Bovine biology series
Part - 9 Small intestine
The small intestine
The small intestine is comprised of three parts: the duodenum, jejunum and ileum. The strange words are derived from Latin, and they help describe either the physical properties or function of these three parts of the small intestine.
I would imagine we can remember back to high school biology when we learned about this part of the digestive system: this is where absorption of nutrients takes place, thus what we eat is transformed into fuel for cellular work.....driving muscular contraction, signaling nerve responses, producing white blood cells when we fight a cold and making all those brain cells work when we had a mid-term exam on our desk!
Let's review this integral part of the digestive system.
We may be astonished at the length of the small intestine, for in the ruminant animal, it can be 140 feet long! Expressed as a ratio of body length to small intestine length, it is about 20 times longer than the length of cow at seven feet. In terms of capacity, the small intestine can hold at most about 15 gallons of digesta. This amounts to slightly more than one gallon per ten feet of small intestine length, or just under a pint (16 fluid ounces) per foot. This digesta will weight, then, about one pound per foot, and if the small intestine is completely full at capacity, then the contents of 15 gallons will weigh about 130 pounds! Or 60 kilograms.
How fast does this digesta move through this long, narrow tube? Well, that is the stuff of nutritional conversations every day, for if the movement is too fast, then absorption is reduced with possible digestible nutrients falling out the back end of the cow in the feces. An economic waste, to be sure. And if the movement is too slow caused by a lack of dry matter intake or impairment of the nervous stimuli that must contract the muscles of the small intestine, initiating peristalsis, then absorption of nutrients is probably in fine shape, but the total nutrient load absorbed in a period of time is insufficient, thus the fuel demands of cells in order to accomplish body functions, including milk production, maintenance of a fetus, keeping the immune system healthy, and so on is compromised.
So it is that when we walk down the free stall alley we smear some fresh manure around with our boot. What we are doing is looking at two things: consistency and content. Consistency tells us something about motility, in that a ration too succulent, lacking in fiber and thus travelling rapidly through the gut is deposited out the back end of cow in a less viscous manner. In other words, as soon as we feed green chop or pasture and back off on expensive Eastern Oregon hay, then we dare not stand too close to Bossy when she lifts her tail! Conversely, a ration that is found in heifer or far off dry cow mangers will take longer in travelling this small intestine, and thus ample opportunity for water reabsorption occurs, and the manure is, well, thicker and less liable to contain excess potential digestible nutrients.
To put some ballpark numbers with this discussion, material in the ration will show up in the feces as early as 12 to 24 hours depending upon particle size and moisture level. Again, the pasture ration and green chop will appear in less than 24 hours. About 80% of the entire ration is through the cow's gut in 3 to 4 days. It is completely out in 7 to 10 days.
Again, the primary function of the small intestine is absorption of nutrients into to bloodstream. But at the duodenum, there is the influence of other exocrine organs. The pancreas secretes pancreatic juice, along with bile from the gallbladder and liver, and enter the duodenum via the bile duct. These collectively are called the bile salts and as alkaline buffers begin to emulsify or put into suspension the lipid material of the ration that has reached the small intestine. These bile salts hydrolyze, or reduce in size these lipid molecules into fatty acids and glycerol. They also function in carbohydrate metabolism by hydrolyzing multiple sugar molecules into shorter molecules, such as the conversion of starch to maltose and then to glucose, which can readily be absorbed into the bloodstream and carried to the liver. One other function of bile salts is that they contain enzymes that hydrolyze protein molecules into peptides and amino acids, thus serving the role of taking large macromolecules, those really big particles of the ration that have survived the work of reticular-rumen microbes, and breaking them down into smaller ones that can fit into the absorptive mechanism of the intestinal wall, and into the bloodstream.
So it is in the first part of the small intestine, the duodenum, that an alkaline substance produced by related digestive organs is transported via the bile duct and during the mixing and motility of the digesta, lipids, carbohydrate and protein portions of the ration are acted upon by these enzymes so that they can be absorbed into the bloodstream.
The second and third portion of the small intestine, the jejunum and ileum, are responsible for further absorptive function of nutrients that are now small enough to be absorbed by the villa that line the small intestine.
It is in these villas that some of the pancreatic juices are found, thus continuing the hydrolyzing action of large molecules. You may remember seeing photographs of these very small, fingerlike villa, jutting out from the membrane of the muscular-walled intestine. These greatly increase the surface area of the intestine, increasing the opportunity for nutrient absorption. These villa structures are continuously being replaced in the healthy gut, thus assuring nutrient uptake. It is diseases like coccidiosis that impair the ability of villa renewal, replacement and repair. Animals never seem to overcome this malady, and we all know that as poor-doers they are usually culled from the herd.
The fate of many of these absorbed nutrients is ultimately the liver, the organ responsible for partitioning nutrients for either immediate use or storage. Some nutrients such as minerals and water, as well as food nutrients, are absorbed directly into the lymphatic system for transport to the venous blood system, then directly to the heart for distribution throughout the body.
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