Coordination in Life Processes
1. What This Chapter Covers
The human body is a more complicated structure than it appears. Respiration, digestion, blood circulation, excretion and the nervous system are all built into it at specific places, carrying out specific functions in a coordinated manner.
The book is explicit about why the earlier chapters were written the way they were: we study each metabolism separately so that we can understand that no metabolism functions alone. Every process depends on other processes to keep the body in good condition.
This chapter takes one example, the digestive system, and follows it from feeling hungry to the release of energy, asking at each stage which other systems are involved.
The index allots it 10 periods across November and December and runs it from page 153 to page 175.
2. Feeling Hungry (Textbook 7.1)
Activity 1 lists six things and asks which of them make you feel hungry: the smell of food, the taste of food, the sight of food, being tired and exhausted, the need of food, and the thought of food.
The major cause lies in the physiology of blood circulation. Levels of substances in the blood are maintained mainly by the digestive system, and one of the major ones is glucose. When glucose levels fall, we get hunger pangs in the stomach.
This involves a series of proteins, some of which are hormones. Ghrelin is secreted from certain cells in the wall of the stomach when it goes empty, and the hunger-generating signals that reach the brain follow from it. The diencephalon in the forebrain and the vagus nerve, the 10th cranial nerve, are believed to carry these signals.
Hunger pangs continue for 30 to 45 minutes. An increase in ghrelin produces the sensation of hunger and the motivation to consume food.
When the stomach is full and no more food is needed, another hormone, leptin, is secreted, and it suppresses hunger.
The book adds an observation everyone has made: because we usually take food at a particular time, we usually start feeling hungry at that time — as in a school lunch hour.
3. Taste and Smell (Textbook 7.1.2)
Taste and smell are intimately entwined, most obviously in how we perceive the flavours of food. Anyone with a severe cough and cold cannot tell certain foods apart. What is affected is the flavour — the combination of taste and smell — because only the taste, not the odour, is being detected.
Taste itself distinguishes chemicals that are sweet, salty, sour, bitter or umami, umami being Japanese for savoury. Interactions between the two senses enhance our perception of what we eat.
Activity 2. Close your nose with your fingers, chew some jeera, then some saunf. Could you recognise the taste, and how long did it take? Wash your mouth and repeat with a piece of apple and then a potato, keeping the nose closed throughout.
Activity 3. Rub a pinch of asafoetida powder or garlic on a handkerchief or tissue. Close your eyes, smell it, and then try to identify different foods with a friend's help. The questions the book attaches are whether garlic has a more intense smell than apple, and how a stronger smell affects the sensation of taste.
How the two senses work
When we smell, airborne substances dissolve in the watery film of nasal mucus. The chemoreceptors in the nose — the olfactory receptors — trigger nerve impulses to the brain, where smell is detected. As food enters the mouth, the taste buds send their own signals. The brain identifies food tastes by picking up slight differences in smell.
To taste a food it must dissolve in saliva; we can only taste food that is in liquid form. Different types of papillae are present on the tongue for different tastes — circumvallate, foliate, filiform and fungiform. Only after dissolved food enters the cup-like taste buds is the sense of taste carried to the brain for analysis.
The book notes that we prefer food that is attractive to the eyes and fragrant to the nose before we taste it, so without knowing it we use sight, nose and tongue together to select food. Ivan Pavlov found that even the thought of food will water the mouth — a conditioned stimulus reflex.
Activity 4 — the role of the palate
Place sugar crystals on your tongue with your mouth open, making sure the tongue does not touch the palate, and time how long it takes to get the taste. Then repeat, this time pressing the tongue against the palate, and time it again.
Taste is identified much faster when the tongue is pressed against the palate. Taste buds are tiny papillae with an opening on top and several taste-sensitive cells inside. Food dissolved in saliva is pressed against that opening when the tongue meets the palate, reaching the taste cells and triggering the signal. This is why we normally slurp while chewing.
The book's flow chart for the sense of taste runs: mouth, tongue and nose; taste buds and olfactory receptors, with the salivary glands feeding them; then the brain.
Read the chapter along this line rather than by section. Each station in the top row is a place where the digestive, muscular, nervous and endocrine systems all act on the same mouthful at the same time — which is the chapter's whole argument.
4. The Mouth as a Munching Machine (Textbook 7.1.4)
Activity 5. Break a chalk piece in two; crush one half to tiny pieces and leave the other whole. Half-fill two cut-down bottles with vinegar, add the crushed chalk to one and the whole half to the other, and observe after half an hour.
The crushed chalk dissolves faster, which shows why food has to be broken into tiny pieces: to increase the surface area for the substances that aid digestion.
The teeth
Activity 6 asks you to look at a model or chart of the jaw and note that the teeth are not all alike. Incisors have sharp edges, canines have sharp and pointed edges, and molars and premolars have blunt, nearly flat surfaces.
The book's Fig-3 gives the arrangement in one half of a jaw as incisors 2, canine 1, premolars 2, molars 3 — the ratio 2:1:2:3 that the fill-in-the-blank question refers to, in which the 1 is the canine.
The surface muscles of the jaw help in biting and chewing and move the jaw up, down, forward and backward during mastication. The teeth cut and grind while tongue movements spread the food out and mix it with saliva, and the muscles of the mouth push the food around the oral cavity. The 5th cranial nerve controls the movement of the jaw muscles.
Activity 7 — the action of saliva
Half-fill a test tube with water and add a pinch of flour, shaking until mixed. Test a few drops in a watch glass with dilute tincture iodine: a bluish black colour confirms starch.
Divide the mixture equally between two test tubes. Add a teaspoon of saliva to one and nothing to the other. After 45 minutes add dilute iodine to both and compare.
Saliva is secreted by three pairs of salivary glands under the control of the autonomous nervous system, moistening the food so chewing and swallowing are easier. Chewing forms the food into a slurry mass, the bolus, which the tongue helps transport into the oesophagus. The enzyme salivary amylase breaks large starch molecules into smaller subunits, usually sugars. The mechanism for swallowing is under nervous coordination, with its control centre in the medulla oblongata of the brain stem.
Activity 8 — the pH of the mouth
Take a strip of pH paper with a colour chart from your chemistry teacher, touch it to your tongue, match the colour and note the pH. Take at least four readings, including some after lunch, and compare with a friend's.
The conclusion is that the saliva secreted makes the medium alkaline, which is what salivary amylase needs to act.
We are diurnal animals, so all our systems are active during the day, including the digestive system, which is ready to receive food. That is why, if we sleep during the daytime, saliva oozes out and wets the pillow, while this does not happen at night. We secrete 1 to 1.5 litres of saliva per day.
5. Through the Oesophagus (Textbook 7.1.5, 7.1.6)
The oesophagus is a long muscular and elastic tube whose upper end connects to the pharynx and lower end to the stomach. Its walls secrete mucus and carry on wave-like movement by contraction and relaxation, passing food to the stomach by peristalsis.
The oesophagus model. Lubricate the inside of a piece of waste cycle tube with oil, insert one or two oil-coated potatoes, and push them along by squeezing the tube. The questions are how you had to squeeze, how the oil helped, and whether the muscles in the oesophageal wall must do something similar.
Mucus lubricates and protects the oesophageal walls from damage and helps the bolus slide down, just as the oil did; the saliva already in the bolus helps too.
The wall has two kinds of smooth muscle: an inner layer of circular muscles and an outer layer of longitudinal muscles.
- Contraction of the circular muscles narrows the oesophagus just behind the bolus, squeezing the food downwards.
- Contraction of the longitudinal muscles in front of the bolus widens the tube, shortening that part of the oesophagus.
Together these produce the wave that propels the bolus into the stomach. Peristalsis is involuntary and under the control of the autonomous nervous system, which is why people are advised not to swallow without chewing properly or to eat in a hurry.
6. The Stomach (Textbook 7.1.7)
When food is in the oral cavity, the nerves in the cheek and tongue are stimulated and carry impulses to the brain. The brain analyses the message and transmits it back through motor nerves to the wall of the stomach, stimulating the gastric glands to produce gastric juice — so the stomach is already preparing before the food arrives.
The walls secrete juice containing hydrochloric acid and other digestive juices, and these secretions are stimulated by the nervous system. Contraction of the stomach muscles squeezes and mixes the food with the acids and juices, turning it into a smooth porridge-like chyme, and some large protein molecules are broken into simple ones here.
The book's Fig-7 names three distinct movements: propulsion, in which peristaltic waves move food from one part to another; grinding, in which the most vigorous peristalsis and mixing occur close to the pylorus; and retropulsion, in which small amounts of chyme are pushed into the duodenum while most of it is forced back into the stomach.
As digestion nears completion the contractions decrease, and the pyloric sphincter at the junction of stomach and duodenum relaxes, releasing the partly digested chyme into the duodenum in small quantities.
Rumination and reverse peristalsis
Watch a cow or buffalo under a tree and you see something moving from throat to mouth before it starts chewing again. That is rumination — the bolus moving back from near the stomach to the mouth, which is reverse peristalsis.
It is normal in ruminants, which have an extra pouch in the stomach to store quickly swallowed food. In human beings it is mainly a protective mechanism to expel unwanted substances from the food canal.
How long food stays
| Percentage emptied | Emptying of stomach | Emptying of small intestine |
|---|---|---|
| 50% | 2½ to 3 hours | 2½ hours |
| Total 100% | 4 to 5 hours | 30 to 40 hours, transit through colon |
The book cautions these are only averages, varying between individuals and after different meals. It also notes the stomach is not a bag of fixed volume but an elastic pouch whose size increases with the food taken in.
Why the stomach does not digest itself
Digestive juices are produced depending on the quantity of food; if the same amount were produced regardless, the stomach walls would be destroyed.
Lab Activity — acid and leaf. Take two similar green leaves. Apply vaseline, petroleum jelly or grease to one and leave the other bare. Add one or two drops of a weak acid to both and observe after half an hour.
The greased leaf is protected. Mucus secreted by cells in the stomach wall forms a thin lining that counters the acid in exactly the way the petroleum jelly did, and that is what protects the stomach from its own secretions. The book notes the HCl is strong enough to digest hard bone.
7. The Small and Large Intestine (Textbook 7.1.8)
When the chyme enters the intestine, its acidic nature initiates the production of hormones — secretin and cholecystokinin — which stimulate the pancreas, liver and intestinal walls to secrete pancreatic juice, bile juice and succus entericus.
Absorption by the villi is a very selective process, and the intestinal walls allow only tiny nutrient particles to pass.
Activity 9. Make a tube from a 10 by 20 cm sheet of chart paper and another from a 20 by 20 cm sheet, and try to insert the larger inside the smaller. You cannot. Now take another 20 by 20 cm sheet, fold it as many times as possible, join the ends into a folded tube, and try again — this time it fits.
Folding packs a much larger area into the same space, and that is exactly what the thousands of finger-like villi on the inner surface of the small intestine do. They increase the surface area so that food retained in the folds stays longer, enhancing absorption.
The book's schematic of a villus labels the epithelium, a network of blood capillaries, the mucus membrane and mucus glands.
The second brain
The digestive tract is unique among internal organs because it is exposed to a large variety of physiochemical stimuli from the external world in the form of ingested food. In response it has developed a rich store of coordinated muscular movements together with a neural apparatus, to ensure appropriate mixing and propulsion during digestion, absorption and excretion.
That neural apparatus is so vast that scientists have nicknamed it the second brain. Research is currently investigating how it mediates the body's immune response — at least 70 per cent of our immune system is aimed at the gut, to expel and kill foreign invaders — and how the trillions of bacteria in the gut communicate with the cells of the gut nervous system.
Technically it is the enteric nervous system: sheaths of neurons embedded in the walls of the alimentary canal, which measures about nine metres from oesophagus to anus. It contains some 100 million neurons, more than in either the spinal cord or the peripheral nervous system, which lets us feel the inner world of the gut and its contents.
Equipped with its own reflexes and senses, the second brain can control several gut functions often independently of the brain, although it is not the seat of conscious thought or decision-making. The book's own test case is the everyday one: if you are tense for some reason you start having loose motions.
Expelling the waste
The book's image is a roll of tea leaves wrapped in tissue paper: press it, open it, and the tissue has absorbed the water. Similarly, waste material reaches the large intestine, peristaltic waves move it into the rectum, the left side of the colon acts like a storage tank, water is reabsorbed, and the remaining hard mass is stored in the rectum before being expelled through the anus.
There are two muscular layers at the exit. The internal anal sphincter is involuntary; the external anal sphincter is under voluntary control.
The book contrasts the two pathways of waste expulsion: expulsion via blood through the kidneys and skin, which removes mainly salts, water and urea; and expulsion of undigested food matter as stool, which happens exclusively through the intestine.
8. Why Digestion Needs Respiration and Circulation
To obtain energy from food, the food has to be oxidised, and for that respiration must go on.
During inhalation, oxygen moves across the walls of the alveoli into the blood, enters the red blood cells and is distributed to the cells of the body. At the same time carbon dioxide moves from blood into the alveoli and is breathed out. Nutrients in the cells are oxidized and energy is released.
Breathing is involuntary and controlled by the medulla oblongata through the autonomous nervous system. The movement of the intercostal muscles and the diaphragm moves the rib cage, inflating and deflating the lungs.
So the conclusion the chapter has been building to: digestion occurs in the food canal, but coordination of respiration and blood circulation is necessary, or the oxidation of food and the transport of substances vital to releasing energy will not take place — which would lead to the shut down of systems that depend on each other.
9. The Book's Annexures
A window in a stomach
At Fort Mackinac on the upper Michigan peninsula, a 19-year-old voyageur, Alex St. Martin, was accidentally shot in the stomach. The wound perforated the abdominal wall and stomach with profuse bleeding, and the army surgeon Dr Beaumont was called. He cleaned the wound, pushed the protruding portions of lung and stomach back into the cavity and dressed it.
Beaumont was surprised to find St. Martin alive the next day. When the wound healed the stomach had fused with the body wall leaving a hole, with part of the wound forming a small flap that resembled a natural valve — which allowed fluids to be drawn out for testing. 16 June 1822 became the beginning of some of the most pioneering experiments in medicine.
For centuries the stomach had been thought to cook food by producing heat, and had been viewed as a mill, a fermenting vat or a stew pan. Beaumont's findings overturned that.
- He measured the temperature of the stomach during digestion and, to his surprise, found no change — it was maintained constant at 100 °F, 38 °C.
- He found that pure gastric juice contains large amounts of HCl, contrary to the previous view that it was simply water, and that even the hardest bone cannot withstand its action. It could effect digestion even outside the stomach, which made HCl a chemical agent rather than a mechanical one.
- He found gastric juice is not stored in the stomach but secreted when food is taken.
- He found digestion begins immediately when food enters the stomach, testing the contents exactly 20 minutes after a meal and finding it already well under way.
- He found that food in the stomach satisfies hunger even though it has not been eaten, by making St. Martin fast until four o'clock and then introducing food through the flap — the sensation of hunger subsided.
Vomiting and belching
When we eat something spoiled or unfit, the digestive mechanism recognises it and refuses to digest it. A disturbance in the stomach walls, working under the involuntary nervous system, expels the chyme with the undigested food — vomiting.
Sometimes we suddenly belch, and some digestive juices move back into the mouth through the oesophagus, giving a burning sensation in throat and chest from the backward movement of acid. These muscular contractions are controlled by the 10th cranial nerve under the autonomous nervous system.
Key words from the chapter
Ghrelin, leptin, taste receptor, chemoreceptors, papillae, food bolus, peristalsis, chyme, pyloric sphincter, villi, medulla oblongata, brain stem, nervous system.
10. Summary
Food has to be broken down into its constituent substances for proper digestion, assimilation and the release of energy, and the human digestive system involves both the muscular and the nervous systems.
A special nervous system in the gut, of nearly 100 million nerves, coordinates muscular activity, blood flow, digestion, absorption of nutrients and other activities of the gastro-intestinal tract.
The hormone ghrelin, secreted in the stomach, generates the sensation of hunger; leptin suppresses it.
Taste can be identified easily only when the tongue is pressed against the palate, and taste and smell are closely related — the chemoreceptors in the nose and on the tongue trigger nerve impulses to the brain, where both are detected.
Saliva maintains an alkaline medium that aids the digestion of starch; the mouth also secretes acid, which protects it from harmful bacteria. Saliva is released by the salivary glands under the autonomous nervous system to moisten food and make chewing and swallowing easier.
The tongue is a muscular and sensory organ that is not only gustatory but also shifts and mixes food in the oral cavity and helps swallowing, and the mechanism for swallowing is coordinated by the swallowing centre in the brain stem.
Contraction and relaxation of gut muscles produce the wave called peristalsis, which travels the entire length of the food canal and is involuntary, under the control of both the autonomous nervous system and the gut nervous system.
Muscular contractions of the stomach churn food into chyme, whose entry into the duodenum is regulated by the pyloric sphincter. The strong HCl makes the stomach acidic so that protein-digesting enzymes can work, and the mucus lining protects the stomach from its own acids.
The coordination of digestion, respiration and circulation is necessary for the utilization and oxidation of food and the transport of nutrients, and muscular and nervous control keeps all of it regulated.
