D2 - Digestion Flashcards
Exocrine glands + examples
Exocrine glands produce and secrete substances via a duct onto an epithelial surface – either:
The surface of the body (e.g. sweat glands, sebaceous glands)
The lumen of the digestive tract / gut (e.g. digestive glands)
Examples of digestive glands include:
Salivary glands – secrete saliva which contains amylase (breaks down starch)
Gastric glands – secretes gastric juices which includes hydrochloric acid and proteases (breaks down protein)
Pancreatic glands – secretes pancreatic juices which include lipase, protease and amylase
Intestinal glands – secretes intestinal juices via crypts of Lieberkuhn in the intestinal wall
Goblet cells – secrete mucus to form a protective layer around the stomach lining
Parietal cells – secrete hydrochloric acid which is responsible for creating a low pH environment in the stomach
G cells – secrete gastrin (stimulates release of stomach acids to increase stomach acidity)
D cells – secrete somatostatin (inhibits release of stomach acids to reduce stomach acidity)
Chief cells – secrete pepsinogen (inactive protease precursor which is activated by acidity to form active pepsin)
Exocrine gland structure
Exocrine glands are composed of a cluster of secretory cells which collectively form an acinus (plural = acini)
The acini are surrounded by a basement membrane and are held together by tight junctions between secretory cells
The secretory cells possess a highly developed ER and golgi network for material secretion and are rich in mitochondria
Exocrine products are released (via secretory vesicles) into a duct, which connects to an epithelial surface
These ducts may arise from a convergence of smaller ductules (each connected to an acinus) in order to enhance secretion
Gastric secretion control
The secretion of digestive juices is controlled by both nervous and hormonal mechanisms
These mechanisms control both the volume of secretions produced and the specific content (e.g. enzymes, acids, etc.)
Gastric secretion - nervous mechanism
The sight and smell of food triggers an immediate response by which gastric juice is secreted by the stomach pre-ingestion
When food enters the stomach it causes distension, which is detected by stretch receptors in the stomach lining
Signals are sent to the brain, which triggers the release of digestive hormones to achieve sustained gastric stimulation
Gastric secretion - hormonal mechanism
Gastrin is secreted into the bloodstream from the gastric pits of the stomach and stimulates the release of stomach acids
If stomach pH drops too low (becomes too acidic), gastrin secretion is inhibited by gut hormones (secretin and somatostatin)
When digested food (chyme) passes into the small intestine, the duodenum also releases digestive hormones:
Secretin and cholecystokinin (CCK) stimulate the pancreas and liver to release digestive juices
Pancreatic juices contain bicarbonate ions which neutralise stomach acids, while the liver produces bile to emulsify fats
Stomach acid
The gastric glands that line the stomach wall secrete an acidic solution that creates a low pH environment within the stomach
The normal pH of the stomach is roughly 1.5 – 2.0, which is the optimum pH for hydrolysis reactions by stomach enzymes
Stomach acid functions
The acid conditions in the stomach serve a number of functions:
Assists in the digestion of food (by dissolving chemical bonds within food molecules)
Activates stomach proteases (e.g. pepsin is activated when pepsinogen is proteolytically cleaved in acid conditions)
Prevents pathogenic infection (stomach acids destroy microorganisms in ingested food)
Stomach acid - neutralisation & protection
The stomach wall is lined by a layer of mucus, which protects the stomach lining from being damaged by the acid conditions
The pancreas releases bicarbonate ions into the duodenum which neutralises the stomach pH (intestinal pH ~7.0 – 8.0)
Certain foods (e.g. antacids) may also neutralise stomach acids, impairing digestion and increasing chances of infection
Proton Pumps
The low pH environment of the stomach is maintained by proton pumps in the parietal cells of the gastric pits
These proton pumps secrete H+ ions (via active transport), which combine with Cl– ions to form hydrochloric acid
Certain medications and disease conditions can increase the secretion of H+ ions, lowering the pH in the stomach
PPIs
Proton pump inhibitors (PPIs) are drugs which irreversibly bind to the proton pumps and prevent H+ ion secretion
This effectively raises the pH in the stomach to prevent gastric discomfort caused by high acidity (e.g. acid reflux)
Individuals taking PPIs may have increased susceptibility to gastric infections due to the reduction of acid secretion
Absorption in small intestine
Once digested food has passed through the stomach, it enters the small intestine for absorption into the blood
The small intestine also releases digestive enzymes to ensure the complete hydrolysis of food molecules
The inner epithelial lining of the small intestine is highly folded into finger-like projections called villi (singular: villus)
Many villi will protrude into the intestinal lumen, greatly increasing the available surface area for material absorption
Features of villi
Microvilli – Ruffling of epithelial membrane further increases surface area
Rich blood supply – Dense capillary network rapidly transports absorbed products
Single layer epithelium – Minimises diffusion distance between lumen and blood
Lacteals – Absorbs lipids from the intestine into the lymphatic system
Intestinal glands – Exocrine pits (crypts of Lieberkuhn) release digestive juices
Membrane proteins – Facilitates transport of digested materials into epithelial cells
Features of epithelial lining to optimise absorption
Tight Junctions
Occluding associations between the plasma membrane of two adjacent cells, creating an impermeable barrier
They keep digestive fluids separated from tissues and maintain a concentration gradient by ensuring one-way movement
Microvilli
Microvilli borders significantly increase surface area of the plasma membrane (>100×), allowing for more absorption to occur
The membrane will be embedded with immobilised digestive enzymes and channel proteins to assist in material uptake
Mitochondria
Epithelial cells of intestinal villi will possess large numbers of mitochondria to provide ATP for active transport mechanisms
ATP may be required for primary active transport (against gradient), secondary active transport (co-transport) or pinocytosis
Pinocytotic Vesicles
Pinocytosis (‘cell-drinking’) is the non-specific uptake of fluids and dissolved solutes (a quick way to translocate in bulk)
These materials will be ingested via the breaking and reforming of the membrane and hence contained within a vesicle
Dietary fibre
Dietary fibre, or roughage, is the indigestible portion of food derived principally from plants and fungi (cellulose, chitin, etc.)
Humans lack the necessary enzymes to break down certain plant matter (e.g. lack cellulase required to digest cellulose)
Certain herbivores (ruminants) possess helpful bacteria in the digestive tract that can break down indigestible plant matter
Dietary fibre - transit
The rate of transit of materials through the large intestine is positively correlated with their fibre content:
Roughage provides bulk in the intestines to help keep materials moving through the gut
Roughage also absorbs water, which keeps bowel movements soft and easy to pass
