Lesson

Another name for the digestive system is the alimentary canal. "Aliment" is the French word that means food. The digestive system (also called the gastrointestinal tract) has three important functions:

1. Breaks complex food substances down into smaller units;
2. Absorbs the smaller units, the nutrients, into the bloodstream as an energy source;
3. Eliminates the waste material (what cannot be digested) from the body.

Digestion begins once food is placed in the oral cavity, which includes the mouth and the accessory organs - the tongue and teeth. Through chewing and through the action of saliva, food is mechanically and chemically broken down. A bolus (small rounded mass of food) is formed. Some related combining forms are:

• or/o
• bucc/o
• dent/i
• stomat/o

The bolus is swallowed via the pharynx. The action of peristalsis, which is the wave-like alternating movements of constriction and relaxation, propels the bolus down the esophagus to the stomach.

Related combining forms are:

• pharyng/o
• esophag/o

The stomach is lined with small folds called rugae that contain digestive enzymes. Once food reaches the stomach, the action of these digestive enzymes, including hydrochloric acid, helps to chemically alter it so that it can then pass slowly into the small intestine. Related combining forms are:

• gastr/o
• chlorhydr/o

When food enters the duodenum (the first part of the small intestine) pancreatic enzymes and bile act to further break down starches, fats, and proteins before they reach the jejunum (the second part of the small intestine). Nutrients in the food are absorbed through the walls of the small intestine. Villi are microscopic structures in the walls of the small intestine that absorb the nutrients, which then pass into the bloodstream.

Related combining forms are:

• cholecyst/o
• duoden/o
• pancreat/o
• enter/o
• jejun/

What is left over then goes into the large intestine, where water is reabsorbed. Waste material is stored until time to be excreted through the terminal portion of the colon, called the rectum, and through the opening of the digestive tract, the anus. The act of excreting the waste is called defecation. The waste product itself is termed feces or stool. Some related combining forms are:

• an/o
• proct/o
• rect/o

Bile and enzymes are secreted by the liver. Bile is stored in a small sac called the gallbladder, which is located below the liver. The bile is secreted whenever food enters the duodenum. The bile helps to break down large molecules of fat through a process called emulsification. Related combining forms are:

• hepat/o
• cholecyst/o
• cboledoch/o

The primary digestive function of the pancreas as an exocrine gland is to secrete enzymes and digestive juices into the small intestine through the pancreatic duct. The enzymes it secretes include amylase, which digests starch; lipase, which digests fat; and protease, which digests protein. Related combining forms are:

• pancreat/o
• prote/o
• lip/o
• amyl/o

For a diagram of the digestive system, see the GI tract or go to the ADAM interactive website to view an anatomical illustration.

Nitrogenous waste products are formed when proteins combine with oxygen. Since nitrogen cannot be converted into a gaseous form and exhaled, it must be dissolved in water and excreted as urea. If the body did not have a mechanism to remove urea from the bloodstream, it would accumulate and create a toxic condition called uremia. The formation of urea occurs in the liver, and then the substance travels to the kidneys via the bloodstream.

The kidneys are essentially a filtering device in which urea and other metabolic waste products, such as creatinine and uric acid, can be separated from the blood and dissolved in urine, which also contains salts, water, and acids. The unique characteristic of the kidneys is the ability to remove the wastes while allowing valuable substances, such as proteins, sugar, and water, to be reabsorbed back into the bloodstream.

A series of steps are involved in this process. Once filtration is completed at the microscopic level of the glomeruli, associated arterioles, and Bowman capsules, reabsorption occurs. Next, urine passes through the renal tubules, through the renal pelvis, and to the two ureters that lead to the urinary bladder. The urinary bladder, which is a muscular, distensible organ, stores the urine until pressure builds up, resulting in the need to urinate. Once urination is initiated, sphincter muscles in the bladder relax, allowing urine to flow through the urethra, the tube that carries urine from the bladder to the outside of the body. Urination is also referred to as voiding and micturition.

Related combining forms are:

• nephr/o
• ren/o
• cali/o and calic/o
• glomerul/o
• pyel/o
• cyst/o
• ureter/o
• urethr/o
• vesic/o