In this lecture we shall condense two semesters of Anatomy & Physiology into a single lecture. Of course, you remember all of this from your A & P course, but I would like to remind of a few key aspects of anatomy and physiology which are considered important to your understanding of nutrition.
It is at the cellular level where nutrients ultimately fulfill their destiny. After digestion and
absorption, nutrients are transported to the various tissues of the body, and are withdrawn from
the circulatory system [or sometimes interstitial fluid] by individual cells as the cells need the
nutrients. For most nutrients, the movement into the cell is active transport rather than passive
diffusion, which means the cells are choosing to aborb the nutrients in response to an internal
When the cell's needs are for energy in the form of ATP molecules, the mitachondria take glucose [or some glucose analog] from the cell contents, and carry out the catabolic pathways which capture [as ATP molecules] the energy which had been stored in the chemical bonds of the energy-rich glucose-like molecules. The mitachondria release the captured energy to the rest of the cell by moving ATP molecules outward across the mitachondrial membrane. Meanwhile, the cell withdraws glucose to replenish the stored glucose in the cell.
When the cell needs to carry out anabolic reactions with nutrients as substrates, the cell first utilizes stored nutrients from the cell, then will withdraw [from serum] the additional nutrients needed for the reactions and for replenishing stored nutrients, usually by active processes which will utilize ATP molecules as the energy source for the active transport. Anabolic reactions which produce proteins are mediated by ribosomes; however, all anabolic reactions which do not produce proteins require enzymes, some of which will have to be manufactured by the ribosomes. Those enzymes, co-enzymes and co-factors which the cell cannot make must be withdrawn from serum.
The higher animals have a “one-way digestive system.” Jellyfish and corals do not, so they must expel undigested reside through the same opening that was used to acquire food. Fortunately jellyfish and coral don't have brains, so they don't think about how disgusting it is to expel undigested residue through their mouth! While not defecating through the mouth is intellectually appealing, that is not the major advantage of the one-way system. The actual advantage is that the digestive system can be divided into distinct regions where different food substances can be digested, making the digestive processes more efficient. The price we pay for having an efficient digestive system is the metabolic cost of using peristalsis to move the contents along the system, and of using muscular sphincters to control the flow of food substances. For the Human population, we can summarize the digestive system as having four major regions as follows, with their major functions:
|complex carbohydrates → sugars|
|stomach||protein → amino acids|
|small intestine||lipids → fatty acids,
|large intestine||cellulose? (via symbionts?),
Now, in annoying detail, we shall travel from food to nutrients, and beyond, in a trip through the
Preparation: We begin by setting out on a hunting and gathering expedition, which costs energy. In Nursing terms, this activity is an ADL, but in nutrition terms is called Dietary thermogenesis, or the thermic effects of food. For contemporary people in civilized countries, we do our hunting and gathering at the grocery store. Once we acquire a food supply in our cave [or mortgaged suburban home], we must prepare and serve a meal. As we eat, we cut the food into bite sized pieces to begin its journey into the digestive system via the mouth, all without any activity that counts as exercise.
Mouth: Chewing uses the jaw muscles and teeth to break food into small bits. As we chew, the tongue gathers food particles from around the teeth and gums, and checks the consistency to determine if it's ready to be swallowed. As the food particles are being examined by the tongue, the tongue is rolling the food against the hard palate to form a bolus or ball. Occassionally, the cordination of the tongue and jaw muscles gets out of synch, and the tongue is trapped between the teeth as the jaw closes painfully on the tongue. As soon as the tongue is satisfied with the consistency of the bolus, the bolus is rolled back across the soft palate to the epiglottis, triggering swallowing by peristalsis in the throat and esophagus. When we began chewing, the stomach was triggered to release digestive enzymes [particularly peptase, for digestion of the protein in the bolus]; and the salivary glands were releasing saliva [particularly, salivary amylase, for digestion of the complex carbohydrates in the food, and water for lubrication during swallowing]. The bolus moves surprisingly slowly down the esophagus, extending the time for salivary amylase to digest complex carbohydrates, because the salivary amylase (a protein) will be digested in the stomach, temporarily suspending carbohydrate digestion.
Stomach: As a bolus approaches the stomach [detected by propriosensors in the esophagus], the stomach relaxes the cardiac sphincter, allowing the food to enter the stomach, then immediately closes the sphincter to retain the food in the stomach. Failure to properly close [note the split infinitive] the cardiac sphincter results in “acid reflux” or, to non-medical people, “heartburn.” Waves of peristalsis and reverse peristalsis agitate the contents, liquifying the food [to chyme and mixing it with peptase and Hydrocloric acid. The acid is made from the Chloride ions in table salt (Sodium chloride, NaCl). Theoretically if one could remove all salt from the diet, it could prevent the formation of stomach acid, but there is no evidence of this in any Humans. The acid (pH = 3.3, comparable to coffee and tomato juice) is needed for effective digestion of protein, but interferes with the digestion of carbohydrates and lipids. Lipids tend to clump into rather large blobs at this pH, in spite of the agitation. Depending on the protein density of the food, this sloshing may continue for hours. When the stomach considers the protein to be sufficiently digested, it relaxes the pyloric sphincter, and triggers the gall bladder to squirt bile into the duodenum. Limited absorption takes place in the stomach, and it appears that only certain chemicals can be absorbed here, unfortunately including nicotine and some street drugs.
Small intestine: The partially digested food, or chyme, enters the duodenum [part of the small intestine] where bile titrates the acidity to circumneutral (pH ≥ 7.0), and emulsifies the lipids [breaks large globules into small globules], which is necessary for proper digestion of fats since fat digestion is a surface phenomenon [you can only digest the surface of a fat globule; this is not an issue for carbohydrates and proteins since they were liquified in the stomach]. Enzymes to digest carbohydrates, lipids and proteins are secreted by the duodenum, so digestion of all digestable food continues. Note, at this point in the system, (1) carbohydrates are already digested to fragments of complex carbohydrates [and some intact complex cardohydrates called fiber] plus disaccharides, and will be slowly digested to disaccharides and “soluable fiber” (undigestable carbohydrate fragments); (2) a few of the lipids are already digested to fatty acids while most are intact, some of which will be digested to fatty acids; (3) proteins have been fragmented to amino acids and polypeptides which will be digested to amino acids. Some absorption takes place in the duodenum. The chyme then moves into the small intestine itself where all texts state that the majority of absorption occurs. Digestion crashes on at a leisurely pace. In spite of its liquid character, and peristalysis, the food material may remain in the small intestines for days. Minimum residence time under moderately severe diarrhea is about 24 to 36 hours.
Large intestine: After loitering in the small intestines, the chyme moves into the colons [ascending, tranverse, descending, and sygmoid]. The chyme turns right as it enters and travels up the ascending colon; to the left is the caecum [where we store bacteria to replenish those lost during episodes of moderate diarrhea, although not many texts agree with me on this one]. Hidden back in the cul-de-sac of the caecum is the dark alley known as the appendix. The appendix serves a far more useful function than getting infected so a surgeon can make his boat payments. Because it is so well hidden, not even severe diarrhea can flush out the bacteria stored here, suggesting that these bacteria are crucial to Human nutrition. Similar structures are found in termites, where the termites store the micro-organisms which digest wood. To me this suggests that we do the same thing. The appendix stores those bacteria we need to digest otherwise undigestable food, such as lettuce from which we extract many important vitamins and phytochemicals. The extraction of these requires digestion of the cellulose in lettuce, which we cannot do. So in exchange for room & board, we hire bacteria to do the digesting for us. [The hypothesis was my own crazy idea, which I thought makes sense. A brief note in Science, a journal of the American Association for the Advancement of Science, vol 326 (23 October 2009, p. 503] summarizes an article in the Journal of Evolutionary Biology, in which (Smith et al, J. Evol. Biol. 22, p. 1984, 2009) hypothesize exactly this].
Back to the Saga of the Traveling Chyme [a possible movie title?]… by taking the long way around (up to the top of the abdomen, across, then back down), the residence time of the mostly digested food is prolonged. This again suggests that something important is happening here. So, in my world, the ascending and transverse colons are the site of symbiontic bacterial digestion of undigestable food and absorption of the extracted nutrients. In the transverse colon large amounts of water are re-absorbed into the circulatory system, thickening the chyme to sludge. The descending and sygmoid colons add squeezing to extract most of the remaining water, dehydrating the sludge-like chyme to the consistency of a soft stool. The rectum finally squeezes hard to form normal fecal material; it can also overdo it (often under sub-clinical to moderate dehydration of the patient) causing constipation. Under severe dehydration of the patient, the rectum can form blockages that would challenge even the best household drain cleaners
[WARNING, … NEVER administer drain cleaners to relieve severe constipation!
Drain cleaners contain a very strong base (sodium hydroxide, NaOH), which is always fatal when taken internally. Unlike strong acids where one acid molecule is destroyed for each Human molecule destroyed, strong bases are not changed as they destroy Human molecules. The base ions (OH) will remain in the interstital fluid, destroying molecules until the life of the individual has ended. This is based on clinical experience, such as a woman who tried to commit suicide by swallowing Drano. She changed her mind, called 9-1-1, and had her stomach pumped. Repeated pumping at the ER, then OR, only removed the stomach contents [which at the time were tested for pH and found to be basic not acidic (remember pH is supposed to be near 3; hers was above 8), and contained part of her stomach's mucosa]. A month later, she returned for treatment of her ulcer caused by the residual Drano in her stomach's epithelial lining. Next month, she returned for treatment of her bleeding ulcers, then again for her perforated ulcers, and finally for her appointment with the coroner after several miserable months at extreme levels of pain. The coroner found numerous other visceral organs severely damaged by the Drano].
At long last we have arrived at the anus, which is a sphincter which allows us to chose when to expel the undigested remains of food intake from a week or so ago. The consistency and color of the fecal material is an important diagnostic tool for assessment of general health of the patient. Normal [healthy] Human fecal material is “formed” (bananna-shaped according Roizen and Oz), and medium brown.
Humans have a closed circulatory system, which means the blood never [almost] leaves the vessels of
the circulatory system. We also have a parallel “open” circulation in the lymphatic
circulation. The cardiovascular system consists of a heart and three types of blood vessels (four if
you count the lymphatic). There are arteries, which carry blood from the heart to the
capillary beds in each of the tissues. Arteries circulate blood under pressure (the systolic
pressure), pushing against the resistance (dystolic pressure) due to friction of blood flow across
the lining of the blood vessels [this would be “back pressure” in Engineering terms].
Veins, transport the blood from the capillary beds back to the heart. Then there are vessels
which neither come from the heart, nor return to the heart; these are called portal vessels. One of these is of particular interest here [the other one is
not “interesting” because I don't want to talk about it]: the hepatic portal vessel which
transports blood rich in nutrients from the mesenteric capillaries to the liver, where the majority
of homeostatic regulation of nutrient density of the serum occurs. The surplus non-energy nutrients
(amino acids, vitamins, minerals) are withdrawn from the serum, and stored in the liver up to the
capacity of the liver to store them, and excess beyond the liver storage are dumped back into the
blood stream to be removed and discarded by the kidneys. The regulatory functions and storage
options of the liver for energy providing nutrients are much more complex. For example, blood
glucose [sugar] goes through the following steps after removal from the blood stream for storage:
excess serum sugar - stored as sugar in liver
excess sugar in liver storage - stored as glycogen in liver/muscles
excess glycogen in liver storage - stored as fat in liver
excess fat in liver storage - stored as fat in adipose tissue
TABLE OF CONTENTS
© 2004-2010 TwoOldGuys
revised: 02 Jun 2010