Digestive System Notes PDF

3.1.1 Gastrointestinal tract 3.1.1 Gastrointestinal tract *By completing this subtopic, you will be able to: Identify all relevant anatomical structures and histological changes throughout the GIT and explain their function.* 3.1.1.1   Overview The digestive system is comprised of two categories...

3.1.1 Gastrointestinal tract 3.1.1 Gastrointestinal tract *By completing this subtopic, you will be able to: Identify all relevant anatomical structures and histological changes throughout the GIT and explain their function.* 3.1.1.1   Overview The digestive system is comprised of two categories of organs: - The gastrointestinal tract (GIT); and - Accessory digestive organs (McKinley et al., 2021). The GIT is a continuous tube that extends from the oral cavity (mouth), through the pharynx (throat), oesophagus, stomach, small intestine, and large intestine, terminating at the anus (McKinley et al., 2021). The accessory digestive organs refer to both organs and glands including the salivary glands, liver, pancreas, teeth, tongue, and gallbladder (McKinley et al., 2021). These structures are depicted in the figure below.  Source: McKinley et al. (2021) Collectively, the GIT and the accessory digestive organs serve a number of functions including: - ***Ingestion** -* the introduction of solids/liquids into the oral cavity; - ***Motility** - *the voluntary and involuntary muscular contractions for mixing and moving material through the GIT; - ***Secretion*** - the process of manufacturing and releasing substances that facilitate digestion into the GIT; - ***Digestion ***- the breakdown of ingested food into smaller structures for absorption from the GIT. Digestion can be *mechanical *(physical breaking down through chewing and mixing) or *chemical *(the use of enzymes to break chemical bonds of large molecules); - ***Absorption** *- membrane transport of digested molecules, electrolytes, vitamins, water from the GIT into the blood or lymph; - ***Elimination** - *the expulsion of indigestible material that is not absorbed (McKinley et al., 2021). 3.1.1.2   Histology of the gastrointestinal tract Four concentric layers, called *tunics*, comprise the walls of the GIT from the oesophagus to the large intestine (McKinley et al., 2021). From innermost to outermost, these include the: ***1. Mucosa** *- mucous membrane lining the lumen of the GIT. It is composed of: - *epithelium *- in contact with contents of lumen; allows for secretion and absorption; - *lamina propria* - loose areolar connective tissue with capillaries for nourishment and absorption; houses mucosa-associated lymphatic tissue (MALT); and - *muscularis mucosae* - smooth muscle that produces local movements of mucosa (McKinley et al., 2021). ***2. Submucosa*** - loose connective tissue with glands, blood/lymph vessels, MALT, and *submucosal plexus* (neural network) that controls secretion (McKinley et al., 2021). ***3. Muscularis** - *2 - 3 layers of smooth muscle responsible for segmentation and peristalsis; also contains *myenteric plexus* (neural network) that controls motility (McKinley et al., 2021). ***4. Adventitia** *or ***Serosa** *- outermost tunic that can be either *adventitia *(areolar connective tissue in portions of GIT located *outside* the peritoneal cavity) or *serosa (*areolar connective tissue covered in serous membrane in portions of GIT located *within* the peritoneal cavity) (McKinley et al., 2021). The following figure depicts the tunics of the GIT and links these layers to *absorption* and *motility*, two important functions of the GIT described in the previous section. 3.1.1.6   Stomach Following passage through the oesophagus, the bolus arrives at the *stomach*, a muscular holding sac located in the superior left abdominal quadrant (McKinley et al., 2021). Here peristaltic movements of the stomach mix the bolus with secretions from the stomach wall and mechanically digest the bolus into *chyme *- a semifluid mass. The protein and fat in chyme undergo some chemical digestion in the stomach, however little absorption occurs in the stomach itself (McKinley et al., 2021). Eventually, the majority of chyme will be emptied (small amounts at a time) into the small intestine.   Structurally, the stomach is shaped like the letter J and comprised of four regions: the *cardia*, *fundus*, *body *and *pylorus *(see figure below).   Source: McKinley et al. (2021) The lining of the stomach is arranged into *gastric folds* or *gastric rugae* that facilitate the expansion of the stomach when filling with food/ fluid (McKinley et al., 2021). The stomach wall itself is comprised of: - - - Refer to the figure below depicting these layers.  Source: McKinley et al. (2021) 3.1.1.3   Peritoneum The peritoneum is the serous epithelial membrane that covers the wall of the abdominal cavity as well as the outside of the abdominal organs (McKinley et al., 2021). It consists of two layers: - ***parietal peritoneum*** -- covers abdominal wall - ***visceral peritoneum*** -- covers outer surface of abdominal organs (McKinley et al., 2021). The space between these two layers is known as the peritoneal cavity (McKinley et al., 2021). Similar to the pericardial cavity, it contains serous fluid to reduce friction between the abdominal wall and the external organ surfaces. There are a few folds within the peritoneum that suspend components of the GIT: - ***mesentery*** (small intestines suspended on it) - ***mesocolon*** (colon suspended on it) - ***greater omentum*** (stores fat and insulates the abdominal organs) (McKinley et al., 2021). 3.1.1.4   Oral cavity and salivary glands Substances enter the GIT through the *oral cavity*, which is lined with stratified squamous epithelium and kept moist by *saliva* (McKinley et al., 2021).* *The oral cavity is the first site of chemical digestion. As food enters the oral cavity, salivary glands located inside and outside the oral cavity are stimulated to secrete saliva which mixes with the ingested material to form a bolus, or wet mass (McKinley et al., 2021).  **Saliva and salivation:** - 1-1.5L produced by salivary glands (parotid, submandibular and sublingual) - Contains 97 % H2O, enzyme salivary amylase (starts *chemical digestion* of starch), IgA & lysozymes etc... - Salivation is triggered by taste, smell, sight or thought of food - Salivation stimulated by parasympathetic nervous system via CN VII -facial nerve and CN IX -glossopharyngeal nerve (McKinley et al., 2021). - At the same time that saliva is secreted, the teeth, and skeletal muscles of the lips, tongue, cheeks and jaw begin *masticating*, or chewing (McKinley et al., 2021). Mastication enables *mechanical digestion*, reducing the bulk of the ingested material into smaller particles in preparation for swallowing (McKinley et al., 2021). Mastication not only increases the surface area for chemical digestion, it also promotes the secretion of saliva from the salivary glands. Once sufficiently broken down, the bolus is pushed against the hard palate by the tongue, moving it towards the section of the pharynx called the *oropharynx. *This process represents the first phase of swallowing known as the *voluntary phase* (McKinley et al., 2021). - - 3.1.1.5   Pharynx and oesophagus - The *pharynx* (commonly known as the throat) is a muscular tube that provides passageway for both air and food (McKinley et al., 2021). The oropharynx and laryngopharynx are lined with nonkeratinized stratified squamous epithelium to provide protection from the abrasive bolus of food as it passes down to the oesophagus (McKinley et al., 2021). - The *oesophagus*, a 25 centimetre long tubular passageway, connects the pharynx to the stomach, passing through the diaphragm at the opening called the oesophageal hiatus (McKinley et al., 2021). As in the pharynx, the nonkeratinized stratified squamous epithelium of the oesophagus protects the oesophagus from abrasion during peristalsis. Upper and lower oesophageal muscular sphincters control the passage of food; they are normally closed, but open during swallowing (McKinley et al., 2021). - Source: McKinley et al. (2021) 3.1.1.6   Stomach Following passage through the oesophagus, the bolus arrives at the *stomach*, a muscular holding sac located in the superior left abdominal quadrant (McKinley et al., 2021). Here peristaltic movements of the stomach mix the bolus with secretions from the stomach wall and mechanically digest the bolus into *chyme *- a semifluid mass. The protein and fat in chyme undergo some chemical digestion in the stomach, however little absorption occurs in the stomach itself (McKinley et al., 2021). Eventually, the majority of chyme will be emptied (small amounts at a time) into the small intestine.   Structurally, the stomach is shaped like the letter J and comprised of four regions: the *cardia*, *fundus*, *body *and *pylorus *(see figure below).   Source: McKinley et al. (2021) The lining of the stomach is arranged into *gastric folds* or *gastric rugae* that facilitate the expansion of the stomach when filling with food/ fluid (McKinley et al., 2021). The stomach wall itself is comprised of: - ***Mucosa** *- composed of *simple columnar epithelium* that folds, forming numerous* gastric pits* and deep *gastric glands *that house unique secretory cells (described below) - ***Muscularis** *(or smooth muscle) - arranged in three layers that assist in blending and churning food during mechanical digestion - ***Serosa** *-- outermost layer called the *visceral peritoneum* (McKinley et al., 2021). Refer to the figure below depicting these layers.  Source: McKinley et al. (2021) Stomach mucosa The *simple columnar epithelium* of the mucosa is comprised of five cell types, each contributing in different ways to digestion (McKinley et al., 2021). *Surface mucous cells*, *mucous neck cells*, *parietal cells* and *chief cells* play a role in producing the 3 litres per day of gastric secretions (or *gastric juice*) required for digestion. This gastric juice is made up of: *mucin, *a mucus that protects the lining of the stomach; *intrinsic factor*, which helps absorb vitamin B~12~; *hydrochloric acid; *and *pepsin*. You will earn more about the production and functions of hydrochloric acid and pepsin in the next activities (McKinley et al., 2021). The fifth cell type, the* G cells* secrete *gastrin* hormone into the blood. Gastrin stimulates stomach motility (mixing and moving), along with gastric secretion (McKinley et al., 2021). You will learn more about gastrin in the next section, when hormones of the small intestine are discussed.   The figure below depicts the five cell types, their distribution in the gastric pit and gland, and their specific functions.  Source: McKinley et al. (2021) After 2-6 hours of mixing in the stomach with gastric juices (a process called *gastric mixing*), the chyme, combined with gastric secretions, is emptied into the first section of the small intestine called the *duodenum* (McKinley et al., 2021). The figures below depict the specific processes of *gastric mixing* and *gastric emptying*. It is in the small intestine that absorption of nutrients from the chyme will peak (McKinley et al., 2021).  Source: McKinley et al. (2021) 3.1.1.7   Regulation of gastric secretion/motility There are three phases that regulate the secretion of gastric juices and the movement of food through the stomach (McKinley et al., 2021). A summary of these phases is provided below. Cephalic phase - Initiated by the sight, smell, taste and thought of food - Leads to parasympathetic (cranial nerve X - vagus) activation and stimulation of gastric secretions and motility (force of contractions) (McKinley et al., 2021). Gastric phase - Activated via stretch receptors and chemoreceptors (monitor pH) in the stomach after the food comes in - Results in increased peristalsis (mixing) of chyme and increased release of gastrin which further increases force of contraction and release of secretions (McKinley et al., 2021). Intestinal phase - Activated by the presence of acidic chyme in the duodenum - Enterogastric reflex and hormones enterogastrones (secretin, cholecystokinin and vasoactive intestinal peptide) decrease stomach secretions, motility and emptying (McKinley et al., 2021). 3.1.1.8   Small intestine As chyme moves through the first section of the small intestine, the *duodenum*, it mixes with other secretions from the *accessory organs* (McKinley et al., 2021). These substances include: - ***Bile***: An alkaline fluid containing water, bicarbonate ions, bile salts, bile pigments, cholesterol, lecithin (a phospholipid) and mucin that aids in the digestion of lipids; it is produced by the liver and then stored, concentrated and released by the gallbladder (McKinley et al., 2021). - ***Pancreatic juices***: A fluid comprised of numerous digestive enzymes that aids in the chemical digestion of starch, triglycerides, protein and nucleic acids; it is produced and released by the pancreas (McKinley et al., 2021). Stimulation to secrete these substances is regulated by two hormones released in the small intestine, *cholecystokinin (CCK) *and *secretin* (McKinley et al., 2021). Moving into the second portion of the small intestine, the jejunum, the chyme undergoes chemical digestion and the nutrients contained within it begin to be absorbed by the small intestine (McKinley et al., 2021). Nutrient absorption continues as chyme moves into the final segment of the small intestine, called the *ileum* (McKinley et al., 2021). There are several histological features that make the small intestine the prime organ for absorption of nutrients. These include: - ***Circular folds***: inward projections of the mucosa and submucosa; - ***Villi***: fingerlike projections of the mucosa containing an arteriole, rich capillary network and a venule; - ***Microvilli***: extensions of the plasma membrane of the simple columnar epithelium lining the small intestine (McKinley et al., 2021). Collectively, these features (depicted in the figures below) increase the surface area of the small intestine, allowing more of the small intestine wall (in particular the villi and microvilli) to make contact with the nutrients of the chyme (McKinley et al., 2021). This increased area for contact enhances absorption of nutrients, as more nutrients can move into the rich capillary network of the villi, and onwards through the bloodstream (McKinley et al., 2021). 3.1.1.9 Pancreas The pancreas is located just inferior to the stomach and is mostly retroperitoneal (McKinley et al., 2021). The pancreas is both an exocrine and endocrine gland: - *exocrine*: glandular cells produce 1.2-1.5 L/day of digestive pancreatic juice - *endocrine*: pancreatic islets (islets of Langerhans) contain cells that produce hormones glucagon and insulin that control blood glucose level (McKinley et al., 2021). Pancreatic juice is drained into the duodenum via the main pancreatic duct and to some extent the accessory duct (McKinley et al., 2021). The pancreatic duct joins the common bile duct that comes from the liver forming the common duct that joins the duodenum (McKinley et al., 2021).  Source: McKinley et al. (2021) The pancreatic juice contains water, salt, bicarbonate and digestive enzymes (McKinley et al., 2021). The alkaline bicarbonate neutralises the acidic gastric chyme, stops the action of pepsin; and creates the proper pH for pancreatic and intestinal digestive enzymes (McKinley et al., 2021). The digestive enzymes include the following: - *pancreatic amylase* à carbohydrate starch breakdown - *trypsin, chymotrypsin and** **procarboxypeptidase** ***à protein breakdown - *pancreatic lipase* à most of triglyceride breakdown - *deoxyribonuclease** ***and*** **ribonuclease* à nucleic acids breakdown (McKinley et al., 2021). - Control of pancreatic juice release - Similar to bile release, the release of pancreatic juice is controlled by secretin and cholecystokinin (CCK) (McKinley et al., 2021; Marieb & Keller, 2017). The process is detailed in the diagram below. -  Source: McKinley et al. (2021) - - 3.1.1.10 Large intestine - Following absorption of nutrients by the small intestine, the now watery chyme moves into the large intestine, a relatively wide tube extending from the ileum of the small intestine to the anus (McKinley et al., 2021). The large intestine consists of the caecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum and anus, that frame the abdominal cavity (McKinley et al., 2021). - The large intestine functions primarily to absorb water and electrolytes (in particular sodium and chloride) from the chyme, leaving the chyme compacted as faeces by the time it reaches the anus (McKinley et al., 2021). Bacterial flora within the large intestine are responsible for chemical breakdown of complex carbohydrates, proteins and lipids that are leftover in the chyme (McKinley et al., 2021). They produce gases such as hydrogen sulfide [[\[MS1\]]](https://lms.griffith.edu.au/courses/22485/pages/3-dot-1-1-gastrointestinal-tract?module_item_id=589671#_msocom_1) and methane which partially accounts for the smell of faeces. This indigenous microbiota also produce B vitamins and vitamin K which are also absorbed by the large intestine. - The large intestine is comprised of teniae coli, thin bundles of smooth muscle that bunch the large intestine into numerous sacs called haustra (McKinley et al., 2021). The mucosa of the large intestine is lined with simple columnar epithelium and is smooth (unlike the small intestine which has villi). A number of intestinal glands extend towards the muscularis mucosa secreting mucin to lubricate material as it passes through the large intestine (McKinley et al., 2021). -  Source: McKinley et al. (2021) - - Process of defecation - Colonic peristaltic activity slowly moves faeces into the rectum where stretch receptors signal parasympathetic nervous system (PNS) centres in the sacral spinal cord (McKinley et al., 2021). The PNS nerves subsequently activate muscles of the rectum and relax the internal anal sphincter. The external sphincter (skeletal muscle) is voluntarily controlled to ensure that defecation can be postponed (McKinley et al., 2021). This process is outlined in the diagram below. -  Source: McKinley et al. (2021) - - 3.1.1.11 Liver and gallbladder - The liver weighs approximately 1.5kgs and is located below the diaphragm in right hypochondrium and has 4 primary lobes (McKinley et al., 2021). Located directly underneath the right lobe is a pear-shaped sac called the *gallbladder* (McKinley et al., 2021). The gall bladder stores bile which is released through the cystic duct into the common bile duct leading to the duodenum. -  Source: McKinley et al. (2021) Liver histology - Liver cells (hepatocytes) are arranged in functional units (roughly hexagonal in shape) called lobules; - Central vein is located in the lobule centre; - Blood-filled spaces in between hepatocytes are called sinusoids (leaky capillaries) -- they join the central vein; - Liver cells are well positioned to take substances such as nutrients and medications from blood for metabolism; - Scavenger Kupffer cells (hepatic macrophages) in sinusoids phagocytose microorganisms & foreign matter from blood; - Bile canaliculi are found between layers of hepatocytes -- they drain bile produced by hepatocytes into bile ducts (McKinley et al., 2021).  Source: McKinley et al. (2021) Liver blood supply The liver receives blood from two sources. The hepatic portal vein brings nutrient blood from the GIT, while the hepatic artery delivers oxygenated blood from the abdominal aorta (McKinley et al., 2021). Branches of these two blood supplies are part of the portal triad, as seen in the diagram above. Blood from both the portal vein and hepatic artery mix in liver sinusoids and flows towards the central vein, which join to form a single hepatic vein which drains into the inferior vena cava (McKinley et al., 2021). The hepatic portal system is shown in the figure below.  Source: McKinley et al. (2021) Activity Complete the following activity to review your knowledge of the hepatic portal system.  Drag the words to their correct position to show the flow of blood from the when it arrives in the liver to when it returns to the heart. This activity will help test your knowledge and prepare for your upcoming assessments. Liver functions The liver performs a large number of important functions including the following: - Carbohydrate metabolism (maintenance of blood glucose level: glycogenesis, glycogenolysis, gluconeogenesis) - Lipid metabolism (synthesis of lipoproteins and cholesterol) - Protein metabolism (conversion of one amino acid into another, synthesis of plasma proteins such as albumin and blood clotting factors) - Processing of drugs and hormones (detoxifying) - Processing and excretion of bilirubin (breakdown product of haemoglobin) into bile - Synthesis of bile acids (needed for digestion of lipids) - Storage (glycogen, iron, vitamins A, B12, D, E, K) - Phagocytosis (via Kupffer cells) - Activation of vitamin D taken in food (together with kidneys) (McKinley et al., 2021). Bile and bile secretion On a daily basis the liver produces about 800-1000 mL, which is stored in the gallbladder (McKinley et al., 2021). The bile is composed of water, ions, bile acids, cholesterol, lecithin (phospholipid), and bile pigments (conjugated bilirubin from erythrocyte recycling) (McKinley et al., 2021; Marieb & Hoehn, 2018). The main function of bile is to assist with the emulsification of large lipid globules, as seen in the diagram below.  Source: McKinley et al. (2021) Control of bile secretion The release of bile from the gallbladder is triggered by the presence of acidic chyme containing fatty and amino acids (McKinley et al., 2021; Marieb & Keller, 2017)). As shown in the diagram below, this triggers the release of the hormones secretin and cholecystokinin (CCK).  Source: McKinley et al. (2021)