Gut 101 - Condensed

Overview

To understand what goes wrong and why, we first need to understand how things work. Understanding constipation requires understanding gut function.

The digestive tract starts at the mouth and ends at the anus with the esophagus, the stomach, the small intestine, and the large intestine in between, all hooked together in a long continuous complex tube. The continuity can be seen in the rollover above; let your mouse pointer hover over the top cat and follow the red trail from one end of the underneath cat to her other end. While it is easy enough to imagine the course as a marble run, no swallowed marbles allowed even though, as we learned in The First Lesson, inside that tube is not inside the body proper.

The digestive system includes organs that are actually inside the body but work closely with the digestive tract – the liver, the gall bladder and the pancreas. Not all those organs are pictured above and our focus is on the digestive tract, not on the internal digestive organs.

The inner lining of the entire digestive tract consists of mucous membrane. Mucous membranes secrete mucous from special glands which are particularly abundant in the gut. Mucous keeps passages moist and serves other purposes including playing a role in immune function. It also makes the digestive tract self- lubricating.

Like the walls of the bladder, blood vessels and other internal organs, the gut wall is composed of smooth muscle cells whose action is involuntary, that is, it is not under conscious control. Once a bite of food is swallowed, the gut's very own nervous system, the enteric nervous system or ENS, takes over control from the central nervous system. A remarkable system, the enteric nervous skystem, governs the digstive tract. The enteric nervous system is a huge affair; its cells make up a majority of the peripheral nervous system!

It is sufficient to know that this vast network of nerves exists, dedicated to informing and supporting the gut. Attention to the health of the ENS plays a role in preventing constipation.

The gut wall is more than a passive barrier between the lumen (the inside space of the intestinal tract) and the inside of the body; it is an active and dynamic organ.

Food

Food provides nourishment for the body to fuel, build, repair and maintain itself and all its parts. Food consists of proteins, fats, carbohydrates and fiber plus vitamins and minerals. And, of course, food contains water so nourishes and hydrates. Food (and water) has kept life going on Earth for millions of years. Food is life.

A quick note about the body's water content. Water is everywhere in the body, it is a given. Even when not specifically mentioned, we should imagine water as part of the equation, whether discussing food or digestion or absorption. Water is the major component of blood and of the interstitial fluid and other body fluids, the fluid outside the blood vessels.

The nourishment in food is locked up in complex molecules that,

before digestion, are too large to get through the barrier of the gut

wall into the blood stream and thus into the body. First food must be

digested.

As we learned in The First Lesson, the inside of the gut is not the inside of the body, it is the inside of the gut. The gut is open to the outer world at both ends and the world is not a sterile place. Stomach acid evolved about 350,000,000 years ago, in species who developed digestive tracts, in order to help protect the eater from incoming pathogens. Under normal circumstances, stomach acid kills incoming bacteria.

Before food can nourish, it must be digested so it can be absorbed into the body.

Mouth

A cat may or may not chew a bite of commercial cat food much

before she swallows it. Cats on commercial diets do not tend to chew enough to make as much use of saliva as when cats were hunting for themselves.

Still saliva moistens and lubricates food and saliva contains buffers which can alter the pH of the mouth’s contents and protect the esophagus. We often see a cat swallow several times in quick succession before vomiting. Stomach contents are highly acidic and the esophagus is not protected from contact with acid as the stomach is protected. When vomiting threatens, more saliva is produced, saliva to be swallowed to temporarily coat the esophagus against the acid bath soon to follow. We experience the same, our mouths 'water' and we swallow extra saliva before we vomit.

Saliva-coated food does not quite touch the walls of the esophagus, it is separated from direct contact by a thin coating of saliva.

The cat swallows the bite and the food moves through the upper esophageal valve or sphincter in the back of the throat into the esophagus.

A note here about oral bacteria: Bacteria live in everyone's mouth and cats are no exception. Bacteria also need to eat, using their own enzymes to digest or ferment food. Oral bacteria do not ferment protein or fat, they ferment carbohydrates. There is often heated debate as to whether kibble does or does not clean teeth and which types of food contribute to dental decay in cats. Bacteria in organized groups form plaque, which then turns to tartar. Their fermentation by-products are irritating to mouth tissue, especially to the gums if the bacteria manage to take up residence below the gumline. The gums response is to inflame which pulls them away from their tight fit against the teeth, leaving more space for tartar buildup. Bacteria have eons of practice at meeting their needs. They also need food to live and reproduce, just as the rest of us do.

Food that does not remain in the mouth is not available to ferment in the mouth.

The cat's jaw is wired for up-and-down movement only, no sideways movement, so their ability to retrieve stray food particles and residue is very limited. You can check this out yourself by trying to run your tongue around the outsides of your teeth without allowing your jaw to shift to the side.

Esophagus

Once in the esophagus, the swallowed food is inched down by peristalsis, a snake- like action of the esophageal wall that moves in one direction. Many people are surprised by the length of the esophagus.

For cats, all solid medications and supplements should be 'chased' with sufficient food or water to ensure they make the long journey successfully and do not become entrapped in the esophagus. Chasing liquid medications is also a nice courtesy to avoid potential irritation of the tender mucosa.

When food or liquid reaches the end of the esophagus, the lower esophageal or cardiac valve opens to allow access into the stomach.

This is a good place to mention the difference between vomiting and regurgitation. Vomiting is the forceful ejection of acidic stomach contents. Regurgitation is a gentler release of non-acidic esophageal contents and the food retains the shape of the esophagus, looking like a sausage or even sometimes mistaken for poop.

Stomach

Once food is in the stomach, the work begins. The stomach

environment is very acidic but the stomach wall is protected from contact with acid by a thick layer of mucous. The strong stomach musculature kneads and blends, with assistance from gastric rugae, folds of the stomach wall which boost action like the fins of a washing machine agitator, churning and kneading and smushing the food mixture into a smooth thick soup called chyme. This action is mechanical digestion which starts the breakdown process.

The rugae, those folds of the stomach wall, also allow the stomach to expand in response to a volume food. Although we think of the stomach as a digestive organ, which is it, it is mainly a storage organ which allows us to eat more at one time than without a stomach. Digestion can be accomplished without a stomach, the small intestine can do the job on its own. But if we were entirely dependent on the small intestine for digestion, we could eat only a very small amount at one time, we would have to eat almost continuously, and digestion would take longer without the stomach's mechanical digestion and predigestion of protein as we learn below. Holiday meals would be impossible and a cat in the wild would be unable to eat the whole mouse while opportunity is at hand, or paw as it were.

Without a stomach's acidic environment, all of us, cats and humans alike, would be more subject to food-borne illness and bacterial overgrowth.

Additional water can be requested from the body stores if there was insufficient water in the food, as well as more gastric acid to maintain a desirable pH. Although the gut wall acts as a barrier, permissible traffic can go both ways.

Stomach acid, in addition to killing incoming bacteria, 'denatures' protein; it unfolds the amino acid chains that make up protein for better access by pepsin. Pepsin is an enzyme synthesized by special glands in the stomach.

Enzymes break down larger molecules into smaller and smaller molecules.

Protein digestion is completed in the small intestine. Carbohydrates and fats wait until the small intestine for their chemical digestion; in the stomach they are blended and smushed by mechanical digestion.

When the chyme is sufficiently liquified and blended into proper consistency, it is released bit by bit through the pyloric valve into the small intestine.

On to Gut 102 – Full Version . . . . . .

Or Gut 102 – Condensed

Gut 101 – Condensed - From Mouth to Stomach

Quick Review

The digestive system includes organs that actually are inside the body but work closely with the digestive tract – the liver, the gall bladder and the pancreas.

Mucous makes the digestive tract self lubricating.

The gut wall is composed of smooth muscle cells whose action is involuntary.

The enteric nervous system is a huge affair; its cells make up a majority of the peripheral nervous system!

Before food can nourish, it must be digested so it can be absorbed into the body.

Food that does not remain in the mouth is not available to ferment in the mouth.

Food provides the nourishment needed by the body to fuel, build, repair and maintain itself and all its parts.

Many people are surprised by the length of the esophagus.

For cats, all solid medications and supplements should be 'chased' with a small amount of food to ensure they make the long journey successfully and do not become entrapped in the esophagus. Chasing liquid medications is also a nice courtesy to avoid potential irritation of the tender mucosa.

Enzymes break down larger moleculels into smaller and small molecules.

Modified from http://commons.wikimedia.org/wiki/Media:Scheme_cat_anatomy-en.svg