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 system, governs the digestive tract. The enteric nervous system is a huge affair; its cells make up a majority of the peripheral nervous system! Older anatomy texts such as Gray's Anatomy make no mention of this vast system because its confirmation is so recent.
A division of the peripheral nervous system (PNS) as opposed to the central nervous system (CNS), the ENS can run on its own if need be. It is worth pausing to ponder this remarkable system. Here is what one pioneer researcher, Michael D. Gershon, MD, wrote eloquently about the ENS in his book, The Second Brain:
Keep the general concept, if not the details, of the enteric nervous system in mind. 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 also contains cells called Interstitial Cells of Cajal or ICCs. These specialized cells act as pacemakers for the rhythm of the gut wall's contractions, contractions whose timing varies from section to section. Research of ICCs is ongoing, science doesn't know everything about them yet, but we could think of them as dance choreographers.
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 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. Food is not always sterile, it certainly was not sterile when cats were on their own in the wild. Yet the actual inside of the body needs to be protected from bacteria. 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.
How to turn food into cat? We do not staple or sew a piece of chicken into a cat. First the chicken must be digested, broken down into the smallest elements, absorbed into the body, and there put to use for the cat’s own purposes. It is not like ship-in-a-bottle, the cat does not recreate chicken after getting the small bits inside, she makes cat out of chicken.
Before food can nourish, it must be digested so it can be absorbed into the body.
A cat may or may not chew a bite of commercial cat food much before she swallows it. Saliva, a mixture of mucous, water, electrolytes and enzymes, has enzymes with antimicrobial properties which can protect against some food-borne bacteria. Cats on commercial diets do not tend to chew enough to make as much use of saliva as when cats were hunting for themselves. A mouse takes more gnawing and the section of mouse the cat is gnawing spends more time in contact with saliva.
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 action of the teeth, tongue and mouth help to shape and prepare the food for swallowing, which is a surprisingly complex and delicately coordinated process. The pre-processing of food in the mouth, even kibble, prepares it for the trip ahead.
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.
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. As the illustrations above depict, it runs from the back of the mouth down through the neck and chest and passes through the diaphragm before connecting with the stomach.
When the esophagus is empty, the walls collapse forming a closed space. They open and expand in response to swallowed foods and liquids.
The esophagus evolved in concert with water and food which not only move along well in response to peristalsis but make it easier for the esophagus to move them along because they are not unyielding like tablets and gelcaps. There's a reason we're told to swallow pills with a full glass of water.
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. Closing securely behind is important to prevent 'heartburn', so nothing from the stomach comes back up into the esophagus. The lining of the esophagus is not designed for exposure to stomach acid or for anything to linger long there.
Still sometimes stomach contents do come back up if a cat vomits. When vomiting is imminent, an excess of saliva is produced. The swallowed saliva temporarily protects the esophagus with a coating of buffered fluid so the esophagus is not burned by stomach acid. We humans do the same; we feel our mouths 'watering' before we vomit.
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.
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 it is, 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.
Denaturing protein is akin to chemical hair straightening since stomach acid relaxes the folds or curls just as hair straightener relaxes the hair. Hair is, after all, made of a special kind of protein called keratin.
Below is a representation of one protein in its folded state, on the left, and in its denatured state on the right. I have reversed the original image for this purpose. You can see how denaturing or unfolding gives enzymes better access to these otherwise complex protein structures. Hair, incidentally, is neither digestible nor fermentable by the cat or her gut bacteria.
The action of enzymes on food is called chemical digestion or enzymatic hydrolysis. Enzymes break down larger molecules into smaller and smaller molecules. They break chemical bonds at various points along the way. Enzymes themselves are constructed of amino acids. The body makes the necessary enzymes, for digestion and metabolic processes.
Chemical (enzymatic) digestion of protein starts in the stomach with the aid of those special glands in the stomach which secrete pepsin. As a chain of amino acids, protein is not a string of beads and enzymes are not scissors but the image below illustrates that pepsin snips longer strings of amino acids into small units called peptides. Pepsin does not alter the amino acids themselves, pepsin alters the length of the protein strands to prepare them for absorption into the body.
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. Thanks to complex fluid dynamics and elegant design, larger bits requiring more work get tossed back by stomach action for further digestion. In the meantime, receptors in the duodenum, the first part of the small intestine following the stomach, detect what it is being received and what the body needs to receive which helps set the pace of the discharge from the stomach.
The stomach doesn't drain like a bathtub, its emptying is an orchestrated process, a process dependent on the chemical mix of the chyme and the needs of the body. If the body's need for nourishment is not immediate, the process can be slowed to keep pace. If the last meal was too long ago, the process can be stepped up. If the chyme has a high fat content, gastric emptying may be slowed to better control the upcoming absorption process.
On to Gut 102 – Full Version . . . . . .
“Cats require purity and simplicity.” – SEM