Normal Digestion

Erin Malone DVM PhD

14 Normal Digestion

Sites of Digestion

There are 5 main “sites” of digestion.

Oral cavity

Chewing and the production of saliva both help with digestion. Chewing breaks down food into smaller components and increases the surface area available for enzymatic digestion. In some species, saliva contains salivary amylase. This compound can start the process of carbohydrate digestion. Dogs, cats, sheep and goats do not have salivary amylase.

People (and many omnivores) have salivary amylase. Chewing starches for a longer period of time makes the food taste sweeter.

Stomach

Protein digestion starts in the stomach. Acid and pepsin break down proteins into polypeptides. Pepsinogen is produced by chief cells and is converted to pepsin by HCl (released from parietal cells). Once activated, pepsin can activate itself. Some animals produce gastric lipase, permitting some fat digestion in the stomach. Salivary amylase is inactivated by the stomach acidity.

Liver/gall bladder

Bile is required to digest fats. Bile salts are released from the liver and help break up fat globules, allowing them to be broken down into smaller components by pancreatic lipase. Without bile, pancreatic lipase can’t do anything.

After monoglycerides and fatty acids enter the cell, they are recombined into triglycerides. The triglycerides are coated in proteins and transported into the lacteals as chylomicrons.

Not all species have gall bladders but they all have biliary systems.

Small intestine and pancreas

The small intestine works similarly across species despite dietary differences. The main function of the small intestine is to move nutrients from the diet into the bloodstream. In a herbivore, this includes the non-cellulose parts of the diet (fats, proteins, carbohydrates from grains or oils, not hay).

Once foodstuffs enter the SI, they are mixed with enzymes and buffers from profuse pancreatic secretions and from the SI brush border. The enzymes (amylase, trypsin, chymotrypsin, lipase, carboxypeptidases, endopeptidases) start breaking down the food products into glucose, amino acids, free fatty acids and monoglycerides. Pancreatic enzymes are released in an inactive form (zymogen) and must be activated. Along with peptidases that can digest proteins directly, intestinal cells produce enterokinase which converts the pancreatic zymogen trypsinogen to trypsin. Trypsin then converts other zymogens to active forms. The process is enhanced by calcium levels in the duodenum. These enzymes work on carbohydrates, fats and proteins.

Motility and digestion

The intestinal wall contracts rhythmically to mix the ingesta with the enzymes (segmentation).

The simpler components are absorbed through capillaries and lymphatics in the intestinal wall where they can be transported to other organs for use or storage. At the same time, the ingesta is moved from the duodenum, through the jejunum and into the ileum. Gradually more and more of the ingesta is exposed to the gut lining and can be absorbed if it hits the right receptors. Most of the simple sugars and proteins are absorbed in the jejunum. If fats are in low concentration, they can also be absorbed in the jejunum. Excess fats are absorbed in the ileum. The ingesta will often be held and mixed in the ileum until the fat content is minimal (see “ileal brake“).

During the process, any structural carbohydrates (hays, grasses, seeds) in the diet are not broken down but are being hydrated by active fluid secretion. Eventually the remaining components are moved into the cecum where microbial digestion will start.

The water exchange that occurs in the SI equals about 1.5x the total extracellular fluid volume of the animal over a 24 hour period. The SI is the main site of water secretion and water absorption in all species. Colonic water absorption occurs (especially horse colons) but colonic water absorption levels are still secondary to the SI.

Digestion by dietary component

Carbohydrates

Carbohydrates are primarily digested by pancreatic amylase in the SI.

https://youtu.be/9HNz_QW838Q

Proteins

Proteins are digested in the stomach (pepsin) and SI (trypsin and chymotrypsin).

Lipids

Lipids must be emulsified by bile salts before they can be broken down in the SI by pancreatic lipase. The breakdown triglycerides are coated with proteins and absorbed into the lymphatic system.

https://youtu.be/NMab4HmJ7bI

Proteins and lipids

Challenge questions

If pandas are really carnivores, how do they eat bamboo?
What makes feces brown? What would feces look like if the animal had no pancreatic enzymes?
Why do some animals eat their own feces? Explain copraphagy and cecotrophy.