Essay: How the Digestive System has Evolved to Cater for different Diets

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  • How the Digestive System has Evolved to Cater for different Diets
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Digestive systems have evolved to cater for the diets of different species, resulting in varied ways of digesting food. The human and the cow, having different dietary needs, thus have different types of mouths, dental structure, stomachs and enzymes secreted to digest their food.

The first stage of a cow’s digestive tract starts with the bovine mouth, which is a hollow cavity through which food enters the body. The tongue, the cheeks, and the soft and hard palate. Most mechanical digestion takes place within the mouth, using teeth to grind food into a smaller, digestible piece called the bolus. The roof of a cows mouth contains a hard dental pad as apposed to the incisors of humans, which they used their bottom incisors and canines to grind against. Cows, like humans, have 42 teeth, yet cows have a large gap between the incisors and the molars and premolars, which crush and grind plant material during initial chewing and rumination. The chemical processes within the digestive system begins with the cow’s saliva, which aids in chewing and swallowing, while also containing the same enzyme in humans, amylase, that breaks down fat and starch, a cow’s saliva is also involved in nitrogen recycling to the rumen, as well as buffering pH levels in the reticulum and rumen.

The second stage in the digestive system of a cow is the oesophagus, which is a relatively straight muscular tube through which the bolus passes from the pharynx to the stomach. Anatomically, in a human, it lies behind the trachea and heart and in front of the spinal column; it passes through the muscular diaphragm before entering the stomach. Contractions of the muscles within the oesophageal wall, called peristalsis, move the bolus down the oesophageal tube and into the stomach. Because cows eat so rapidly, swallowing much of their food without chewing it sufficiently, the oesophagus functions bi-directionally in ruminants, allowing them to regurgitate their cud (improperly chewed food) for further chewing.

The third stage within the bovine digestive system, is the stomach,, which aids in digestion in both mechanically, by mixing and crushing the food, and also chemically, using microorganisms to digest food. A cow has one stomach with four compartments: the rumen, reticulum, omasum, and abomasum, the stomach connects the oesophagus, to the first portion of the small intestine.. The first compartment of the stomach, and also the largest, is the rumen, which acts as a fermentation vat for partially digested food by hosting microbial fermentation. It is lined with papillae for nutrient absorption and divided by muscular pillars into the dorsal, ventral, caudodorsal, and caudoventral sacs. Home to a population of microorganisms that include bacteria, protozoa, moulds and yeasts, it is these microorganisms living in a symbiotic manner, that allows the cow to eat and digest large amounts of varied feed. These microbes ferment and break down plant cell walls into volatile fatty acids, these fatty acids are absorbed directly through the rumen wall and supply 60 to 80 % of the energy needed by the cow. While a bovine stomach contains multiple compartments, the human stomach is one, J-shaped muscular bag. As apposed to using microorganisms to digest food, as bovines do, the human stomach uses gastric enzymes, hormones or compounds such as pepsin, or hydrochloric acid to digest food.

The second compartment within the stomach is the reticulum, the main function of the reticulum is to collect smaller bolus particles and move them into the omasum, while the larger particles remain in the rumen for further digestion. The reticulum, is mainly involved with rumination or ‘cud-chewing’. When cattle ruminate, they are regurgitating a bolus of incompletely chewed food, for the microbes within the reticulorumen to digest fibre rapidly and efficiently, it must be further chewed and mixed with saliva.

The omasum is the third compartment within the stomach, it is spherical and connected to the reticulum by a short tunnel. The omasum is coined, the butcher’s bible, due to the many folds within the omasum which resemble the pages of a book. These folds of its mucosa increase the surface area, and trap digesta particles, aiding to the efficiency of the omasum. The omasum primarily aids in the absorption of water, magnesium and the volatile fatty acids that have not yet been absorbed in the bloodstream, from rumen fermentation. from feed and water.

The abomasum is the “true stomach” of a ruminant, as it is the compartment that is most similar to a stomach in a non-ruminant. The abomasum produces hydrochloric acid and digestive enzymes, such as pepsin (which breaks down proteins), and receives digestive enzymes secreted from the pancreas, such as pancreatic lipase (breaks down fats). These secretions help prepare proteins for absorption in the intestines. The pH in the abomasum generally ranges from 3.5 to 4.0. The chief cells in the abomasum secrete mucous to protect the abomasal wall from acid damage. The small and large intestines follow the abomasum as further sites of nutrient absorption.

The 4th stage within the bovine digestive system, is the small intestine, which is a tube up to 150 feet long with a 20-gallon capacity in a mature cow. Semi-digested food entering the small intestine mix with secretions from the pancreas and liver, which elevate the pH from 2.5 to between 7 and 8. This higher pH is needed for enzymes in the small intestine to work properly. Bile from the gall bladder is secreted into the first section of the small intestine, the duodenum, to aid in digestion. Active nutrient absorption occurs throughout the small intestine, including rumen bypass protein absorption. The intestinal wall contains numerous “finger-like” projections called villi that increase intestinal surface area to aid in nutrient absorption. The same muscular contractions from the oesophagus, called peristalsis aid in mixing digesta and moving it to the next section.

As apposed to using microorganisms to digest food, the human stomach uses enzymes and pancreatic juices, which are are produced by particular cells in the pancreas. Inside the pancreatic juice are three important enzymes which are responsible for breaking down carbohydrates, fats and proteins. These enzymes are called amylase, lipase and protease. After the pancreatic juice is created in the pancreatic ducts, it travels through the pancreas and into the major and minor papilla where the juice enters the duodenum and is mixed with the chyme. Bile is produced by the ducts inside the liver. Once the bile is produced, the bile travels through the ducts until it reaches the cystic duct and becomes stored in the gallbladder. When digestive occurs, the bile travels back through the cystic duct and down through the common hepatic duct to the duodenum, where it is mixed with the chyme and pancreatic juices. A small portion of the pancreas, (1-2%) called islets of Langerhans, contain alpha cells which secrete glucagon and beta cells which secrete insulin. Insulin works by increasing the uptake of glucose from the blood across cell membranes and into the cells of the body, and so takes glucose out of the bloodstream. Insulin is released from the beta cells in your pancreas in response to rising glucose in your bloodstream. Once in the cells, the glucose is used as the energy to fuel the cells doing their different jobs or is stored in the liver or muscle cells as glycogen. As a result, the glucose level of the blood drops, which then triggers the pancreas to switch off the release of insulin.

The large intestine is the 5th stage within a cows digestive system, absorbs water from material passing through it and then excretes the remaining material as feces from the rectum. The cecum is a large blind pouch at the beginning of the large intestine, approximately 3 feet long with a 2-gallon capacity in the mature cow. The cecum serves little function in a ruminant, the colon is the site of most of the water absorption in the large intestine.

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