The Digestive System

Types of Digestive Systems

Regarding the digestive system, the domestic animals are classified into two main groups:

Monogastric (“one stomach”)

Monogastric (Non-ruminants) such as pigs, dogs and cats have a simple stomach, which can hold relatively little food, and they must therefore take in food in a concentrated form. The digestive process is totally dependent on acids and enzymes. They do not however, have an enzyme which can digest cellulose, so this substance cannot be utilised by these animals.

The stomach of a monogastric

Herbivores

Non-ruminants e.g. the equine family.

The horse has a relatively small stomach, but its large intestine (caecum) can accommodate a large volume of food. Cellulose digestion takes place under the influence of micro-organisms in the caecum of the horse.

The digestive stomach and lower digestive tract of the horse

Ruminants e.g. cattle and sheep Food reaches the rumen where digestion takes place under the influence of micro-organisms.

The nutritive value of grass is relatively low. so the herbivore is forced to take in large quantities of it. The end products of digestion by the micro-organisms are of great importance to these animals.

The Organs and Structures of the Digestive Tract

Lips and Tongue

In Horses: The lips are strong, sensitive and mobile. During grazing the lips collect the grass, which is then cut by the incisor teeth. When feeding on concentrates the tongue is also utilised to collect the food.

In Cattle: The lips have a very limited mobility. The tongue has hard protrusions pointing backwards on its dorsal surface. The tongue is curled around the grass and brought into the mouth where it is pressed against the dental pad by the incisors. The incisor teeth cut off the grass with a quick upward jerk of the head.

In Sheep: Sheep have split upper lips, which make short grazing possible. The grass is taken between the incisors and dental plate and cut off with an upward jerk of the head. The upper lip of goats is not split but flexible to allow for browsing.

Teeth

The teeth are classified as follows:

• Incisors
• Canines
• Premolars
• Molars

The function of the incisors is to cut the food. In carnivores the canine teeth help to grip the food or to tear it off. The function of the molars and premolars is to chew food to a fine consistency. During grazing ruminants do not chew their food thoroughly but swallow it after a few cursory chews. Later, when chewing the cud (rumination), the molars and premolars are used. The mastication of food is necessary to:

• Expose a greater surface of the food to digestive enzymes in the stomach and small intestine to improve digestion;
• To thoroughly soak it with saliva and thus facilitate swallowing.

By examining the teeth of an animal, the age (in years and/or months) can be determined. It cannot, however, be determined with absolute accuracy. Various changeable factors influence the eruption of both temporary and permanent teeth as well as the wear of the teeth.

In general, the temporary deciduous incisor can be distinguished from the permanent incisors by the following. Each temporary incisor has:

• A definite neck
• Is smaller, smoother and whiter
• Various ridges and shallow grooves while the permanent incisors only have one or two clear grooves.

In the following table, the average age of teeth eruption in the various domestic animals is given.

Key:

Di = Deciduous incisor

I = Permanent incisor

Dc = Deciduous canine

C = Permanent canine

Dp = Deciduous premolar

P = Permanent premolar

M = Molar

Salivary Glands

The salivary glands secrete the fluids, which mix with food during the mastication process, and this facilitates swallowing. Ruminants secrete large volumes of saliva (cattle approximately 55 litres per day). This saliva contains sodium bicarbonate which serves to neutralise the acids formed in the fore stomach. Saliva of humans and pigs contains the enzyme ptyalin, which breaks down carbohydrates to maltose. Because ptyalin is active only in an alkaline medium, its action is inhibited almost immediately in the acid medium of the stomach. This enzyme thus has limited significance to digestion.

Oesophagus

The oesophagus connects the mouth cavity and stomach. It is a tubular structure lined on the inside by a mucous membrane, which again is surrounded by involuntary muscle.

Stomach

Simple Stomach: The functions of the stomach are as follows:

• Digestive processes under acid conditions.
• Produces the “intrinsic factor” necessary for the absorption of vitamin B12 by the small intestines.

The stomach wall is lined with a membrane containing many small glands. These glands are responsible for the secretion of hydrochloric acid, which creates the acid conditions in the stomach. Other glands in the stomach wall secrete various digestive enzymes. by the small intestine.

Compound stomach (ruminants): In the stomach of the ruminant four definite compartments occur i.e. the rumen, reticulum, omasum and abomasum. The first three develop as evaginations of the oesophagus whilst the abomasum can be likened to a simple stomach. Secretions of the abomasum also resemble that of the simple stomach.

Definition: Evagination – An out-pouching of a layer or part of (in this case the oesophagus)

Small Intestine

The ducts of the pancreas and gall bladder open into the small intestine near the junction of the stomach and small intestine. The small intestine consists of the duodenum, jejunum and ileum and goes over the large intestine at the ileo-caecal valve. As a rule, the length of the small intestine is approximately 7 metres long. In ruminants the small intestine can be much longer between 25 to 28 meters in sheep.

Large Intestine

Except in the horse, in which cellulose digestion takes place in the large intestine, water, which is necessary for the digestive processes, is absorbed here mainly in the caecum. The function of the last part of the large intestine i.e. the rectum is mainly to collect faeces before it is passed out through the anus.

The Digestion of Food in an Animal with a Simple Stomach

Digestion in the Stomach

In farm animals with a simple stomach, such as the pig and dog the following enzymes help with the digestion of food:

Pepsin: The cells of the mucosa of the stomach secrete the enzyme pepsin for protein digestion. Before pepsin can have any proteolytic effect, it has to be activated by the presence of hydrochloric acid. The activated pepsin then breaks down proteins to peptides. Food does not, however, stay in the stomach long enough for this process to be completed and enzymes in the small intestine has to complete this process.

Lipase: Only small quantities are found in the stomach juices. This is an enzyme which breaks down fats to fatty acids.

Hydrochloric acid (HCI): Activates pepsin and rennin and curdles milk.

Digestion in the Small Intestine

Pancreatic juice: The following enzymes are the most important ones formed/produced by the pancreas:

Trypsinogen: This is changed to trypsin by enterokinase, which is secreted by the small intestine. Trypsin breaks down protein to peptides and amino acids. Before trypsin can react, the medium in which it is dissolved must be alkaline and the proteins must be digested to a certain stage.

Pancreatic lipase: This enzyme hydrolyses fats to fatty acids and glycerol. Most of the fat is absorbed in this form in the small intestine.

Pancreatic amylase: This breaks down carbohydrates to maltose. The presence of bile possibly improves the action of amylase.

Intestinal juice: Intestinal secretions also contain various enzymes, which, as is the case in the previous group, are responsible for the final steps in the digestion of:

• Proteins and peptides to amino acids,
• Compound sugars to glucose and fructose.

The Digestion of Food in a Ruminant Stomach

The Digestion of Milk in the New-born Ruminant

Rennin: The enzyme rennin is found in the stomach juices of calves and possibly also in other young ruminants and causes milk to curdle. It is also activated by hydrochloric acid.

In the new-born calf or lamb, digestion in the stomach is similar to that of the monogastric animal. By virtue of the oesophageal groove, milk consumed by-passes the rumen and reticulum and arrives directly in the abomasum. The milk is then coagulated by rennin and digested as in animals with a simple stomach. At the age of two weeks, calves and lambs will start to nibble and chew pieces of roughage. This intake of roughage is necessary for the development and functioning of the fore-stomachs i.e. rumen, reticulum and omasum.

Digestion of Cellulose

After thorough mixing of the food in the rumen the cellulose is broken down by ruminal organisms to fatty acids. These organisms have an optimal activity and multiply only at a pH of 5, 0 to 7, and 0. The fatty acids that are produced, of which acetic acid and butyric acid are the most important, are inclined to increase acidity (lower pH) of the rumen, which would have an adverse effect on the micro-organisms. The sodium bicarbonate secreted by the salivary glands act as a protective agent by neutralizing the acids and keeping the pH constant.

Click here to view a video that explains cellulose digestion.

The Fatty Acids thus Formed are Then

• Directly absorbed through the ruminal wall into the blood, or
• Moved down to the abomasum and small intestine to be absorbed there, or
• Taken up by the ruminal organisms themselves.

Small quantities of sugar (2 percent) stimulate a more effective digestion of cellulose. When too much sugar is present this process is inhibited because the organisms would rather attack sugar than cellulose. If enough proteins are present, however, up to 6 percent of sugar can be fed with good results (Usually in the form of molasses or bagasse – derivatives of sugar cane).

Carbohydrates can be absorbed directly by the organisms for the production of glycogen or can undergo fermentation by yeast cells, which are one of the types of ruminal micro-organisms. Gasses, especially carbon dioxide (CO2), are formed by this process and they collect on top of the fluid layer in the rumen.

The yeast cells synthesize vitamins such as vitamin B1 (thiamine) and vitamin B12 so that these vitamins need not be included in ruminant rations.

Digestion of Proteins

The micro-organisms in the rumen of the ruminant can make use of two sources of nitrogen to build up their body protein. They either use free nitrogen in the form of non-protein nitrogen or protein nitrogen from their rations. Some micro-organisms have the ability to synthesise protein out of free nitrogen and build up their own systems with these proteins. The organisms are themselves later digested by the ruminant in the abomasum.

The ruminant then utilizes the synthesized microbial protein to its own benefit. The ruminant cannot use nitrogen unless enough easily digestible sugar is available to convert it to protein. This is why urea, which is a good source of non-protein nitrogen, can be used successfully under the right conditions. In the absence of enough sugar, however, urea is converted to ammonia, which is very poisonous. Sugar as such need not be present in the ration. The ruminant can produce its own sugars if enough carbohydrate is present.

The appetite of the ruminant is directly related to the activity of the ruminal microbes. Conditions, which might detrimentally affect these microbes, are:

• Sudden changes in the ration
• Lack of green feed
• Lack of water
• Changes in the pH of the rumen
• Poor quality feed e.g. during the winter
• Phosphate deficiency
• Dosing with antimicrobial drugs

Luminal movements and the activity of ruminal organisms are also interdependent. If there are not enough active organisms in the rumen, the ruminal movements will cease (rumen stasis). This condition is commonly known as “dry gall sickness”.

Treatment for these conditions is:

• Sufficient water
• Acetic acid (vinegar) – to restore the pH of the rumen
• Sugar or glycerol – an energy source for the organisms
• Brewer’s yeast – replacement of yeast cells in the rumen
• Fresh ruminal fluid can also be dosed A simple preventative measure is to make any change in the diet gradually so that the micro-organisms can adapt to the changes.

Eructation

The oesophageal opening is not situated at the highest point in the rumen so that the surface of the fluid layer lies above this opening. During the eructation movement of the rumen, the reticulum relaxes to take a greater volume of fluid. The height of the fluid is thus lowered, and it allows gas to escape.

The Kidneys

The functions of the kidneys:

• The kidneys excrete substances, which are formed by metabolism, especially nitrogen- and sulphur-containing substances.
• Conserve the water equilibrium of the body, especially with respect to plasma volume.
• Regulate the acid-based equilibrium (pH) of the body by excretion of non-volatile acid and base radicals.
• Control blood pressure.
• Excrete poisonous substances and medicines, which had been taken in by animal.
• Form specific substances such as ammonia and hippuric acid. The following diagrams show the cross-sectional structure of the kidney and its internal structure.

The structure of the kidney

The microscopic structure of the kidney

The following could result from chronic kidney damage:

• Increase in the volume of urine
• Decrease in the concentration of urine
• Protein in the urine
• Presence of glucose and other substances in the urine
• Dehydration

Substances such as glucose can, however, be excreted in the absence of kidney damage e.g. in the case of diabetes mellitus in human beings. The reason for this is that blood glucose reaches such high levels that total resorption cannot take place.

The urine produced, leaves the kidneys through the urethras. The urethras join the bladder, where urine is stored, until passed out through the single urethra.

The Endocrine System

The endocrine system is a system of ductless glands, which secrete chemical substances, called hormones, directly into the blood system. Via the blood circulatory system, they come into contact with the target organ(s) on which they have a specific effect.

Hormones have the following general characteristics:

• They regulate reactions rather than initiating them
• They are effective in minimal quantities
• Their levels fluctuate according to demand

The latter characteristic is necessary for integrated systems, which handle the different requirements of growth, sexual differentiation, reproduction and adaptation to environmental changes.

Sheep Endocrine