Digestive System Function

Function Of The Digestive System

The are seven main functions:

  1. Ingestion – eating/consuming food and liquid
  2. Propulsion – the method of moving food through the digestive system which is called peristalsis.   This occurs via the contractions and relaxations of muscles around the walls of the digestive organs causing food and waste materials to move forward through the system.
  3. Secretion of digestive enzymes and other substances liquefies, adjusts the pH of, and chemically breaks down the food.
  4. Mechanical digestion is the process of physically breaking down food into smaller pieces. This process begins with the chewing of food and continues with the muscular churning of the stomach. Additional churning occurs in the small intestine through muscular constriction of the intestinal wall. This process, called segmentation, is similar to peristalsis, except that the rhythmic timing of the muscle constrictions forces the food backward and forward rather than forward only.
  5. Chemical digestion is the process of chemically breaking down food into simpler molecules. The process is carried out by enzymes in the stomach and small intestines.
  6. Absorption is the movement of molecules (by passive diffusion or active transport) from the digestive tract to adjacent blood and lymphatic vessels. Absorption is the entrance of the digested food (now called nutrients) into the body.
  7. Defecation is the process of eliminating undigested material through the anus.

Digestions and nutrient absorption occur in the stomach and small intestines.  After absorption, the resulting waste material moves through our large bowels, or more commonly known as the colon, before it is released from the body.  Let’s understand what happens in the colon:

An illustration of the colon is shown below.  Imagine your belly-button in the center of the illustration with the Cecum, which is the transition from your intestines into the colon, being on your right side.

So we have 5 main sections of the colon as follows:

  • Ascending colon
  • Transverse Colon
  • Descending colon
  • Sigmoid
  • Rectum

The main function of the colon is to absorb water and to breakdown any remaining materials by the action of bacteria.  Material continues to moves along the colon via peristalsis and this waste material becomes thicker as water is absorbed back into the body.  Thick or formed stool is easier for our bodies to expel.

Let’s focus on the 2 areas that directly affect LARS; the Sigmoid and the Rectum.  When these are working normally, both the Sigmoid and the Rectum are essentially holding tanks for waste material.  The signaling for when the material is moved is controlled by our brains which depends on information from nerves, or sensors inside our colons .  

The Sigmoid is capable of holding waste for 7 hours or longer and once it is full or when the rectum is empty, as determined by our nerves, the material is moved into the rectum.  Nerves in our Rectum now tell our brains that waste is present and now need to expel it from our bodies. Typically, the Rectum can store this waste for 2 hours or more giving us time to plan ahead to find a toilet.

Once we’re ready to expel this waste, we can consciously relax and open our sphincter muscles causing normal evacuation of solid waste via peristalsis as the Rectum muscles contract and relax.  Once the waste is eliminated, the colon sensors inform our brains that the process is completed, our Rectum is empty, and then peristalsis stops and we have a general sense of completion.

However after a LAR, there is major disruption to the nerves and muscles in the Sigmoid and Rectum. Surgeons have typically assumed that Colon function, as a result of a LAR, returns naturally over time however for the majority of cases, normal bowel function as described above does not return.  

Instead, the following may occur depending heavily on the specific surgery, amount of Rectum and/or Sigmoid removed, and general success of the surgery, ie which nerves or muscles were compromised and to what extent; damaged sensors and muscles in the Rectum heavily influence the brain’s ability to control the necessary actions to push out the waste and monitor whether the waste is fully ejected.  The normal control “circuitry” is simply impaired and LARS patients then suffer incontinence, clustering and tenesmus and a consequence of the disruption. Because of the various factors noted above, we can expect that the loss of function and severity will be different between patients and likewise, the treatments to this condition cannot be universally applied. The treatments therefore require trial/error before the symptoms can be reduced to tolerable levels.