Negotiated Study One: Further System Research - The Digestive System and The Liver

The Digestive Tract

"Man is a machine fueled by food. If food is to be utilized it must be broken down into molecules small enough to be absorbed. 

This is the duty of the digestive tract, a 26ft long tube from mouth to anus in which food is subjected to a coordinated thermal, mechanical and above all, chemical assault called digestion. 

Mouth

• Food enters the mouth where it is warmed or cooled to the optimal temperature for digestion. 
• Solid food is torn and ground by the teeth into small pieces, which are easier to swallow and digest.
• Dry food is now mixed and lubricated with saliva to ease its passage through the esophagus
• Each day the salivary glands produce about three pints of saliva which contains the starch-digesting enzyme ptyatin. This however has little chance to act before food is swallowed.

Esophagus

• Food leaves the mouth when we decide to swallow, a process which once begun is beyond conscious control.
• When the ball of food leaves the mouth it passes through the esophagus, where powerful waves of muscular contraction squeeze it down to the stomach.

The Stomach

• The adult stomach is a distensible sac which can hold up to two and a half pints
• It serves to store food so that it can be slowly pushed on to the duodenum at the rate required, while the gastric juice it produces starts the break down of the proteins in the food and kills any contaminating bacteria. 
• The activities of the stomach are controlled by hormones and nerves
• The sight, smell and even the thought of food will stimulate the production of the gastric juices
• There are three major components of the gastric juices - digestive enzymes, hydrochloric acid and mucus
• Digestive enzymes are compounds which cause the chemical breakdown of food.

• The rate at which food leaves the stomach is controlled by its chemical composition
• Carbohydrate rich food leaves the stomach in a few hours
• Protein rich food is retained longer
• While it takes many hours before fatty food is gone
• Proteins and fats, which are more difficult to digest than carbohydrate s, cause the duodenum to release into the blood varying amounts of the hormone entrogastrone, which reduces gastric activity, so that the duodenum receives food at the rate it can deal with, rather than at the rate the food is eaten.

The passage of food through the stomach

Duodenum

• As soon as acid is detected, the hormone secretin is released. This stimulates the pancreas to secrete a watery alkaline juice that neutralizes the corrosive hydrochloric acid.
• The hormone pancreozymin the causes the pancreas to start the somewhat slower production of enzymes to break down proteins and start the digestion of the carbohydrates, the fats and the nucleic acids.
• The presence of fats causes the duodenum to release yet another hormone, chlocystokinin, which makes the  gall bladder contract. As a result, bile, a thick green alkaline solution of salts and pigments made in the liver and stored in the gall bladder, is poured out and mixes with food. 
• Bile salts are necessary for the efficient digestion of fats, which they emulsify so that they are more easily acted on by enzymes.

• By the time it leaves the duodenum almost anything we have eaten looks the same - a watery mixture of partly digested food, enzymes and other excretions.
• This mixture still needs further processing before it can be absorbed
• Both of these processes occur in the next and longest part of the small intestine

• The logic of absorption and digestion is best understood when it is realized that a human being is a kind of fancy shaped doughnut, whose digestive tract is the holes in the middle

The Small Intestine

• When the chyme reaches the small intestine, although some food substances are ready to be absorbed, further processing is required for most. 
• Food protein has already fragmented into chunks called peptones and these are now broken into the absorbable amino acid
• Sugars and starches, similarly fragmented in the duodenum, are broken down into individual monosaccharides from which are built.
• Fats emulsified by the action of bile are split into particles which can be absorbed. 
• All this is achieved by the enzymes contained in the intestinal secretion
• About five pints of this secretion in produced each day, but only in response to food. 
• The food is slowly pushed along the small intestine by rhythmic contractions, which also break up any remaining lumps.
• As this final breakdown of food is occurring the end produced are being absorbed across the intestinal wall into the body.
• the small intestine is a narrow tube which is about twenty one foot long and the amount of absorption required is enormous
• the volume total of food, drink and gastrointestinal secretions is some two and a half gallons per day pf which only about one fifth of a pint is finally lost in the feces. 
• To facilitate the absorption of nutrients as well as water, the wall of the small intestine is folded to increase its surface area
• Not only do broad, spirally arranged folds create a series of permanent ridges down its length, but also these are in turn covered on the inner wall by minute finger like projections called villi.
• The villi are just visible to the naked eye and are said to give the intestine a velvety appearance
• The villi increase the absorptive surface of the intestine by about five times what it would be if smooth, just as a piece of velvet has a mush larger surface area than a piece of smooth cotton cut to the same dimensions. 

• Villi themselves are further subdivided their surface being covered in the microvilli, so small that only the most powerful microscope can detect it

• One square inch of intestinal wall is covered by something like twenty thousand villi and more than ten thousand million microvilli

• Although this enormous surface makes the absorption of food highly efficient, it also increases the area of cells exposed to wear and tear

• Once absorption is complete, a watery mix containing mainly fibrous waster, indigestible cellulose, some salts, and unwanted breakdown products remain, and this mow passes through the ileocecal valve into the large intestine.

The Large Intestine

• On a normal western diet, about twelve ounces of chyme enters the cecum every day. The continuing reabsorption of water reduces its weight by two thirds, forming the feces.
• These weight however, are extremely variable - much reduced on a refined diet and considerably increased by the consumption of roughage

• Most of the large intestine is colonized, permanently and harmlessly, by bacteria which use the indigestible residues as nutrients and my synthesize some of the bodys victim supplies
• there is a continual loss of these bacteria into the feces
• Like the stomach, the large intestine, particularly the descending colon and the rectum, has use as a store
• Although the presence of increasing amounts of feces causes a reflex desire to defecate, the large intestine can store considerable amounts of fecel matter

The Liver

• The  liver is a central chemical processing plant of staggering complexity. Weighing three pounds, it is the largest single gland in the body and with more than five hundred functions so far indentified, it is easily the most versatile.
• Unlike other organs, the liver is served by two distinct blood supplies. The heptatic artery supplies t with fresh oxygenated blood, while the hepatic portal vein transports blood to it for processing.
• This portal vein supply comes from the stomach and intestines carrying absorbed nutrients which the liver extracts, processes and stores, and from other main abdominal organs - the spleen, the gall bladder and the pancreas.
• When the portal vein enters the liver it branches repeatedly, its smaller branches, running with branched of the hepatic artery and bile ducts along the connective tissue - lined tubes that surround the liver lobules
• The lobules are tiny filters of which is, in effect a chamber of hepatic cells honeycombed by an interconnecting maze of sinuses, or caverns, that lead to a central draining vein.
• The central veins, in turn, join, finally leading to the hepatic vein that takes blood from the liver. 
• The cells of the lobules are washed by arterial blood, bringing them necessary oxygen, and portal blood, from which they remove nutrients, bacteria and old red blood cells.
• At the same time, into this cascade of blood passing through the microscopic sinuses, the secrete glucose, proteins, vitamins, fats and most of the other compounds required by the body. 

• Running between the individual cells that line the sinuses is yet another network of tunnels, much smaller than the sinuses, which drains outward, carrying bile toward the local branch of the bile duct, at the edge of the lobule.
• Bile, a solution of organic compounds in water, is produced by the hepatic cells. Some of the contents of the bile pigments that give it its green colour, cholesterol and bile salts produced from cholesterol are, in part, waster products from the breakdown of old red blood cells.
• The economy of the body is such that the waste products are sued as essential substances for the digestion and absorption of fats in the intestine.
• Part of the bile salts and cholesterol in the bowel is absorbed and recycled and part is lost in the 'drain' of the intestine, with the pigments in the bile contributing to the colour of the feces.
• Eventually, the pint of bile produced each day reaches the gall bladder, a small muscular sac underneath the liver, where it in stored until it is needed in digestion

• When hemoglobin is broken down in the liver much of the iron that it contains it reutilized
• Excess amino acids from hemoglobin, or, more generally, from all the other proteins that are broken down, are either used by the versatile hepatic cell for building new proteins or further broken down to provide energy.
• Poisonous ammonia produced during this process is converted to urea, a non toxic compound, and returned to the blood for transport to the kidneys.
• From there it is excreted as urine.

• Perhaps what is most surprising of all is that the  site of this immense chemical plant - factory, warehouse and sewage works, is the simple hepatic cell
• The liver has another property that makes it different from most of the rest of the body. If it is damaged, or part is removed, it can regrow to a large extent to continue its work as before.

• The liver is the largest organ in the body
• Liver tissue is composed of a compact mass of multisided units, the hepatic lobules.
• Each lobule consists of a central vein surrounded by plates of liver cells.
• The liver receives blood from two sources; 80 percent which carries digested food materials, arrives from the intestines via the portal vein; the remaining 20 percent is oxygenated blood from the heart, which enters through the hepatic artery.
• An exchange of materials takes place between liver cells and the blood, which then passes into the central veins and returns to the general body circulation via the hepatic vein.
• When an adult is at rest, about two and a half pints of blood flow through the liver each minute.