Human Digestive and Respiratory Systems

Think of the digestive system as a long, one-way tunnel with a few helper factories along the way. Food goes in at the mouth, is broken down by size (mechanical digestion) and by chemicals (chemical digestion), the useful parts cross the wall into the blood, and the leftover leaves at the anus. The tunnel itself is called the alimentary canal (digestive tract), and the factories (salivary glands, liver, pancreas) are the digestive glands.

Food moves along the canal by peristalsis: waves of squeezing and relaxing by smooth (involuntary) muscle in the wall. That is why you can swallow even while standing on your head, and why you cannot control digestion by will. Only the first step (chewing and swallowing) and the last step (passing stools) are under voluntary control.

Teeth belong to the mouth's job of mechanical digestion. Humans are heterodont (teeth of different kinds), thecodont (each tooth sits in a socket in the jaw bone) and diphyodont (two sets in a lifetime: 20 milk teeth that start to appear at about six months, replaced by 32 permanent teeth). The last four permanent teeth, the third molars, are the wisdom teeth, which appear between roughly 17 and 25 years of age.

Dental formula shows the teeth in one half of the upper jaw over one half of the lower jaw, in the order incisors, canines, premolars, molars. Adult human: 2 1 2 3 / 2 1 2 3 per half, so 16 teeth in each half and 32 in all (written as 2123/2123). Milk teeth: 2 1 0 2 / 2 1 0 2 per half = 20 in all, with no premolars.

Look inside one tooth. The visible part is the crown, covered by enamel, the hardest substance in the body (made of calcium phosphate, and it has no living cells, so it cannot repair itself). Below it is dentine, then the pulp cavity full of blood vessels and nerves, and the part in the socket is the root. Tooth decay begins when bacteria in plaque turn the sugar you eat into acid; the acid dissolves enamel, and if it is left untreated the germs may reach the pulp and cause infection and pain. Fluoride strengthens enamel, but too much fluoride in water causes fluorosis.

Digestion cannot happen by squeezing alone; chemicals must break big food molecules into small ones that can cross the gut wall. These chemicals are made by glands. Learn the glands in the order food meets them: salivary glands in the mouth, gastric glands in the stomach, then the liver and pancreas which pour their juices into the first part of the small intestine (duodenum), and finally the intestinal glands in the small intestine's own wall.

Enzymes are biological catalysts, mostly proteins. They lower the activation energy of a reaction, so it happens fast at body temperature; they are not used up; and each works on one substrate (the lock and key idea). Their speed changes with temperature and pH. Pepsin likes acid (pH about 2), trypsin and amylase of the intestine like alkali. A quick naming habit: the enzyme often carries the name of its substrate: lipase (lipids), protease (proteins), lactase (lactose), maltase (maltose), sucrase (sucrose), amylase (starch).

The liver deserves a closer look because it is also the body's chemical factory. It sits in the upper right of the abdomen, weighs roughly 1.2 to 1.5 kg, and is the largest internal organ and the largest gland. Besides making bile, it stores glycogen and vitamins (A, D, B12) and iron, converts toxic ammonia into urea, detoxifies drugs and alcohol, makes plasma proteins (albumin, fibrinogen, prothrombin) and the anticoagulant heparin, and helps break down old red blood cells. It can regenerate itself, which is why part of a liver can be donated by a living person. Bile leaves the liver through the hepatic duct, is stored in the gall bladder, and reaches the duodenum through the bile duct, which joins the pancreatic duct.

An animal like us follows holozoic nutrition, which has five steps in a fixed order: ingestion (taking food in), digestion (breaking it down), absorption (moving the useful products into blood or lymph), assimilation (cells using them for energy and growth) and egestion (throwing out what cannot be used). The first step of the five is ingestion. Plants are autotrophs; most animals are heterotrophs; fungi feed saprophytically on dead matter; and parasites such as tapeworm and Cuscuta feed on a living host.

Now follow one meal. In the mouth teeth grind, saliva wets, and salivary amylase turns starch to maltose. In the stomach (acidic) the churned food becomes chyme, and pepsin starts on proteins. When chyme enters the duodenum, three things join it: bile (emulsifies fat), pancreatic juice (amylase, lipase, trypsin) and the bicarbonate that makes the surroundings alkaline. The small intestine then finishes the job with its own enzymes. The three big nutrient classes end up as simple, absorbable molecules.

Absorption is where the small intestine shows its engineering. Its inner wall is folded, covered with millions of finger-like villi, and each villus has a surface of still tinier microvilli. This multiplies the surface area many times so that nutrients cross quickly. Each villus contains a network of blood capillaries and one central lymph vessel called a lacteal. Glucose and amino acids enter the capillaries and go to the liver through the hepatic portal vein; fatty acids and glycerol are re-built into fat and enter the lacteals, finally reaching the blood through the lymph. The large intestine absorbs the leftover water and salts, and the bacteria living in it make some vitamins, notably vitamin K and some B-complex vitamins.

Why are villi so important? Absorption is by diffusion and active transport, and both work faster when the surface is larger and the wall is thin. Villi give a huge surface, a thin one-cell-thick wall, and a rich blood supply that keeps carrying the nutrients away. That is the same design logic as alveoli in the lungs.

A few more connected facts. The gastro-intestinal wall is made of smooth, involuntary muscle that contracts slowly and continuously. Goblet cells in the lining of the digestive tract make mucus to lubricate food and protect the wall. Three gut hormones are worth knowing: gastrin (stomach: stimulates gastric juice), secretin (stimulates the pancreas to release bicarbonate-rich juice) and cholecystokinin or CCK (makes the gall bladder contract and pancreas release enzymes). In ruminants such as cow, buffalo, goat and sheep the stomach has four chambers (rumen, reticulum, omasum, abomasum) and the animal chews the cud (rumination) to digest cellulose with help of microbes. Diarrhoea is loss of water and salts through loose motions, treated by oral rehydration solution (ORS); constipation is hard stools, prevented by roughage and water.

Every cell needs oxygen to release energy from food and must get rid of the carbon dioxide that this produces. The respiratory system is the delivery gate for both. Do not mix up two words: breathing is the physical movement of air in and out of the lungs; respiration is the chemical release of energy inside cells. Breathing serves respiration.

Why does air rush in? Air always flows from high pressure to low pressure. The lungs have no muscle of their own. When the diaphragm and rib muscles enlarge the chest cavity, the pressure inside the lungs falls below the outside pressure, so air flows in. When they relax, the cavity shrinks, the pressure rises and air is pushed out.

Now the gas exchange itself. In the alveoli, oxygen is at higher concentration than in the blood arriving there, so it diffuses across the thin wall into the blood; carbon dioxide, at higher concentration in the blood, diffuses out into the alveoli and is breathed out. Ideal exchange needs a large surface, a thin wall and a moist lining, which is why alveoli are built as they are. Most oxygen is carried in red blood cells by the iron-containing respiratory pigment haemoglobin (about 97 per cent, the rest dissolved in plasma). Carbon dioxide is carried mostly (about 70 per cent) as bicarbonate in the plasma, about 20 to 25 per cent bound to haemoglobin, and a small part dissolved in plasma. Haemoglobin picks up carbon monoxide about 200 times more eagerly than oxygen, which is why CO is so dangerous.

A spirometer measures how much air the lungs move. These figures are for a healthy adult and are approximate, but the names and the formula are what the exam wants.

Other animals solve the same problem differently. Amoeba, sponges and Hydra breathe by simple diffusion over the whole body. The earthworm breathes through its moist skin. Insects have a network of air tubes (tracheae) opening by pores called spiracles, so their blood does not carry oxygen. Fish use gills, frogs use skin, mouth lining and lungs, birds use lungs with air sacs, and mammals use lungs. In plants, gas exchange happens through stomata, lenticels and root surfaces. Haemoglobin (iron) is the pigment of vertebrates, while many crustaceans and molluscs use blue, copper-based haemocyanin.

Food is like coal in a store room: it holds energy, but the cell cannot spend it directly. In cellular respiration the cell breaks down glucose step by step and captures the released energy in a small, ready-to-spend molecule called ATP (adenosine triphosphate), the energy currency of the cell. When ATP loses one phosphate and becomes ADP, energy is released, and the cell uses it for muscle contraction, active transport, making proteins, nerve impulses and everything else that needs energy.

ATP-driven reactions are endothermic (energy-requiring) ones: ATP supplies the energy and the reaction runs. The ATP factory of the cell is the mitochondrion ('powerhouse of the cell'), a double-membrane organelle whose inner membrane is folded into cristae.

Notice the logic. Only the last stage needs oxygen directly, and oxygen's job is to take away the spent electrons so the chain can keep running. If oxygen is missing, the cell cannot continue past glycolysis, and it has to find another way to keep glycolysis going. That alternative is anaerobic respiration (fermentation).