Digestion is the process by which the large, complex, insoluble food materials ingested are broken down by a series of mechanical and chemical processes into soluble forms which can be absorbed into the blood and used by the cells of the body. The substances ingested into the body are usually in their larger, complex molecular forms, for example proteins, starch (carbohydrates) and fats as a constituent unit of the larger food product. However, for these materials to be utilized by the body, they must be broken down into simpler forms by the digestive system.
Digestion, therefore, is an essential process which breaks down nutrients (useful substances in food) from the complex polymeric forms to their simpler monomeric forms. This occurs through mechanical digestion and chemical digestion (including hydrolysis and the action of enzymes).
As previously stated, digestion occurs in two parts. It begins with mechanical digestion, which is followed by chemical digestion.
Mechanical Digestion
Mechanical digestion is the physical breakdown of food into smaller bits and pieces to increase the surface area on which digestive enzymes can act.
Mechanical digestion begins in the mouth where it is carried out by the teeth.
The teeth are responsible for chewing, formally known as mastication.
Mouth- The mouth is an organ consisting of the oral cavity inside the head and containing the teeth, gums and tongue. It is the main site of mechanical digestion and is the starting point of the digestive process overall. Ducts lead in from the salivary gland which secretes saliva containing salivary amylase, beginning chemical digestion by breaking down starches/polysaccharides into disaccharides (such as maltose). The tongue (which is also in the mouth) is one of the main agents in deglutition, as it forces the chewed food down the oesophagus by pressing up against the soft palate, propelling it towards the stomach.
Oesophagus- A muscular tube leading from the mouth to the stomach. The walls contain circular smooth muscle tissue which, through wave like motions known as peristalsis, move food (the bolus) down to the stomach. The muscles contract behind the bolus and relax in front of it. This is why you can still swallow even if you are upside down.
Stomach- The stomach is a highly internally-folded muscular bag containing an enzyme-acid mixture known as gastric juice. The stomach’s internal folds, known as rugae allow the stomach to expand after the consumption of food. The stomach also contains a mucus membrane which allows it to withstand the effects of the hydrochloric acid it contains. The gastric juice within it consists of pepsin, rennin and hydrochloric acid (which creates an acidic environment of pH 2). The function of the acid is to create the ideal environment for the function of the enzymes (pepsin, which breaks down protein into polypeptides and rennin, which breaks down milk proteins in young mammals) and to work as an antiseptic. The stomach also performs mechanical digestion through stomach churning, where it continues to mix the food with the gastric juice to form chyme.
Liver- The liver is an accessory organ to the digestive process. It produces a substance known as bile which helps in breaking down fats through emulsification. Because lipids are hydrophobic, they tend to form large globules by clumping together. The bile is capable of splitting these globules into smaller droplets to increase the surface area on which the pancreatic lipase can act. The bile can force the fat to ‘dissolve’ in aqueous environments forming micelles that helps in both digestion through lipase action and absorption (fat soluble vitamins). The liver also converts excess glucose to glycogen for storage.
Gallbladder- The liver secretes this bile into the gallbladder where it is stored until needed. The gallbladder contracts to release bile via the bile duct into the duodenum.
Pancreas- The pancreas is another accessory organ to the digestive process, as it secretes enzymes which assist in breaking down certain macromolecules. The pancreas secretes pancreatic juice- a clear, alkaline mixture of enzymes. Pancreatic juice comprises trypsin and chymotrypsin, which break down proteins; pancreatic lipase, which breaks down fats; and pancreatic amylase, which breaks down disaccharides into monosaccharides. The pancreas also serves an endocrine function in the body of regulating blood sugar levels through the secretion of hormones (insulin and glucagon) into the blood stream. Insulin reduces blood sugar when it is too high and glucagon increases it when it is too low.
Small intestine- The small intestine is 20 feet long and is divided into the duodenum, jejunum and ileum. The first section, the duodenum, is the site at which digestion is completed. That is, it is the last site where enzymatic activity occurs to break down food. The duodenum secretes its own mixture of enzymes, intestinal juice- an alkaline mixture of mucus, lactase, erepsin, enterokinase, lipase and amylase. The alkalinity of the intestinal juice allows it to neutralize the acidic gastric juice coming from the stomach. The enzyme mixture pancreatic juice is also secreted into the duodenum, as well as bile, where they continue to break down food. The jejunum and ileum both contain numerous structures on their walls known as villi and microvilli, which allow for them to absorb certain products of digestion. The jejunum absorbs mainly fatty acids, glycerol, amino acids and monosaccharides, while the ileum absorbs vitamins, bile salts and remaining nutrients unabsorbed from the jejunum.
Large intestine- The large intestine connects directly to the end of the ileum at the ileocecal valve. It is about 5 feet in length and is responsible for the reabsorption of water and mineral ions including sodium and chloride, the formation and temporary storage of faeces, hosting a population comprising over 500 species of bacteria and facilitating the bacterial fermentation of indigestible materials. The large intestine, divided into the appendix, cecum, ascending colon, transverse colon, descending colon and sigmoid colon, propels food along through a series of mass muscular movements called peristalsis. The bacteria population of the large intestine consists mostly of anaerobes which thrive mostly in oxygen-free environments and ferment undigested food materials into short chain fatty acids while releasing gases like methane. They also produce vitamins like vitamin K.
Rectum- The rectum is the final point at which undigested waste materials arrive before being evacuated from the body. It connects the sigmoid colon to the anus. Its lining also absorbs water and electrolytes such as sodium and potassium. Anaerobic bacteria continue to ferment the waste, while the waste is thickened through further water absorption and mixing with mucus secreted by the walls of the rectum. It is about 12-16 cm in length.
Anus- This is the opening from which the waste from the digestive system is excreted. Muscles, known as the anal sphincters, are responsible for controlling when waste is evacuated. Receptors are also found there to determine whether the contents are solid or gas.
As stated above, absorption of digested materials occurs in the small intestine and large intestine.
In the small intestine, digested materials are absorbed via the lining (mostly in the ileum) through special structures known as villi. The ileum is adapted to absorb food molecules as efficiently as possible.
It has a large surface area, as it is about 5 metres in length for the average adult.
The lining of the ileum is covered in massive numbers of tiny finger-like projections called villi- which greatly increase surface area for absorption (through diffusion or active transport)
Each villus contains a capillary bed and a lacteal so that the absorbed nutrients can be transported away as quickly as possible.
Each villus has a wall, or epithelium that is one cell thick, allowing digested food to pass through to the capillaries and lacteal easily.
The epithelium of the villi are covered in even smaller projections known as microvilli, which increase the surface area available for absorption even further.
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