D.2: Digestion Flashcards
Regulation of gastric secretions
- sight/smell of food results in the brain sending nerve impulse via the vagus nerve from the medulla
- gland cells are stimulated and secrete components of gastric juice
- if peptides are detected by chemoreceptors (food is in stomach), the brain sends impulse via vagus nerve to endocrine cells in duodenum and part of the stomach nearest to the duodenum, stimulating them to secrete gastrin (hormone)
- gastrin stimulates two types of endocrine glands (in stomach wall) to secrete acid or pepsinogen
- hormones secretin and somatostatin inhibit gastrin if pH levels become too low
- When digested food (chyme) passes into the small intestine, the duodenum also releases digestive hormones:
Secretin and cholecystokinin (CCK) which stimulates 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
Exocrine glands produce and secrete substances via a duct onto an epithelial surface: either:
(note: exocrine glands secrete into ducts, unlike endocrine glands, which secrete into the bloodstream)
- The surface of the body (e.g. sweat glands, sebaceous glands)
- The lumen of the digestive tract / gut (e.g. digestive glands)
Examples of exocrine glands
- Salivary glands – secrete saliva which contains amylase (breaks down starch) (composed of water, electrolytes, salivary amylase, mucus, lysozyme)
- Gastric glands – secretes gastric juices which includes hydrochloric acid and proteases (breaks down protein) (composed of water, mucus, enzymes such as pepsin or rennin ( both types of protease), hydrochloric acid)
- Pancreatic glands – secretes pancreatic juices which include lipase, protease and amylase (composed of water, bicarbonate, enzymes including: amylase, lipase, carboxypeptidase, trypsinogen)
- Intestinal glands – secretes intestinal juices via crypts of Lieberkuhn in the intestinal wall
Where do exocrine glands secrete into?
Exocrine glands secrete into ducts, unlike endocrine glands, which secrete directly into the bloodstream
Structure of an exocrine gland
(mention: acini, basement membrane (etc.), tight junctions, organelles, how exocrine gland functions, ductules)
exocrine glands are composed of a cluster of secretory cells which collectively form an acinus (plural = acini)
(acini surround duct branch)
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 (to provide ATP for protein synthesis or other cell activities)
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
Stomach acid pH
around 1.5 – 2.0
What functions do the acid conditions in the stomach serve?
- 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 structure and neutralization in the stomach
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
Where is stomach acid secreted from?
(hint: NOT exocrine glands)
Acid is secreted by the parietal cells of the stomach
Maintenance of the pH environment of the stomach, and PPIs
- The low pH environmentis maintained by proton pumps in the parietal cells of the gastric pits
- proton pumps secrete H+ ions (via active transport), which combine with Cl– ions to form hydrochloric acid (HCl)
- Certain medications and disease conditions can increase the secretion of H+ ions, lowering the pH in the stomach
- 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
Structural features of intestinal villi which facilitate the absorption of digestive products: MR SLIM
- 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
structural features of the epithelial lining of villi
- tight junctions
- microvilli
- mitochondria
- pinocytotic cells
Tight Junctions (epithelial lining of villi)
- 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 (epithelial lining of villi)
- 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
(Large number of) Mitochondria (epithelial lining of villi)
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)
- pinocytosis
