Digestive System Glossary

antidiuretic hormone (ADH)

Substance that helps control how much water is either sent to the bladder (in urine) by the kidneys or retained in circulation. In a release controlled by the brain’s hypothalamus, ADH increases the permeability of both the distal nephron tubule and the n


Substance produced by the liver that facilitates the digestion of fats. Bile can be released either directly by the liver or by the gallbladder, which stores and concentrates this substance.

calorie (cal)

The amount of energy necessary to raise the temperature of 1,000 grams of water by 1°C. Sometimes written as Calorie to distinguish it from a standard calorie, which is defined as the amount of energy needed to raise the temperature of 1 gram of water by


An organic molecule that always contains carbon, oxygen, and hydrogen and that, in many instances, contains nothing but carbon, oxygen, and hydrogen. Carbohydrates usually contain exactly twice as many hydrogen atoms as oxygen atoms. The building blocks o


The soupy mixture of food and gastric juices that passes from the stomach to the small intestine.


A type of accessory molecule that binds to the active site of an enzyme, thus allowing the enzyme to bind to its substrate. Many vitamins are important coenzymes.

digestive system

The organ system that transports food into the body, secretes digestive enzymes that help break down food to allow it to be absorbed by the body, and excretes waste products. This system consists of the esophagus, stomach, and large and small intestines p

digestive tract

A muscular passageway for food and food waste that runs through the human body from the mouth to the anus.

essential amino acid

In nutrition, one of nine amino acids that the body cannot make and that hence must be supplied by food.


A dietary lipid that is solid at room temperature (e.g., butter, the fat in a piece of bacon).

fatty acid

A molecule, found in many lipids, composed of a hydrocarbon chain bonded to a carboxyl group.


In skeletal muscle, a single elongated muscle cell, containing hundreds of long, thin myofibrils that run the length of the cell. In nutrition, one of the three principal classes of dietary carbohydrate, defined as a complex carbohydrate that is indigesti


Organ of the body that stores and concentrates the digestive material bile, which is produced by the liver. Bile facilitates the breakdown fats by digestive enzymes.


An organ or group of cells that secretes one or more substances.


Knotted network of capillaries in each of the kidneys’ nephrons that receives blood and lets some smaller blood-borne materials pass out of it (and into the surro

high-density lipoprotein (HDL)

A type of protein that transports fat or lipid molecules (usually cholesterol) from various tissues in the body to the liver. Sometimes known as the “good cholesterol,” HDLs help remove, and possibly neutralize, the LDLs or low-density lipoproteins that c


The filtering organs of the urinary system that produce urine while conserving useful blood-borne materials.

large intestine (colon)

That portion of the digestive tract that begins at the small intestine and ends at the anus. The large intestine serves largely to compact and store material left over from the digestion of food, turning this material into the solid waste known as feces.


A member of a class of biological molecules whose defining characteristic is their relative insolubility in water. Examples include triglycerides, cholesterol, steroids, and phospholipids.


Organ that is central to the body’s metabolism of nutrients and that serves as a major storage site for blood.

low-density lipoprotein (LDL)

A type of protein that transports fat or lipid molecules (usually cholesterol) from the liver and small intestine to various tissues throughout the body. Sometimes known as the “bad cholesterol,” LDLs can initiate heart disease by coming to reside within


An element essential to the functioning of a living organism.

muscle tissue

Tissue that has the ability to contract. In humans, one of the four principal types of tissue.


Functional unit of the kidneys, composed of a nephron tubule, its associated blood vessels, and the interstitial fluid in which both are immersed.


A substance found in food that does at least one of three things: provides energy, provides a structural building block, or regulates a physical process. There are six classes of nutrients: water, minerals and vitamins; and carbohydrates, lipids, and prot


The study of the relationship between food and health.


A dietary lipid that is liquid at room temperature (e.g., olive oil, canola oil).


In digestion, a gland that secretes, into the small intestine through ducts, digestive enzymes along with buffers that raise the pH of chyme. In nutrient metabolism, a gland that secretes, directly into the bloodstream, the hormones insulin and glucagon,


Waves of contraction carried out by two sets of muscles in the digestive tract that help digest material and push it through the tract.


In humans, the passageway at the back of the mouth that links the mouth with both the food-transporting esophagus and the air-transporting trachea. In some animals, the pharynx can be everted, or turned inside-out, and used to obtain nutrients.

polyunsaturated fatty acid

A fatty acid with two or more double bonds between the carbon atoms of its hydrocarbon chain.

prostate gland

In human males, a gland surrounding the urethra near the urinary bladder that contributes fluids to the semen.

A large polymer of amino acids, composed of one or more polypeptide chains. Proteins come in many forms, including enzymes, structural proteins, and hormones.

red blood cell

The blood cells, also known as erythrocytes, that transport oxygen to and carry carbon dioxide from every part of the body.

saturated fatty acid

A fatty acid with no double bonds between the carbon atoms of its hydrocarbon chain.

simple sugar

The smallest and simplest form of carbohydrates, which serve as energy-yielding molecules and as the building blocks or monomers of complex carbohydrates. In nutrition, one of the three principal classes of carbohydrates. Simple sugars can either be monos

small intestine

That portion of the digestive tract that runs between the stomach and large intestine.

A complex carbohydrate that serves as the major form of carbohydrate storage in plants. Starches—found in such forms as potatoes, rice, carrots, and corn—are important sources of food for animals. In human nutrition, one of the three principal classes of


An organ that performs digestion and that serves as a temporary, expandable storage site for food.


A lipid molecule formed from three fatty acids bonded to glycerol.


Two tubes in which urine is transported from the kidneys to the urinary bladder.

Tube in which urine flows from the bladder to the outside of the body in both males and females. In males, the urethra also transmits semen.

urinary bladder

Hollow, muscular organ that serves as a temporary, expandable storage site for the waste product urine.

urinary system

The organ system that eliminates waste products from the blood through formation of urine. In humans, this system consists of the kidneys, where the urine is formed; and the ureters, urinary bladder, and urethra, which transport the urine from the kidneys


A chemical compound found in foods that is needed in small amounts to facilitate a chemical reaction in the human body.