In vertebrates, the digestive system is a hollow duct connecting the mouth, in the rostral part of the body, with the anus, in the caudal part of the body. Although there are major differences when comparing the digestive system between species, mostly due to the type of food they eat, we are going to describe the general organization of the digestive system of mammals having omnivorous diet.
The digestive system is constituted of cephalic area, axial part and large glands (liver and pancreas).
The cephalic area includes the oral cavity, salivary glands, teeth, as well as the tongue and the pharynx, which connects the digestive and respiratory systems, and bears the tonsils. The oral and nasal cavity are separated by the palate, excepting at the pharynx level. Both, the hard palate (anterior) and the soft palate (posterior) are covered by the same type of epithelium than the rest of oral cavity: stratified squamous epithelium. When comparing different animal species, the cephalic part shows the higher structural variability of the digestive system. The functions of the cephalic area are mechanical digestion, secretion of digestive enzymes like amylase, swallowing, as well as taste sensing.
The axial part of the digestive comprises the esophagus, stomach and intestine (small and large). Ingested food goes through the esophagus to the stomach. In the stomach, low pH and degradation enzymes produce partial digestion. After that, food moves to the small intestine for further break down. Water and nutrients are absorbed through the epithelium and into the blood and lymphatic vessels. Finally, leftover products are pushed into the large intestine and defecated through the anus.
The histological organization of esophagus, stomach and intestine is formed by four layers. The mucosa is the most internal layer, facing the lumen. It consists of an epithelium that covers the internal surface of the digestive tube, showing different morphological features related to protection, secretion and absorption functions, that it performs along the digestive tube. The epithelium rests on the basal lamina, and basal lamina rests on the lamina propria. Lamina propria is highly irrigated smooth connective tissue containing many cells of the immune system like macrophages, plasmatic cells, lymphocytes, and some others. Epithelium, basal lamina and lamina propria form the mucosa. In the lamina propria, there is a discontinuous layer of smooth muscle cells known as muscularis mucosa, located in the outermost part of the mucosa (far away from the epithelium). Muscularis mucosa makes possible the movement of the mucosa.
The submucosa is the irregular dense connective tissue surrounding the mucosa. It contains many exocrine glands, also known as submucose exocrine glands. It is also highly irrigated by blood vessels and innervated by a dense network of nerves that form the Meissner's plexus, which controls the motility of the mucosa and the secretory activity of exocrine glands.
The muscularis, or muscle layer, covers the submucosa. It is made up of smooth muscle cells, except the upper part of the esophagus that shows skeletal striated muscle fibers. Muscle layer is organized in two sublayers, an inner layer with circularly arranged cells, and an outer layer with longitudinally arranged cells. Between both layers, there is a neurovegetative neural plexus, known as Auerbach's plexus, that drives the muscle layers contraction resulting in the peristaltic movements along the digestive tube.
The serose, or adventitia, is the outer layer. It is made up of loose connective tissue with many adipose cells. This layer sets the border between the digestive tube and the coelomic cavities, and it is dorsally bound to the mesentery.
Two glands release their secretory products into de digestive tube: liver and pancreas. Both share the common bile duct as an excretory duct that ends up in the small intestine. Liver releases bile fluid containing biliary acids needed for digestion and absorption of fat. The exocrine part of the pancreas releases many enzymes for digestion. Both glands also have endocrine function by releasing hormones to the blood. Hepatocytes of the liver are able to synthesize and release both exocrine and endocrine compounds, whereas the endocrine and exocrine functions are performed by different parts in the pancreas. The Langerhans islets are the endocrine structures of the pancreas.