Digestion & Physiology Part 3: The Small Intestines

The small intestine is named in reference to the fact that the intestinal diameter is small compared to the large intestine. It is about 20m long, narrow in diameter, and has an average volumetric capacity of about 50ltrs, relating to a 500kg horse. Food moves through the small intestine fairly quickly, so digestion needs to occur rapidly, or it will pass on to the large intestine undigested. 

 

The small intestine is where most nutrients are digested and absorbed so it is a significant site of chemical digestion. The stomach has a poor ability to digest starch and consequently most starch passes undigested into the small intestine. Here, starch is digested by amylase (digestive enzyme) secretion into the small intestine via the pancreatic ducts, however while this process can be very efficient, resulting in up to 90% of starch digested, the amount of amylase available is small, which is one of many limiting factors on how much starch is digested and absorbed.

 

Taking a closer look at starch, it is a variation of a chain of glucose molecules (a type of sugar), making it about 3 times more resistant to digestion than ordinary sugars. This again limits the amount of starch digested and absorbed in the small intestine, increasing the risk of a starch overload in the large intestines and with that potentially causing conditions such as colic or laminitis.

 

When talking about equine nutrition, it is a key fact to note, that horses have a limited ability to digest and absorb starch in the small intestines.

 

Image Credit "Horse Digestive Tract": www.exploringnature.org 
A very special thank you to Sheri Amsel for allowing EEN to use this artwork.

 

Looking at the small intestine, it is broken up into three main parts: the duodenum (illustrated in yellow), which is closest to the stomach, the jejunum (illustrated red), followed by the ileum (illustrated red / orange).

 

As illustrated above, the duodenum is the part at the end of the stomach, leading into the jejunum. It is only a very small section, about 1 metre long. It is primarily designed to increase the pH of the feed content. 

 

The feed at this point no longer looks anything like what was given to the horse. It is now in a sort of semi digested state, after leaving the stomach, called chyme. The normal pH of the jejunum is around 7 to 7.4. Remembering the stomach has a pH of 2, a significant increase in pH has to occur in this short section of the small intestine (from 2 to 7). This happens through large quantities of bicarbonates being pumped into the duodenum from the pancreas via two pancreatic ducts.

 

The jejunum is the next section of the small intestine and it specialises in absorbing carbohydrates and proteins. It has a considerable amount of villi and micro-villi brush borders. Villi secrete enzymes into the small intestine, digesting the feed. They are also the point where the digested nutrients are absorbed into the body for use. These villi are microscopic in size, and they look like tiny fingers poking out of the wall of the body of the intestines. The jejunum is a much larger section, about 18 to 20 m long. It is here, where much of the nutrients are digested and absorbed so the chyme needs to stay in here for as long as possible to allow time for digestion and absorption to occur. Chyme passes through this section at an average rate of about 30cm per minute.

 

At the end of that section you have the ileum which is the last section of the small intestine. It is about a meter and a half in length with a small amount of absorption occurring here.

 

The small intestine has 4 main functions:

 

  1. It is involved in both chemical and physical digestion of the different components of the feed through the secretion of enzymes and physically through the constant mixing of the feed particles in the small intestine. This mixing causes the particles to collide and rub against each other, breaking down into smaller, more digestible units.

  2. It is a point of absorption for vitamins, minerals, and bile-salts which are recycled to be reused in buffering against the acidic chyme leaving the stomach.

  3. Electrolytes of sodium, chloride, potassium, and water are being secreted into the digestive tract to assist in the digestion of the nutrients and their transport into the body.

  4. The small intestine mixes together all the nutrients from the feed along with digestive enzymes and the microbes residing in the small intestine, all of which are designed to improve digestion and absorption.

 

The small intestine pushes the mixed and digesting nutrients along the digestive tract towards the large intestine through continuous peristaltic waves of muscle contractions and relaxations.

 

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