In this lecture we shall condense two semesters of Anatomy & Physiology into a single lecture. Of course, you remember all of this from the A & P course you didn't take, but I would like to explain a few key aspects of anatomy and physiology which are supposed to help you understand Nutrition, at the level of a Community College student. [WARNING! Will Robinson, I have a rather high opinion of what Liberal Arts Community College students expect to learn!]
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absorption, nutrients are transported to the various tissues of the body, and are withdrawn from the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 by individual cells as the cells need the nutrients (either to replace stored
nutrients, or for current needs). For most nutrients, the movement into the cell is active transport
rather than passive diffusion, which means the cells are choosing to absorb the nutrients in response
to an internal need, and spending metabolic energy to
absorb them. At the cellular level, metabolism is “all of the
chemical reactions necessary to maintain Life.” These reactions are grouped into
anabolic reactions which “join smaller substrate molecules together
to make larger molecules, at the expense of energy,” and catabolic
reactions which “break larger molecules into smaller molecules, releasing energy.”
When a cell needs energy to do physical or chemical work, it must use the energy stored in ATP molecules, yet ATP can only be stored for a very short time because the ATP molecule is not stable. ATP molecules are synthesized (anabolism) in the mitachondria using energy from the breakdown (catabolism) of sugar (glucose) to Carbon dioxide and Water. The mitachondria take Glucose from the cell and produce ATP's which are released from the mitachondrion to the cell. The cell will replace the glucose by taking some from the blood stream. Because this is the only known mechanism by which cells can get usable energy, the number of mitachondria you own will determine how fast (or slow, since each mitachondrion will slowly make ATP even when demand is minimal) you can use energy. If you were a plant, your cells could also make ATP in the chloroplasts using energy from sunlight.
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 the blood stream] 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 the blood stream as needed.
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 muscle contractions (called peristalsis) to move the contents along the system, and of using muscular valves (called 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 (Activity of Daily Living), 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. 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 (nerves which serve to monitor the status of the body, creating self-awareness)], 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 of the sphincter to close properly results in “acid reflux” or, to non-medical people, “heartburn.” Waves of muscle contractions in the stomach walls 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, carbohydrates are fragments of complex carbohydrates [and some intact complex cardohydrates called fiber] plus disaccharides; some lipids are intact and some digested to fatty acids; and proteins have been fragmented to polypeptides and some 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 reckless 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 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 lettuce, which we cannot do. So in exchange for room & board, we hire bacteria to do the digesting for us. [This hypothesis used to be just one of my crazy ideas… 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… 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 mild to moderate dehydration of the patient) causing constipation. Under severe dehydration of the patient, the rectum can form blockages.
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 [the higher number, normally equal to about 120]), pushing against
the resistance (dystolic pressure [the lower number, normally equal to 80]) 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. These vessels operate at virtually no pressure (80), but merely flow pushed by arterial
blood flow. 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 intestines to the liver,
where the majority of homeostatic regulation of nutrient density of the
blood stream occurs. (Homeostasis is the “tendency of living creature to maintain a reasonably
consistent internal environment,” although the external environment may undergo large changes.)
The surplus non-energy nutrients (amino acids, vitamins, minerals) are withdrawn from the blood, 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: 9 Aug 2010