© 1999 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999

8.2.3 Navigational Alimentography

Medical nanorobots may also access the human body via the alimentary canal -- a ~6 meter long musculomembranous digestive tube which begins at the mouth and ends at the anus. With its accessory glands and organs, the alimentary system controls the intake, mastication, digestion, absorption and elimination of foods.

An approximate quantification of the gastrointestinal system is given in Table 8.8. (Passage times are for normal digestion and exclude pathological conditions such as constipation or diarrhea.) The general plan as shown in Figure 8.16 is topologically very simple. The alimentary canal is formed by the mouth, pharynx, esophagus, stomach, small intestine, large intestine, and rectum. Associated with the canal are accessory structures and glands including the salivary glands, liver, gallbladder and pancreas. The tube itself has four layers in radial aspect. Starting at the luminal surface these include the mucosal (superficial epithelium and cells secreting mucus and digestive juices), submucosal (connective tissue with blood and lymphatic vessels), muscular (smooth muscle in circular and longitudinal layers) and serous (parietal and visceral peritoneum) layers. The entire tract, glands, and muscle are innervated from the involuntary or autonomic system and are also influenced by hormones, some from the tract itself and some secreted by glands elsewhere in the body.

It is instructive to imagine the "view" from a medical nanorobot as the nanodevice navigates the entire length of the alimentary system. The journey begins in the mouth (the sphincter oris) where food (typical viscosity 0.01-100 kg/m-sec^3309-3313) is chewed and mixed with saliva secreted by one of the three pairs of major salivary glands (Fig. 8.17). (There are also the minor salivaries consisting of the labial, buccal, molar, lingual, and palatine glands ).⁹³⁵ After consulting its map, the nanorobot first explores a duct (~1 mm wide, ~25 mm long) in the anteroinferior dental arch leading into the walnut-sized submaxillary salivary gland situated mostly under the lower jawbone. The gland is organized as several thousand separate small secretory units called acini. Each acinus with its microduct forms a single parenchymal unit called a salivon.⁹³⁵ The lumens of salivons flow together to form progressively larger ducts (all surrounded by contractile myoepithelial cells and a basal lamina), finally reaching the main salivary duct that empties into the mouth. Passing under the tongue, our mobile nanodevice traverses multiple ducts (~0.5 mm wide, ~5 mm long) leading from the sublingual, the smallest gland located in the floor of the mouth. After climbing up to the soft palate in the roof of the mouth, the nanorobot reaches a pair of ducts (~1 mm wide, 15-20 mm long) leading from the parotid glands -- the largest of the three glands, lying anteroinferior to each ear.

The six major salivary glands together typically produce 500-1500 cm³/day of saliva in response to mechanical, thermal, and chemical stimuli applied to the mucous membrane of the mouth, or as the result of psychological or olfactory stimuli.^749,751,1975 Unstimulated flow is 10-50 cm³/hr during waking hours and 1-2 cm³/hr during sleep,^749,873 but may rise as high as 290 cm³/hr during eating.⁵⁸⁵ Salivary fluid specific gravity is 1.01-1.02, mean viscosity is ~ 4 x 10^-3 kg/m-sec but up to 7.4 x 10^-3 kg/m-sec in bulimic patients,³³⁰⁶ and pH averages 6.8 (range 5.6-7.6).^585,751 Plenty of chemical energy (Section 6.3.4) is available to power a nanorobot exploring these ducts -- including 1.96 (1.13-2.81) x 10^-4 gm/cm³ of glucose, 7.5 (2.5-9.0) x 10^-5 gm/cm³ of cholesterol, and 2-4 x 10^-3 gm/cm³ of protein for stimulated secretions,^585,749 or 1 x 10^-3 gm/cm³ of glucose and 8 x 10^-3 gm/cm³ of cholesterol for unstimulated secretions.⁹⁴³ Saliva is 99.4% (99.1%-99.6%) water,⁵⁸⁵ but duct secretions vary enough in composition along the length of each duct and between glands to be useful in crude chemonavigation (Section 8.4.3). For example, the submaxillary glands secrete 2.7 (0.8-6.0) x 10^-3 gm/cm³ of mucopolysaccharide (mucin) derived from mucous cells within the glands, plus a small amount of the important digestive enzyme ptyalin (salivary amylase) from serous cells. By comparison, the parotid glands are purely serous glands that secrete no mucin, while the sublingual glands secrete mostly mucus.^585,863 The salivary microbial population is ~ 10⁶/cm³.³⁶⁰

After food has been chewed, moistened, and reduced to a semiliquid state, the tongue rolls it into a 1-10 cm³ bolus³³¹² and pushes it backward into the pharynx (Fig. 8.11). During the act of deglutition (swallowing), the uvula is pushed backward against the posterior pharyngeal wall, closing the nasal passages; the larynx elevates and the epiglottis folds back, forming a protective ledge that covers the windpipe. The esophagus opens to receive the food, and contracting muscular waves convey the bolus to the stomach. These waves are strong enough to oppose at least several g's of negative vertical acceleration; esophageal manometry shows normal contractions of up to ~190 mmHg in the distal esophagus.²¹²² With each swallow, 2-5 cm³ of air are ingested.²¹⁸⁰

Like the mouth and pharynx, the esophagus is lined with mucus-coated stratified squamous epithelium which is part of the mucous membrane. Figure 8.18 illustrates a cross-section of the esophagus, showing a number of longitudinal pleats to allow radial stretching. Note that when no food is present, the tube is almost completely squeezed shut. The tube readily distends to a width of 1-2 cm to permit the passage of a bolus of food or drink. The movement of food from the bottom of the esophagus into the stomach is controlled by the opening and closing of a muscular ring called the lower esophageal or cardiac sphincter. At the approach of the bolus, the cardiac sphincter relaxes, the opening widens, and the food is shot into the stomach. Normal sphincter pressure is 10-50 mmHg.^2122,2180

The stomach is an irregularly pear-shaped bag located at the level of the lowermost ribs with its upper end just below the heart. It has a central portion (the body), a balloon-like portion to the left (the fundus), and a constricted portion to the lower right (the pylorus, or lower quarter of the organ). The upper part of the stomach is usually puffed out a bit with gas, even when the stomach is empty, so it protrudes slightly above the cardiac sphincter. However, when the stomach is filled with food in a man standing erect, the organ assumes an almost vertical position with a tubular shape.

When greatly stretched, the stomach of a normal male can store as much as 1000 cm³ of food and fluid, up to 1500 cm³ for very large people but as little as ~60 cm³ in newborns;^863,870 however, 500 cm³ usually gives a person a sense of fullness.⁸⁶³ The inner wall of the empty stomach has longitudinal expansion pleats called rugae. As the stomach fills with food, the rugae flatten out and disappear, leaving a ~600 cm² smooth mucous membrane surface when the organ is full.

The uppermost epithelial layer of the stomach lining is the mucosa, several millimeters thick (Fig. 8.19). Almost all of the epithelial cells that line the surface are simple columnar mucous cells that secrete mucus. The gastric mucus is especially viscous, ~50-100 microns thick,³¹⁶⁵ and is highly resistant to both the digestive juices and the acid secreted by the stomach itself. Absorption through the almost impermeable stomach wall is slight, although the stomach does absorb small amounts of water, electrolytes, certain drugs such as aspirin, and alcohol.

The mucosa contains the secreting cells of the stomach, arranged in small tubular units to form the gastric glands. Within these glands, the chief or zymogenic cells are simple columnar cells forming a continuous lining for the tubule, and secrete important protein-digesting enzymes such as pepsin and rennin (chymosin), and lipid-digesting enzymes such as lipase. The parietal cells are scattered along the tubule and secrete ~0.5% hydrochloric acid solution.⁷⁴⁹ Both the movements of the stomach and the flow of gastric juice may be stimulated by the nerve plexuses and by hormones such as gastrin and histamine.⁷⁵¹

Each day ~35 million gastric glands secrete 1000-3000 cm³ of gastric juice;¹⁹⁷⁵ a residuum of 50 cm³ is always present in the stomach, even after lengthy fasting.⁷⁴⁹ Secretion rates in young adults average 77 cm³/hr (male) and 70 cm³/hr (female) while fasting, 54 cm³/hr (male) and 38 cm³/hr (female) while sleeping, and 114 cm³/hr (male) and 99 cm³/hr (female) after eating.⁵⁸⁵ (The gender distinction is at least partly due to differences in mean body size.) Gastric juice is 99% water with specific gravity ~1.006 (1.004-1.010) and pH ~2.0.^585,751 Glucose content of gastric juice is 0.33-1.19 x 10^-3 gm/cm³.⁵⁸⁵

If the stomach has been empty for some time, it contracts and produces uncomfortable sensations (hunger pangs). Soon after food is taken, the pangs cease and gastric peristalsis begins as a ring of constriction around the middle of the stomach. This ring progresses toward the pylorus, growing measurably deeper as it moves. These mixing waves recur at ~3/minute, each taking ~1 minute to travel from its origin to the pylorus, so there are usually 3-4 moving waves of contraction simultaneously present on the human stomach. This kneading of the food can produce a gurgling sound because of the gas that is usually trapped in the stomach.

The well-churned food mixture, now called chyme, is ejected through the pyloric valve into the duodenum of the small intestine. Nervous and hormonal signals (e.g., enterogastrone) arising mainly from the duodenum, but also partly from the stomach, control the degree of contraction of the pyloric sphincter and thereby control the rate at which the chyme is emptied from the stomach into the duodenum of the small intestine.

About 8-10 cm downstream from the pyloric sphincter, the cruising nanorobot pauses to investigate a small elevation called the duodenal papilla. The orifice at the summit of the papilla is surrounded by muscle fibers which form the sphincter of Oddi, beyond which lies the 6-mm-wide ampulla of Vater and the common bile duct, a tube ~3 mm wide and ~60 mm in length that carries various glandular secretions directly into the duodenum. About 10 mm up the ampulla of Vater the pancreatic duct (duct of Wirsung) veers to the left side of the body and runs ~15 mm to the pancreas, where it continues on for the length of the organ, another ~90 mm. In some patients, the narrower accessory pancreatic duct (duct of Santorini, draining the head of the pancreas) runs ~15 mm from the pancreas and empties into the duodenum ~25 mm upstream from the duodenal papilla. Through the ~2 mm wide pancreatic duct passes ~1000-1500 cm³/day of pancreatic juice.^853,1975 This juice contains pancreatin, a mixture of the three digestive enzymes trypsin (which digests protein), lipase (which digests fat), and amylase (which digests starch). The specific gravity of the fluid is 1.008, mean viscosity is 1.61 x 10^-3 kg/m-sec (up to 5.8 x 10^-3 kg/m-sec in patients with chronic pancreatitis³³¹⁵), the pH is 7-8, and the glucose content is 0.85-1.8 x 10^-4 gm/cm³.⁵⁸⁵ Juice flows on signal from the hormone secretin, manufactured by the mucous membrane of the duodenum which sends its message as soon as partially digested food enters from the stomach.

Moving ~55 mm up the common bile duct from the ampulla of Vater, the nanorobot passes through the sphincter of Boyden and next encounters the cystic duct, a ~2 mm wide tube ~35 mm in length branching to the right that leads through the spiral valve directly to the gallbladder. The branch to the left (of the body) continues as the ~25 mm long common hepatic duct, which then bifurcates to form the left and right hepatic ducts (~2 mm wide) leading directly to the liver (Section 8.2.5), each duct up to ~50 mm in length. Some patients also have a few direct connections between the liver and the cystic duct, called the ducts of Luschka.⁹³⁵

The gallbladder is a 7-10 cm long pear-shaped bag composed of muscle and membrane lodged in a hollow on the underside of the right lobe of the liver behind the lower ribs, that serves as the reservoir for bile that is secreted continuously by the liver. The cystic duct carries bile both to and from the gallbladder, depending upon whether the sphincter of Oddi is open or closed. The liver secretes 800-1000 cm³ of bile each day,⁸⁵³ but the bile duct delivers only ~500 cm³/day of bile to the duodenum.⁸⁷⁰ The excess is shunted to the gallbladder, which holds up to ~50 cm³ of tenfold bile concentrate (more than 90% of the water and salts are reabsorbed from the excess flow). The gallbladder empties by contraction of the smooth muscle contained in its walls, when stimulated by the hormone cholecystokinin, which serves as the signal that bile is needed. This signal in turn is triggered by the passage of chyme over the duodenal papilla in the small intestine. Only about 10% of the bile is permanently lost in the feces; 90% is reabsorbed and returned to the liver.

Bile is a bitter yellowish fluid that helps to emulsify and digest fats to hasten their absorption from the intestines, to activate the pancreatic enzyme lipase, to stimulate intestinal movements, and to inhibit fermentation of the bowel contents. The specific gravity of bile is 0.998-1.062; absolute viscosity ranges from 0.843-2.342 x 10^-3 kg/m-sec; and pH averages 7.5 (6.2-8.5) for hepatic bile, 6.0 (5.6-8.0) for gallbladder bile.⁵⁸⁵ Hepatic bile contains 1.7-5.2 x 10^-4 gm/cm³ sugars and 1.2 (0.8-1.7) x 10^-3 gm/cm³ cholesterol, while gallbladder bile contains 8 x 10^-4 gm/cm³ sugars and 6.3 (3.5-9.3) x 10^-3 gm/cm³ cholesterol,⁵⁸⁵ plus 0.33% lipids.⁷⁴⁹ Thus the various biles are chemonavigationally distinguishable.

The nanorobot resumes its journey down the small intestine. The small bowel extends from the pylorus of the stomach all the way to the large intestine and occupies the greater portion of the abdominal cavity. About 90% of all digestion and absorption takes place in the small intestine, including up to 6 liters/day of the 8-10 liters/day of water that flows into it from swallowed saliva, ingested water, the acid fluid secreted by the stomach, bile and pancreatic juice, as well as fluid secreted by the upper small bowel itself. Food is passed along by muscular contractions in waves known collectively as peristalsis, with waves progressing arhythmically for distances varying from 10-100 cm in length, and occasionally over the entire length of the small intestine. Food is also broken up by rhythmic segmentation contractions within the irregular peristaltic motions, which are ringlike contractions of the circular muscle ranging in frequency from 10-30/minute, with higher rates at the upstream end of the bowel.

The small intestine is a continuous tube with three well-defined sections -- the duodenum, jejunum, and ileum (Table 8.8). The total length is commonly reported as ~7 meters, but this measurement is for tissues taken from cadavers which have lost all muscle tone. In the living body, the small intestine is only 3-5 meters in length.⁸⁶³ In the duodenum and jejunum the submucosa elevates into a series of permanent transverse pleats called circular folds, plicae circulares, or Kerkring's folds (Fig. 8.20). These ridges do not disappear when the wall is distended by the passage of food, and may stand up to ~10 mm high in the mucosa. Some folds wrap all the way around the intestine, while others extend only partly around. Starting a short distance past the pylorus, the circular folds are numerous and high in the distal portion of the duodenum and in the proximal portion of the jejunum, after which they gradually become less numerous and smaller; by mid-ileum, they fade out completely. Thus the number and depth of the folds may be used as crude navigational surface markers of range from the pylorus. The folds increase the local absorptive area by about threefold and further enhance absorption by causing the chyme to spiral as it passes downstream,⁸⁵³ crudely analogous to a spinning bullet passing down a rifled bore. A nanorobot traversing the folded intestinal surface must travel at least ~3 times farther than another nanorobot pursuing a more linear axial course through the luminal (chymous) contents of the tube.

The duodenum is the first short part of the small intestine, arranged in a horseshoe shape enclosing the head of the pancreas. The acidic chyme is neutralized following exposure to the alkaline pancreatic juices and bile fluid. Hence the pH of the chyme increases along the length of the duodenum, a fact which may be useful for chemonavigation (Section 8.4.3). The duodenal glands (Brunner's glands) are found only in the duodenum. They are tortuous and branching; their mucus-containing secretion has a pH of 5.8-7.6, a specific gravity of 1.01, and a highly variable cholesterol concentration of 3.61 (0-31.5) x 10^-4 gm/cm³.⁵⁸⁵ Total unstimulated secretory volume is ~30 cm³/hr, rising to 181 cm³/hr (male) and 126 cm³/hr (female) with secretin stimulation.⁵⁸⁵

In the jejunum, fats, starches, and proteins are broken down to their smallest components (or small peptides) and are absorbed by the lining cells of the bowel. Of particular interest, absorption of sugars takes place chiefly in the upstream portion of the small intestine, specifically in the duodenum and upper jejunum. Hence the concentration of glucose in the chyme peaks and then sharply declines in the jejunum because starches of all molecular sizes are enzymatically reduced to the simplest sugars there, prior to absorption, although the disaccharides are not as readily absorbed. Cholesterol is also absorbed mainly in the jejunum.

In the ileum, water is absorbed (~0.07-0.40 cm³/sec) along with calcium, other minerals, and vitamins (especially vitamin B₁₂). Bile is recaptured and returned to the liver via the hepatic portal vein and the lymphatic thoracic duct systems. Fat is also absorbed more rapidly in the ileum than in the duodenum or jejunum. These spatially differential absorptive properties of the small intestine produce numerous luminal chemical gradients that may be useful for both axial and radial chemonavigation by nanorobots traveling with the chyme flow.

Four additional somatographic features will be encountered by nanorobots crawling across the surface of the small intestine. First, lymphoid tissue is present in the form of nodules or follicles throughout the luminal wall. These may appear as solitary nodules that may extend into the submucosa all over the intestine, averaging 0.4-2 mm in diameter, up to 3-5 nodules/cm² of mucous membrane, ~2400-4500 per patient.⁸⁴⁸ The nodules become larger and more numerous as the nanorobot travels downstream. Aggregated nodules are found chiefly in the lower ileum, penetrating the mucosa, always located on the side of the intestinal wall opposite to the line of attachment of the mesentery. These oval-shaped aggregates, called Peyer's patches, are slightly elevated areas 12-38 mm in length and 8-25 mm wide, oriented with the long axis pointing downstream.⁸⁷⁴ Each patch is composed of 8-60 nodules in varying degrees of fusion.⁸⁷⁴ There are 20-30 patches per patient.⁸⁷⁴

Second, the mucosal surface is pockmarked with ~100 million pit-like structures ~50 microns in diameter called intestinal glands or crypts of Lieberkuhn (Fig. 8.21). Their depth ranges from 100 microns in the mucosa near the headwaters of the small intestine to 700 microns far downstream in the large intestine.⁸⁷² The glands collectively secrete 2-3 liters/day of fluid.⁸⁵³ The secretion is an alkaline fluid (pH ~ 7.6) somewhat resembling pancreatic juice, but includes considerable cellular debris resulting from the sloughing of cells from the mucous lining of the intestine. The secretion also contains mucus and digestive enzymes including enterokinase to activate the trypsinogen of the pancreatic juice (converting it to trypsin), peptidases to break down protein fragments left behind by pepsin and trypsin, maltase to break maltose into glucose, sucrase to break sucrose (table sugar) into halves forming glucose and fructose, lactase to break lactose (milk sugar) into glucose and galactose, and two nucleic acid digesting enzymes (ribonuclease and deoxyribonuclease). The secretory activity of the intestine is influenced by nervous, hormonal (e.g., enterocrinin), and mechanical stimuli, and again provides many navigationally-useful chemical gradients.

Third, there are ~5 million villi studding the entire mucosa of the small intestine (Fig. 8.21). Each villus is a fingerlike outgrowth of the mucous membrane averaging ~200 microns wide and ~1000 (range 500-1500) microns in length. (For comparison, a micron-scale nanorobot is about the size of one of the stipple dots in Figure 8.21.) Villi are taller and more numerous in the duodenum and jejunum (~20-40/mm²) than in the ileum (~15-30/mm²),⁸⁴⁸ and collectively increase the effective absorptive surface of the small intestine to ~10 m². They are leaf-shaped in the proximal duodenum, change to tongue-shaped by the upper jejunum, and finally become slender and finger-shaped in the ileum⁹³⁶ -- probably all morphologically distinguishable for navigational purposes using a high-resolution navigational network (Section 8.3.3). Villi have a skin of simple columnar epithelial cells adapted for nutrient absorption and contain a capillary loop, a central lymphatic (lacteal) and connective tissue. Each villus is also provided with a small lengthwise strip of smooth muscle. During fasting, the villi lie flat on the mucosal surface and are inactive; when exposed to the intestinal contents during digestion, they become erect and perform a lashing movement driven by rhythmical shortenings and lengthenings. These movements accelerate the flow of blood and lymph, and mechanically assist absorption by stirring the liquids of the gut in the immediate neighborhood.

Fourth, the surface area covered by the absorptive mucosal cells of the villi is also increased many times, estimated to be 14- to 39-fold, by the striated or brush border which consists of minute processes or microvilli projecting vertically into the lumen (not shown in Figure 8.21). These microvilli are extremely regular in size and evenly spaced: ~1 micron long, ~0.1 micron in diameter,⁸⁴⁸ and numbering ~1000 microvilli/cell.⁹³⁸

Our touring nanorobot now leaves the ileum and enters the headwaters of the large intestine (the colon) through the ileocecal valve (valvula Bauhini), moving into the cecum. The ileocecal sphincter normally remains mildly contracted to slow the passage of chyme into the cecum. Immediately after a meal, a gastroileal reflex forces any chyme that remains in the ileum out into the cecum, and intensifies ileal peristalsis; the stomach hormone gastrin also relaxes the valve.⁸⁵³ Thus ileocecal-resident nanorobots can usually infer the taking of a meal, even though the ileocecal sphincter contracts more tightly if the cecum becomes distended. In many animals, the bulbous cecum is a storehouse where fermentation can take place. In humans, the cecum is of no particular use. At the bottom of the cecum (~20-30 mm from the ileocecal valve) a small appendage is attached, the vermiform appendix, a twisted and coiled vestigial tube that also serves no useful purpose but often becomes clogged and infected, causing acute appendicitis (see also Section 1.2.2 footnote).

There is almost no digestion in the large intestine. The task of the colon is to absorb most of the remaining fluid and electrolytes from the chyme. Each day ~2000 cm³ of loose watery material is dehydrated and compacted in the ascending colon, and temporarily stored in the transverse colon. All but ~100 cm³ of the 500-1500 cm³ of the water that enters the colon daily is absorbed, mostly in the cecum and ascending colon.^853,2180

Structurally, the colonic tube is shirred into sacculations known as haustra, ~16 mm long segments running along the tube axis. Each haustrum is defined internally by opposing semilunar folds (corresponding to the creases on the outer surface) that act as mechanical baffles to hold the feces steady against lateral accelerations. There are mixing movements comprised of haustral contractions of the recesses of the colon, somewhat resembling the segmenting contractions of the small bowel. Three or four times a day mass peristalsis occurs, longwave contractions that begin at the middle of the transverse colon and drive the more solid contents of the transverse colon into the descending colon and sigmoidal colon. Food in the stomach triggers this reflex action. The large intestine is generally quiet between meals except for mild peristalsis at 3-12 contractions/minute.⁸⁵⁵

The mucous epithelium of the colon has a single layer of columnar cells, some of which have become specialized as unicellular slime glands (goblet cells); the colon coats feces with a thin layer of mucus to ensure smooth movement. (Amoebas move readily through the colonic mucus layer,³³¹⁶ which has an experimentally-measured mean thickness increasing from 107 microns in the ascending colon to 155 microns in the rectum.³³¹⁷) The large intestine has no villi and no circular folds,⁷⁵⁰ although absorptive cells on the colonic surface have a thin striated border (microvilli).⁹³⁵ Intestinal glands are present, but they secrete mostly mucus. Lymphoid tissue is present as scattered solitary nodules but there are no aggregated nodules.

The bacterial population²¹⁹⁸ is also of navigational significance. From the esophagus to the ileocecal valve, the upper tract is relatively sterile with few live bacteria. That's because gastric acids whose main purpose is to break down food more rapidly also act as a disinfectant to prevent the growth of microbes. The human colon, however, contains at least 400 species of bacteria,^360,3050 with ~15 types of these accounting for the majority of the intestinal microflora. These bacteria ferment any remaining carbohydrates and produce surplus vitamins (e.g., vitamins K and B₁₂) and useful amino acids which are absorbed in the colon, so the relationship is quite symbiotic. About one-third of the material in the colon is microbial mass, and up to half of the stool consists of bacteria.

Having passed through the sigmoidal colon, the traveling nanorobot finally enters the rectum. The muscular wall of the rectum is stronger than that of the colon, and that of the anal canal is thicker still. The rectum has 3-4 crescent-shaped folds (Houston's valves) that support the weight of the fecal matter and prevent its movement toward the anus, where its presence excites a sensation demanding discharge. The anus is guarded by two sphincters, internal and external. The internal (involuntary) sphincter is smooth muscle tissue formed by a thickening of the circular fibers of the muscular coat. The external (voluntary) sphincter is striated muscle tissue formed by a separate muscle encircling the lower end of the rectum. The act of defecation is preceded by a voluntary effort consisting of relaxation of the external anal sphincter and usually compression of the abdominal contents by means of straining efforts. Peristaltic waves then appear in the colon. The entire distal colon, from the splenic flexure to the anus, may be emptied at one time. The normal frequency of bowel movements is 3-12 per week among the general population.²¹⁸⁰

The 100-200 grams of feces excreted daily are 65-75% water, 5-10% fatty material, and 20%-50% bacterial bodies, with a normal pH of 7.0-7.5.^585,749,2180 Even while fasting, 7-8 grams of feces are excreted daily. (Further details are in Chapter 26.) The dark brown color of the normal stool is due chiefly to stercobilin and urobilin, the reduction products of the action of bacteria on the bile pigment bilirubin; the high-molecular-mass fraction most responsible for fecal viscosity is bacterial peptidoglycan.³³⁰⁸

Defecation is also accompanied or preceded by the escape of intestinal gases, or flatus. Normal volume is 500-1500 cm³/day, with normal frequency 6-20 per day.²¹⁸⁰ Up to ~500 cm³ of flatus is nitrogen and some oxygen from swallowed air -- it takes ~10 minutes for swallowed air to descend to the colon.⁸⁷³ Flatus contains other gases such as carbon dioxide, inflammable hydrogen and methane. The unpleasant fecal odor is due mainly to skatole and indole, produced by bacterial action in breaking down amino acids, but hydrogen sulfide and methyl mercaptan (more pronounced on a high-protein diet) also contribute. All of these chemical substances may be quantitatively measured in real time (Section 4.2) by medical nanorobots traversing the alimentary canal.

A simple map of the ~0.65 m² cylindrical luminal surface of the gut (Table 8.8) to ~(200 micron)² villiary resolution requires just ~16 million pixels or ~130 million bits assuming 8-bit pixels. This data store can be compactly encoded on ~0.005 micron³ of hydrofluorocarbon memory tape (Section 7.2.1.1) and may be carried aboard a micron-scale nanorobot in a storage device only ~0.01 micron³ or ~(0.2 micron)³ in volume. A complete map of the ~10 m² absorptive surface to ~(20 micron)² cellular resolution (excluding the brush border) requires ~25 billion pixels or ~0.2 terabits assuming 8-bit pixels.

Last updated on 19 February 2003