A description of the digestive process –
Abbreviated form first:
Food gets put into the mouth. Chewing breaks the food mechanically into smaller particles. The salivary glands produce saliva, which moistens the crushed-up food. Saliva also contains enzymes which begin to break down carbohydrates. The chewed food is swallowed, using the action of the tongue. The food gets moved down the esophagus into the stomach. Stomach acids and the enzyme pepsin break down proteins. After the food leaves the stomach and enters the small intestines, gall from the gall bladder and enzymes from the pancreas are added, emulsifying the fats in the food and breaking them down into fatty acids. The proteins are enzymatic broken down into peptides and amino acids. Enzymes break carbohydrates down into sugars. The fats, proteins, and sugars are absorbed in the small intestines. In the large intestines, a host of bacteria ferment undigested foodstuffs. Water and minerals get absorbed. The indigestible remains are stored in the rectum for egression, ‘bowel movements’.
In more detail, the individual stages and processes of digestion are:
The teeth cut and grind the ingested food into smaller particles. Salivary glands excrete saliva, consisting mainly out of water, mucus and the enzyme amylase. Amylase starts to turn the starch contained in the food into maltose. This explains the sweet taste that we can experience when chewing grains or rice. The food gets softened and formed into a ball shape, called a bolus. The tongue then pushes the bolus toward the throat to be swallowed.
Thorough chewing facilitates better digestion in all following stages, and allows time for hormones to be created which control the feeling of satiety.
When we swallow, a skin flap prevents food travelling into the trachea (windpipe) and guides it into the esophagus. The bolus moves down the esophagus to the stomach, transported by a muscular movement called peristalsis. Not much digestion takes place in the esophagus. The food passes then through the hiatal valve, also called esophageal sphincter, at the top of the stomach. The hiatal valve prevents the acidic chyme from the stomach to re-enter the esophagus. Malfunction of the hiatal valve, hiatus hernia, leads to reflux, acidic heart burn, or GERD.
Depending on the composition, foods will stay in the stomach for less than 2 to more than 4 hours, whereby fatty foods stay for longer than mainly carbohydrate foods. When food enters the stomach, its walls start secreting a mix of diluted hydrochloric acid and enzymes, particularly pepsin. Contractions of the stomach walls churn and mix the food and the acid, and reduce the size of food particles even further. This mix is called chyme. The enzyme pepsin breaks proteins into smaller particles, i.e. peptides and amino acids. It works optimally in the acidic environment (pH 1 to 2) of the stomach. The strong stomach acid also kills bacteria.
Enzymatic digestion of carbohydrates is interrupted in the stomach, because the enzymatic action depends on a specific pH value (acidity).
A strong lining of mucus protects the stomach wall from being ‘digested’ (if this protection fails the stomach can develop gastric ulcers).
The pyloric valve, another sphincter muscle, controls and regulates the exit of the chyme into the small intestines.
The small intestine consists of the duodenum, the jejunum, and the ileum. It has a total length of about 7.5 meter. Its walls are lined with finger-like protrusions, called villi (singular: villus), which in itself are covered with even smaller protrusions, called microvilli. This results in a very large surface area of the intestinal tract for optimal absorption of nutrients.
The duodenum is the first, 30 cm long, part of the small intestines. It is lined with a layer of mucus to protect it from the acidity of the chyme. The pyloric sphincter releases small amounts of chyme into the duodenum. Here, the chyme gets neutralized by alkaline bile salts from the liver and gall bladder, and by pancreatic juice containing bicarbonate. The pancreas also excretes enzymes into the duodenum, particular amylase, trypsin, and lipase.
Most of the absorption of nutrients from the food takes place in the jejunum and ileum which have both a similar structure, whereby more absorption of fats (lipids) takes place in the ileum.
Processing of carbohydrates in the small intestines
Carbohydrates in food are starch, cellulose (both polysaccharides), and sugars – particular sucrose, lactose (both disaccharides) and fructose. Most carbohydrates are too big to be absorbed directly and have to be broken down by enzymes. This can involve several steps.
Cellulose cannot be metabolized by human enzymes, but, as insoluble fibre, has positive effects on the colon. Cellulose gets fermented by bacteria in the large intestine.
The enzyme amylase breaks down starch into the sugar maltose (a disaccharide).
Maltose, lactose, and sucrose are split into monosaccharides, which is done by enzymes on the walls of the microvilli (called ‘brush border’ hydrolases). The enzyme maltase, for instance, splits maltose into two molecules of glucose. Lactose is split into glucose and galactose. Sucrose is split into glucose and fructose. Glucose and galactose get actively transported away from the inside of the small intestines, through the cell walls of the microvilli, together with one sodium ion. Fructose gets absorbed differently. The monosaccharide then gets absorbed by the blood capillaries which run through the microvilli.
The blood from the capillaries gets transported to the liver. The liver is the main regulator of the blood sugar levels and processes the glucose into glucagen for intermediate storage, which the liver can break down later and release as glucose. Excess glucose is synthesized into fatty acids and glycerol, to make triglycerides for storage in fat cells.
The sodium transport through the walls of villi also facilitates water extraction from the small intestines. This transport depends on the presence of glucose. In case of diarrhoea, it is therefore important to supply the body with salt and sugar.
Processing of proteins in the small intestines
The enzyme trypsin from the pancreas digests proteins into small to medium peptides. Peptidase, an enzyme that hydrolyses peptides, is fixed on the walls of the cells of the microvilli, and splits these small peptides into amino acids. Sodium-dependent amino acid transporters, analogue to the sugar transport, move the amino acids through the cell walls. A sodium independent process brings the amino acids into the inner part of the villi in contact with the capillaries, to be absorbed by the blood.
Breaking virtually all proteins down into their building blocks, the amino acids, prevents any pathogens entering the blood through the digestive tract.
Processing of fats (lipids) in the small intestines
Lipids in the digestive tract are mainly triglycerides, phospholipids, cholesterol, and fat soluble vitamins.
Bile from the liver emulsifies the fat particles into microscopic droplets so that the enzyme pancreatic lipase can hydrolyse the fats (triglycerides) into two fatty acids and one mono-glyceride.
Assisted by bile, these products permeate into the cells of the microvilli. There, they are re-synthesized into triglycerides and combined with cholesterol and other substances to form particles called chylomicrons. These chylomicrons end up in the lymphatic vessels in the microvilli, from where the lymphatic system transports them into the blood stream.
Re-synthesis of the fatty acids into triglycerides allows the body produce lipids with more suitable properties.
Most of the bile is reabsorbed in the ileum to recycle cholesterol.
Absorption of water and electrolytes in the small intestines
Large amounts of water are absorbed in the small intestine, not just the liquid ingested with food and drink, but also the amount of saliva and the excreted digestive juices. As mentioned before, the absorption of water depends on the transport of sodium which only works together with sugar. The small intestines absorb also electrolytes as calcium, iron, phosphorous, sodium, potassium, chloride, and trace elements.
The small intestine are comparatively free from bacteria. This helps the food to get digested, and not fermented.
A shincter muscle, called the ileocecal valve, separates the small intestines from the large intestines, the bowels. A disturbance in the function of the ileocecal valve can lead to bacteria travelling back into the small intestines and can cause a lot of symptoms.
The parts of the large intestines are the cecum – a small dead-end with the appendix; the colon – ascending, traverse, and descending; and the rectum – leading to the anus. The task of the large intestines is to reabsorb more water and electrolytes from the digestion tract and to compact and store the residues of the digestive process, now called feces (faeces), until they get released from the body. The large intestines also contain large amounts of bacteria which ferment undigested remains of carbohydrates, fats, and proteins. Some of these bacteria produce vitamin K and certain B vitamins; other bacteria produce gases – flatulence or wind.
The colon excretes bicarbonate and mucus, to neutralize the effects of the bacterial fermentation, and to lubricate the movement of the feces.
At the end of the process, the body releases the feces, ideally at least once a day.
Back to the bowl again – the toilet bowl it is.