Digestion in Mammals

This tissue consists of thin columns of elongated cells arranged at right angle to the basement membrane. The cells possess nuclei at their bases, and they are mostly associated with regions such as the walls of the intestines and stomach, where secretion and absorption are the major functions. Columnar epithelium tissues are adapted to withstand wear and tear.

Epithelial tissue cells are interspaced by the goblet cells that are responsible for secreting mucus which helps to protect the stomach from digestive enzymes and acidic content of the gastric glands. Some epithelial tissues are ciliated while others have microvilli that increase the surface area for absorption. Part of the surface that is occupied by the microvilli is called brush border.

This epithelium tissue contains secretory cells that are closely packed. Secretory cells secrete materials such as mucus into the cavity or a space lined by it. For example, in the stomach and the small intestine the mucus protects and lubricates the lumen of the intestine and stomach. In some parts, glandular epithelium is folded in various ways to form glands such as gastric glands whose major function is to secrete enzymes and fluids. Glandular epithelium can be individual cells, such as goblet cells or aggregates of glandular cells, such as multicellular glands like exocrine and endocrine glands. Examples of glandular epithelial tissues found in the digestive system include simple tubular glands such as crypts of Lieberkuhn found in the intestines, simple branched tubular glands such as gastric glands found in the stomach walls (gastric mucosa), and compound saccular and compound tubular glands of the salivary glands.

Mechanical and chemical digestion of food starts in the mouth. Mechanical digestion is achieved by teeth through mastication (chewing). During mastication, the food is mixed with saliva, a watery mixture of mucus and amylase secreted by the salivary glands in response to thought, smell, taste or sight of food. Saliva is a neutral or very weak alkali with the pH ranging between 6.5 and 7.5. Saliva has the following functions with regard to digestion: it lubricates food so that it can move through the oesophagus easily; it catalyses the hydrolysis of starch into maltose using the enzyme called salivary α-amylase; it maintains pH of the mouth between 6.5 and 7.5. This level is optimum for the action of salivary amylase to function which is accomplished by its constituent mineral salts (example NaHCO3).

In the oesophagus the bolus is moved by a series of wave-like movement caused by involuntary contraction and relaxation of its circular and longitudinal smooth muscles. This process is described as peristalsis. The contraction of circular muscles (inner muscles) causes the oesophagus to become narrow and long. The contraction of longitudinal muscles (outer muscles) causes the oesophagus to become wide and short. These contractions and relaxations of circular and longitudinal muscles push the bolus down the alimentary canal. Additionally, the peristalsis waves of contractions assist in mechanical digestion.

The stomach is a highly elastic muscular organ. It has two valve-like rings of smooth muscles called sphincters that can open and close. One sphincter is called cardiac sphincter. It is located between the oesophagus and the stomach. The second sphincter is called pyloric sphincter. It is positioned between the small intestine and the stomach. The stomach wall consists of a layer of mucous membrane called gastric mucosa, it is highly folded and is equipped with small pits (gastric pits) leading to gastric glands, in which gastric juice is secreted.

Gastric juice has the following components:

1. Water. This is a solvent involved in hydrolysis by which food substances are broken down.
2. Hydrochloric acid (HCl). This is an acid produced by parietal cells of the gastric mucosa. Hydrochloric acid activates prorennin and pepsinogen into rennin and pepsin respectively, also kills any bacteria that might have entered the stomach through food.
3. Mucus. This protects the stomach from its own digestive enzymes and lubricates the wall for easy passage of food to the small intestine.
4. Pepsinogen. It is produced by the chief cells of the stomach wall. It is a precursor or inactive form of pepsin. Pepsinogen is activated by hydrochloric acid in the stomach to form pepsin; an enzyme responsible for the breaking down of polypeptides into peptides.
5. Prorennin. This is a precursor of rennin, the enzyme that catalyses the conversion of soluble milk protein into an insoluble milk protein. In other words, rennin is an important enzyme in coagulation or curdling of milk in the stomach. The coagulated milk is semi solid and it can be retained in the stomach for a relatively long time for proper digestion. This is very important in lactating young mammals.

The intestines are contained within the abdominal part of the body and they constitute the longest and the massive part of the alimentary canal. The word intestine comes from a Latin word meaning "gut or internal". The intestines are of two types namely; the small intestine which forms a vital part in the digestion and absorption of foods. The second type is the large intestine which among other functions is mainly important in re-absorption of water from the food residues before they are egested.

This part of the intestine has smaller diameter compared to that of the large intestine and for this reason it is called a small intestine. The diameter of the small intestine of human being is approximately 2.5 cm while the large intestine has a diameter of about 7.6 cm, which is about three times wider than the small intestine.

The small intestine is the longest of all parts of the alimentary canal. It has the length of about 9 metres in a living person.

The small intestine muscles are tight in the living body which makes it slightly shorter, unlike in a dead person where it becomes somewhat longer (about twice as much longer as its normal size in a living person). This is because death makes the small intestine to lose its muscle tone and stretches as it become loose. Unlike the large intestine, the small intestine has numerous folds and projections called villi on its lining which are important in increasing the surface area for digestion and absorption of food to take place.

The surface area of the small intestine of a human is about 200 m2 which is surprisingly closer to 100 times the surface area of our body skin. Schematically, the small intestine is normally surrounded by the large intestine in three sides.

The small intestine is not straight but rather a coiled tube which literally has three major distinct regions namely duodenum, jejunum, and ileum. The sectioning of the small intestine is based on its internal structure and function in digestion and absorption of foods.

Duodenum

This is the initial part of the small intestine immediately bordering with the pyloric sphincter of the stomach on the fore or proximal side and with the jejunum on the distal end. This part of the intestine is curved and it assumes a C-shape. The mid region of duodenum is an important part in which secretions from the pancreas and gall bladder together with intestinal wall secretions meet. The acidic chyme that enters the duodenum is subjected to chemical digestion by the secretions coming from the pancreas, intestinal wall, and the liver.

Ileum

This is the final section of the small intestine in most of high vertebrates, including mammals, reptiles and birds. It is the longest region of the small intestine stretching from the distal portion of the jejunum through proximal portion of caecum of the large intestine at the ileocecal sphincter which is also known as a valve. In human being, it range between 2 and 4 metres long and pH ranging between 7 and 8 (neutral to slightly alkaline). In comparison to the other two parts of the small intestine ileum, is characteristically thicker; more vascularised and has more mucosal folds. Parasympathetic and sympathetic nerve fibres provide extrinsic innervations to the small intestine. The wall of ileum secretes intestinal juices (saccus entericus) which contain mucus, sodium hydrogen carbonate (NaHCO3) and digestive enzymes. Mucus and sodium hydrogen carbonate are secreted by Brunner's glands, which are found in the interstitial wall. The role of mucus is to lubricate the interstitial wall and prevent corrosion, whereas sodium hydrogen carbonate helps to neutralise acidic chyme.

The ileum as a site for absorption

Ileum is a site for digestion and absorption of digested food in the alimentary canal. The absorption is done through diffusion or active transport aided by various adaptive features including presence of circular folds, villi, and microvilli. These features increase the surface area for absorption of nutrients. Villi are finger-like projections on the intestinal wall which are equipped with smaller folds called microvilli on their surface. The surface occupied by microvilli is called a brush border. Folds are deep ridge like structures found on the mucosa and submucosa wall. These folds are important in disrupting the straight movement of food into meander-like movement which ultimately delays food movement in the ileum, thus availing more time for digestion and absorption to take place.

The villi are numerous hair-like projections found on the surface of the folds. The function of the villi is to increase the surface area of the intestinal epithelium for an effective absorption.

Normally in 1 mm2 there can be as many as 40 villi. Mucosal epithelium composed of absorptive cells which covers the villi. The villi are equipped with a large number of blood vessels that carry sugars, minerals, vitamins, and amino acids to the liver for processing. Each villus is supplied with venules, arterioles, capillaries and lymphatic capillaries. Moreover, there are lacteal vessels in the villi which are important for the transportation of fat soluble substances (fatty acids and glycerol) into lymph vessels.

The mucosa between the folds contains deep tissues lined by cells that lead to a tubular intestinal gland called crypt of Lieberkühn. This gland secretes slightly alkaline juice which is triggered by irritation on the mucosa caused by acidic chyme.

Microvilli are cylindrical extensions of the epithelial mucosa cells. They are much smaller than the villi, measuring about 1 μm. Due to their small size, a mass of microvilli appears as fine bristle brush called brush borders. The surface of microvilli contains enzymes which accomplish digestion of proteins and carbohydrates. The brush borders increase the surface area of the membrane plasma to increase absorption. Microvilli increase the surface area for absorption. The surface of the villus is lined with the epithelial cells having large number of mitochondria to provide energy for active transport of nutrients, such as amino acids and glucose which are taken against their concentration gradient.

These nutrient-rich blood from the small intestine are carried to the liver via the hepatic portal vein. Presence of longitudinal and circular muscles in the villus enhances contraction and relaxation which brings villus into contact with the food.

This is the last part of the alimentary canal that extends from a blind pouch called caecum through anus. It surrounds the small intestine in three sides. This type of the intestine is involved in the final stage of water absorption, synthesis of certain vitamins, formation and expulsion of faeces to outside the body. The region between the ileum and caecum has a sphincter known as ileocaecal sphincter and it is responsible for the movement of undigested food materials from the ileum to the large intestine.

Caecum

This is a sac-like structures of about 6 cm suspended interior to the ileocecal valve. Undigested food materials from the ileum are first received by this region of the large intestine where absorption of water and salts is continued.

In this region a winding tube called appendix is attached and its function is not clearly known hence it is considered as a vestigial organ. However, appendix is reported to have immunological function because it contains a group of white blood cells. Recent studies have also shown appendix to have bacteria reservoir which are important in repopulating the enteric bacteria in the early stages of individuals suffering diarrheal illness. As the faeces pass along the large intestine, some water is absorbed into the blood stream.

Colon

This region of the intestine borders with caecum and it is made up of four sub regions namely the ascending, transverse, descending and sigmoid regions. Food residues from caecum enter the ascending region of the colon first which is on the right side of abdomen and travel up through the first bend of the colon to the transverse region of the colon. The residue continues through the second bend to the descending colon which is on the left side of the posterior abdominal wall and enters the sigmoid colon.

Rectum

After food residues have passed through the sigmoid colon, the remaining intestinal content is stored as faeces in the rectum which measures about 20 cm long. Rectum is located interior to the pelvis and it produces some mucus material which are added to the faeces in order to lubricate it for easy passage to the outside by the process called defecation. The desire for defecation is caused by the presence of a large quantity of faeces in the rectum. The walls of the rectum have curved contours and lateral bends which create internal transverse folds called rectal valves. The function of these valves is to separate faeces from gas in order to prevent simultaneous passage of faeces and gas.

Anus

This region is also called anal canal and constitutes the final part of the large intestine. The length of the anal canal is between 3.8 to 5 cm and it opens to the exterior of the body at the anus. It has two types of muscles; the internal anal sphincter and external anal sphincter. The former first type of muscle is made up of smooth muscles, and its contractions are involuntary, while the later is made up of skeletal muscles and they are under voluntary control. Under normal conditions, these two types of muscles make the sphincter remain closed except when defaecating.