattle, sheep and goats.
Refresh your memory by looking at the diagram of the ruminant stomach in Lecture 4. You will see that it is made up of four compartments, the reticulum, the rumen, the omasum and the abomasum. The rumen is by far the largest compartment, and the abomasum is the true stomach, the equivalent of the stomach in the non-ruminant.
The natural food for ruminants is grass, and as we said in the last lecture, grass is a food high in fibre. It is a fibrous food, and the fibre consists of the cellulose which is the cell wall of the plant cell. This cell wall must be broken down before the contents of the cell can be made available to the animal. The other big difference between ruminants and non-ruminants is in their actual eating habits. Ruminants have no incisor teeth in the top jaw, only a toughened area of gum called the hard pad. They do not bite off the grass but simply tear it and swallow it without chewing. Non-ruminants bite off their food, chew it well, and then swallow it, so that the food reaching the stomach is broken down into small particles. The food reaching the rumen is not broken down at all, but is raw grass pulled off and swallowed. The task of the compound stomach is to break down this raw grass into small particles, destroy the cellulose of the plant cell walls and release the nutrients in the cells, which then pass into the true stomach, or abomasum and are digested in the same way as those of the non-ruminant animal. This breakdown of grass and cellulose is done in two ways, by mechanical action, and by the action of micro-organisms, chiefly bacteria.
As you know from Lecture 4, the reticulum, rumen and omasum have muscular walls and various projections sticking out of their inside linings; the honeycomb of the reticulum, the papillae of the rumen and the long, muscular projections of the omasum. The food passes down the oesophagus into the reticulum and from there into the rumen. Once in the rumen, it is churned and mixed by the muscles of the walls, passed back into the reticulum, back to the rumen, into the omasum, back to the rumen and so on, all the time being churned and shredded by the rough inside linings of these organs. These are all mechanical actions designed to break up the tough, fibrous grass.
The other mechanical action which the ruminant can perform is the action of cudding, or rumination. By using the thick, muscular wall of the rumen and the action of the diaphragm the animal can ‘cough up’ a lump of food from the rumen back into the mouth where it can be thoroughly chewed. You often see cattle lying down and chewing their cud, and, in fact, this can occupy one third of the animal’s day. The purpose of chewing the cud is to grind the food into small particles with the back teeth, the molars and premolars, to mix it with saliva so that when it is swallowed it is fairly well broken down and softened. The normal pattern of the ruminant is to graze actively for a spell, simply collecting grass and swallowing it, followed by a period of rest, either standing up or lying down, then it coughs up, or regurgitates, the food it has eaten, breaks it down, then swallows it again.
The rumen of an adult cow is a very large organ holding a wheelbarrow-load of food, and this has to be thoroughly chewed. While this is going on, gallons of saliva are being added to soften the mixture in the rumen together with any water drunk by the animal.
ACTION OF THE MICRO-ORGANISMS
The rumen contains vast numbers of micro-organisms, chiefly bacteria, which live there feeding, growing and reproducing all the time. They are known as the rumen flora. These micro-organisms attack the cellulose in the rumen and break it down into fatty acids, and simple sugars which they use for food. The fatty acids are absorbed through the walls of the rumen, converted into the simple sugar glucose, and used by the animal. At the same time, large quantities of the gases, carbon dioxide and methane are produced, and expelled through the animal’s mouth by eructation, or belching. Sometimes the gases get trapped in the rumen resulting in a condition called bloat in which the animal literally blows up like a balloon; this is a dangerous condition and, unless treated, can cause the death of the animal.
Although only the rumen has been mentioned, because of the mixing of the food between the rumen, reticulum and omasum, the process of bacterial breakdown and the absorption of the fatty acids take place in all three. The micro-organisms of the ruminant cannot digest proteins and fats; their importance in ruminant digestion is their ability to break down cellulose. This breakdown provides food in the form of the fatty acids, directly to the animal, and also releases the contents of the cells for normal digestion in the abomasum and small intestine.
This system is a good example of symbiosis; two organisms living together with each being of some benefit to the other. The ruminant animal provides the carbohydrate (cellulose), protein and other nutrients for the bacteria to live on, and the bacteria break down the cellulose for the ruminant.
Although there are a great many fatty acids, the three that are the result of bacterial digestion in the rumen are acetic acid, butyric acid and proprionic acid. These are very complicated organic acids, which will be discussed in more detail in the Feeds and Feeding Course.
In the ruminant, the breakdown of cellulose takes place in the rumen, reticulum and omasum. In the horse this breakdown takes place in the caecum, with the end products, the fatty acids being absorbed through the wall of the caecum. A very small amount of the microbial breakdown of fibre takes place in the caecum of the pig, but it must be stressed that this is hardly significant; the pig can digest only a small amount of fibre.
- 2. ABSORPTION AND UTILISATION
As we have already said, the end products of microbial digestion in the ruminant, the fatty acids that are absorbed through the wall of the rumen, reticulum and omasum, are converted into glucose and carried into the bloodstream. The remaining products pass into the abomasum and from then on they undergo the same treatment as in the case of the non-ruminant. No actual food is absorbed in the stomach and most absorption takes place in the small intestine. The food or chyme at this stage is a fluid broken down by mechanical chewing and churning, and acted upon by gastric juice, succus entericus, pancreatic juice and bile. Lining the inside of the small intestine are the villae, small projections filled with blood, capillaries which lead to the arteries and veins, and lacteals which lead into the lymphatic system. These are illustrated in the diagrams on the next page.
Figure 1: A Single Villus
Figure 2: A Section of the Small Intestine
The villae are bathed in a solution of chyme, and the nutrients which are dissolved in the chyme, either as ions or molecules, pass through the walls of the villae, these walls being made up of semi-permeable membranes. This passage is achieved in a number of ways, as we discussed in Lecture 2. It would be a good idea to look these up and refresh your memory.
The simple sugars, in particular, glucose in the chyme pass through the walls of the villae and into the blood capillaries which carry them to the liver via the hepatic portal vein.
The fats in the chyme, which have been broken down into glycerol and fatty acids, pass in this form through the walls of the villae where they are re-formed into fats and enter the lacteals which lead
into the lymphatic system. From there they enter the blood system and are carried either to the liver or direct to the tissues.
It is important to remember that the sugars, mainly in the form of glucose, pass directly to the liver. Here they are converted by the action of an enzyme into a substance called glycogen which is then stored in the liver. When required in the bloodstream, the glycogen is converted back into glucose and transported around the body. The liver acts as storage and regulatory organ, the amounts of sugar in the blood being controlled by the hormones, insulin and glucagon which are produced by the pancreas, situated close to the liver.
To sum up, the end product of the digestion of carbohydrate is the simple sugar glucose, which is either circulated around the body by the bloodstream or stored in the liver in the form of glycogen. The end product of the digestion of fats is the breakdown into glycerol and fatty acids; the reforming into fats which are either circulated around the body in the bloodstream or stored in the body to act as a reserve.
The protein part of the food is broken down into amino acids by the digestive process, and these pass into the blood capillaries and are carried to the liver.
Mixed with the chime are vast numbers of the rumenal micro-organisms which are continually being flushed through from the rumen complex into the small intestine where they are digested along with the protein from the food.
After passing through the small intestine, where most of the absorption takes place, the remainder of the chyme enters the large intestine made up of the caecum and the colon. In the ruminant and most non-ruminants there is very little in the way of nutrients to absorb at this stage, and the primary function of the colon is to absorb water from the chyme back into the body. Once this has been done the remainder, which is all waste products, passes into the rectum and out through the anus.
UTILISATION OF THE END PRODUCTS OF DIGESTION
If you set fire to a bale of hay, the result is the production of carbon dioxide, water vapour, and energy in the form of heat. After the bale has completely burned, some solid matter in the form of ash is left, and this ash consists mainly of minerals. The animal does this exactly with the food that it eats. It burns it up, producing carbon dioxide which escapes from the body by breathing, water vapour, and energy which is used to heat the body, repair tissues keep the body functions like breathing, the beating of the heart and the involuntary muscular movements going. It is also required for movements like walking and running. In order to achieve this, the animal needs fuel in the form of glucose and oxygen which it gets from the air by breathing. The glucose is carried to the cells where energy is needed, particularly the muscle cells by the bloodstream; the oxygen required is also carried to the cells by the bloodstream, and the reaction takes place inside the cells.
The waste products, the carbon dioxide and any residues are carried away from the cells by the bloodstream, and the carbon dioxide is passed out of the body via the lungs. Every time you breathe out you are expelling carbon dioxide from the body. The energy is used up on the spot. The process sounds a simple one, but getting the right amount of glucose and oxygen to the right place is very complicated.
Glucose is the basic fuel for the body and the amount in the bloodstream is regulated from the liver. Any excess sugar is stored in the liver in the form of glycogen. However, the liver has only a limited capacity for storing glycogen, and any further excess of glucose is converted into fat, and stored under the skin and around some of the body’s organs such as the kidneys and intestines. This fat acts as a
further reservoir that can be converted back into glucose and used when required. As people get older they take less exercise but eat the same amounts of food. They require less energy but still provide themselves with plenty of glucose. The result is that they get fat.
The fats in the body that have been produced by digestion are also converted into glucose, and in fact, produce more energy than carbohydrates. Any excess fats are stored as fat in the body.
The proteins have a different role to play. They are broken down by digestion into amino acids, passed into the bloodstream and are carried around the body.
Some of these amino acids are then built back into proteins and used to form muscle tissue or meat. The proteins form the lean meat while the fat on the meat is stored as carbohydrates. Proteins also form hair, wool, feathers, hooves, horns and fingernails, and they also replace enzymes and hormones that have been used up. Any proteins that are not required for these tasks are taken to the liver where the nitrogen is removed and the remainder is circulated back into the bloodstream and used to produce energy exactly like the carbohydrates. The nitrogen is converted into ammonia gas which is then turned into a chemical called urea; this is then passed out of the body via the kidneys as urine. Excess protein cannot be stored in the body. The body takes what it needs and the rest is used to produce energy or is passed out as urine.
Figure 3: The Utilisation of the End-products of Digestion can be summarised
You must always remember that the breakdown, absorption and utilisation of food is a very complicated process. You have been given a simple version of what takes place.