Chapter 41 (3)

In mammals, teh digestive system consists fo the __ and various __ that asecrete digestive juices through ducts into the canal. THe __ of the mammalian digestive system are three pairs of salivary glands: __, __, __.

Food is pushed along the alimentary canal by __, alternating waves of contraction and relaxation in the smooth muslce slining the canal. It is __ that enables us to process and idgest food even while lying down. At some of teh junctions between speciealized compartments, the musclular layer forms ringlike valves called __. Acting like drawstrings to close ff the alimentary canal, __ regulate the passage of material between compartments.

__ and the initial steps of __ occur in the mouth, or __. __ begins as teeth of various shapes cut, smash and grind food, making the ofod easier to swallow and increasing its surface area. Meanwhile, the presence of food stimulates a nervous reflex that causes the __ to delvier saliva through ducts to the __. Saliva may also be released before food enters the mouth, triggered by a learned association between eating and the time of day, a cooking odor, or another stimulus.

Saliva initiates __ while also protecting hte __. __, an enzyme in saliva, hydrolyzes starch and glycogen into smaller polysaccharides and the disaccharide maltose. __, a slippery glycprotein i saliva, prttoects the lining of the moth from __.

__ also lubricates food for easier swallowing. Additional components of saliva include buffers which help rprevent thooth decay by neutralizing acid,a nd antibacterial agents, which protect against microorganisms that enter the mouth with food.

The __ aids digestive processes by evaluating ingested material and then enabling its further passage. When food arrives at the oral cavit, the __ plays a critical role in distinguishing which foods should be processe d further.

After food is deemed acceptable and chewing commences, tongue movements manipulate the food, helping shape it into a ball called a __. During swallowing, the tongue provides further help, pushing the __ to the back of the oral cavity and into the pharynx.

The __, or throat region, opens to two passageways: the __ and the __. The __ connects to the stomach, whereas the __ leads to the lungs. Swallowing must therefore be carefully choreographed to keep food from entering and blcoking the airway.

When you swallow, a flap of cartilage called the __ prevents food from entering hte __ nby covering the __- athe vocal cords and the opening between them. Guided b the movements of the __, the upper part of the respiratory tract, this swallowing mechanism directs each __ into the entrance of the __.

If the swallowing reflex fails, foods or liquids can reach the windpipe and cause __, a blockage of the trachea. The resulting lack of airflow into the lungs can be fatal if the material is not dislodged by vigorous couphing or a forced upward thrust of the __ (__).

The esophagus contains both __ and __ muscle. The __ is situated at the top of the esophagus and is active during swallowing. Throughout the rest of the esophagus, __ functions in __. The rhythmic cycles of contraction move each __ to the stomach. As with other partso fthe digestive system, the form of the esophagus fits its funciton and varies among species.

The __ is located just below the __ int eh upper abdominal cavity. A few nutrients are absorbed from the stomach into the bloodstream, but the stomach primarily stores food and continues digestion. With accordion-like folds and a very elastic wall, it can stretch to accommodate about 2 L of food and fluid. The stomach secretes a digestive fluid called __ and mixes this secretion with the food through a churnign action. This mixture of ingested food and digestive juice is called __.

Two components of __ carry out chemical digestion.
o One is __, which disrupts the ECM that biidns cells together in meat and plant material. The concentration of __ is so hight that the pH of __ is about 2, acidic enough to dissolve iron ails. This low pH not only kills most bacteria but also denatures proteins in food, increasing exposure of their peptide bonds.
o The exposed bonds are attacked by the second component of gastric juice a __, or protein-digestive enzyme, called __. Unlike most enzymes, __ works bestin a strongly acidic environment. By breaking peptide bonds, it cleaves proteins into smaller polypeptides. Further digestion to individual amino acids occurs in the small intestine.

Why doesnt gastric juice destroy the stomach cells that make it?

The components of gastric juice are produced by cells int eh gastric glands of the stomach. __ secrete hydrogen and chloride ions, which form __. Using an ATP- driven pump, the __ expel hydrogen ions into the lumen at very high concentration. Theree the hydrogen ions combiene with chloride ions that diffuse into the lumen through specific membrane channels.

meanwhile, __ release pepsin into the lumen in an inactive form called __. HClconverts __ to active pepsin by clipping off a small portion of the molecule and exposing its active site. Through these proceses, both HCCl and pepsin form in the lumen of the stomach, bot w/in the cells of the gastric glands.

After __ converts a small amt of __ to __, a second chem process helps activate the remaining __.

__, like HCl, can clip _- to expose the enzyme's active site. THis generates more __ which activeates more __, forming more active enzyme. THis series of events is an example of __.

Cells are vulnerable to gastic juice as well as to acid-tolerant pathogens in food. However, the stomach lining protects against self-digestion by secreteing __, a viscous and slippery mixture of __, cells, slats, and water. In additon, cell division adds a new epithelial layer every threee days, replacing cells eroded by digestive juices.

Despite these defenses, damaged areas of the stomach lining called __ may appear.

__ by gastric juice is accompanied by the churning action of the stomach. This coordinated series of muscle contractions and relaxations mixes the stomach contents about every 20 seconds. As a result of mixing and enzyme action, what begins a recently swallowed meal becoems the acidic, nutrient-rich broth known as __.

Most of the time, the stomach is closed off at both ends. The __ between the esophagus and the stomach normally opens only when a __ arrives. Occassionally, however, a person experiences __, a backflow of __ fromt eh stomach into the lower end of hte esophagus. This resulitng irritation of hte esophagus is commonly but inaccurately called __.

The __ licated where the stomach opens to the small intestine helps regulate the passage of __ into the small intestine, allowing onyl one squirt at a time. The mixture of acid, enzyme and partially digested food typically eaves the stomach 2-6 hrs after a meal.

Most enzymatic hydrolysis of macromolecules occurs int eh __. Over 6 m long in humans, this organ is the ___'s longest compartment. Its name refers to its small diameter, compared with that of the large intestine. The first 25 cm or so of the small intestine forms the __,a major crossroad in digestion. It is here that __ fromt eh stomach mixes with digestive juices from the __, __ and __, as well as from gland cells of the intestinal wall itself. Hormones released by the __ and __ control the digestive secretions into the __.

The __ aids chemical digestion by producing alkaline solution rich in __ as well as several enzymes.
o The __ neutralizes the acidity of __ and acts as a buffer.

Among the pancreatic enzymes are trypsi and chymootrypsin, _- secreted into the __ in inactive forms. In a chain reaction similar to activation of __, they are activated when safely located int eh extracellular space within the __.

Digestion of fats and other lipids begins in the small intestine and relies on the produciton of __, a mixture of substances that is made in the __. __ contains bile salts, which act as __ (emulsifiers) that aid in digestion and absorption of lipids. _- is stored and concentrated int eh __.

The liver has many vital function sin addition to __. It also breaks down toxins that enter the body and helps balance nutrient utilization.

Bile production itself is integral to another task of the liver:__

In producing __, the liver incorporates some pigments that are by-products of RBC disassembly. These __ pigments are then eliminated from teh body with the feces.

The epithelial lining of the __ is the source of several digestive enzymes. SOme are secreted into teh lumen of the __, wheras others are bound to the surface of epithelial cells.

WHile enzymatic hydrolysis proceeds, __ moves the mixture of __ and digestive juices along the small intestine. Most digestion is completed in the __. The remaining reguions of the small intestine, called the __ and __, function mainly in the absorption of nutrients and water.

To reach body tissues, nutrients in the lumen must first cross the lining of the __. Most of his absorption occurs in __. This organ has a huge surface area- 300 m2. Large folds in teh lining have figher-like projections called __.

in turn, each epithelial cell of a villus has on its apical surface many microscopic appendages, or __, that are exposed to the instenial lumen. The many side-by-side __ give the intestinal epithelium a brush-like appearance- reflected by __ is an adaptation that greatly increases the total capacity for nutrient absorption.

True or False:
Depending on the nutrient, transport across the epithelil cells can be passive or active.
True False

Although many nutrients leave the intestine through the bloodstream, some products of fat (__) digestion take a dfifferent path. After being absorbed by epithelial cells, fatty acids and __ (glycerol joined to a single fatty acid) are reombined into __ w/in those cells.

These fats are then coated with phospholipids, cholesterol and roteins forming water soluble globules called __. These globules are too large to pass through the membranes to capillaries. Instead, they are transported into a __, a vessel at the core of each villus.

__ are part of the vertebrate __, which is a network of vessels that are filled with a clear fluid called __. Starting at the __, lymph containing the __ passes into the larger vessels of the lymphatic system and eventually into large veins that return the blood to the heart.

In contrast with the __, the capillaries and veins that carry nutrient-rich blood awaty from the villi all converge into the __, a blood vessel that eleads directly to the liver. From the liver, blood travels to the heart and then to other tissues and organs.

This arrangement serves two major functions . What are they?
o Because the liver can interconvert many organic molecuels, blood that leaves the liver may have a very different nutrient balance than the blood that entered via the hepatic portal vein.
o The liver is the primary site for the detox of many organic molecules, including drugs, that are foreign to the body.

The alimentary canal ends twith the __, which includes the colon, cecum, and rectum. The small intestine connects to the large intestine at a T-shaped junction, where a sphincter controls the movement of material. One arm of the T is the 1.5 m long __, which leads to the rectum and anus. The other arm forms a pouch called the __.

The __ is important for fermenting ingested material, especially in animals that eat large amts of plant material. Compared with many other mammals, humans have a relatively small __.
- The __, a finger-like extension of the humanc ecum, has a minor and dispensible role in immunity.

What is a major function of the colon?

The __, the wastes of the digestive system, becomes increasingly solid as they are moved along hte colon by __. It takes 12-24 hrs for material to travel the length of the colon. If the lining of the colon ins irritated- by a viral or bacterial infection- less water than normal may be reabsorbed resulting in diarrhea. The opposite problem, constipation, occurs when teh feces move along the colon too slowly. An excess of water causes the feces to become compacted.

True or False:
- A rich flora of mostly harmless bacteria resides in the human colon, contributing approx 1/3 of the dry weight of feces.
True False

besides bacteria, feces contain undigested material, including cellulose fiber. Although it has no caloric value to humans, fiber helps move food along the __.

The terminal portion of the large intestine is the __, where feces ar stored until they can be eliminated. Between teh rectum and the anus are two sphincters, the nner one being __ and teh outer being __. Periodically, strong contractions of the colon create an urge to defecate.