Anatomy Terms Starting With G
Anatomy Glossary: G
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Gallbladder
/ GAWL-blad-er / · Old English gealla, bile; bladdre, bag
Gallbladder is a small, pear-shaped muscular pouch on the inferior surface of the liver that stores and concentrates bile between meals and releases it into the duodenum in response to dietary fat.
The liver produces bile continuously, but the body requires it in quantity only when fat enters the small intestine. Between meals, the gallbladder concentrates bile by actively absorbing water and electrolytes across its mucosal lining, raising bile salt concentration several-fold. When fat reaches the duodenum, enteroendocrine cells release cholecystokinin, which triggers gallbladder contraction and relaxation of the sphincter of Oddi, delivering concentrated bile into the duodenum.
Gallstones form when bile components, most often cholesterol, precipitate within the gallbladder and can obstruct the bile duct, causing intense pain and jaundice.
Bile salts are recycled rather than lost after each meal. About 95 percent of secreted bile salts are reabsorbed in the terminal ileum and returned to the liver through the portal circulation, a cycle completed two to three times per meal.
Fun Facts About Digestive System →The gallbladder makes bile. Bile is produced by hepatocytes in the liver, and the gallbladder only stores and concentrates it.
In the domestic pig (Sus scrofa), the gallbladder anatomy closely resembles that of humans and is routinely used in surgical training models. Bile released during a fatty meal emulsifies lipid droplets, increasing the surface area available to pancreatic lipase and accelerating fat digestion.
Glomerulus
/ gloh-MER-yoo-lus / · Latin glomerulus, small ball of yarn
Glomerulus is the tuft of fenestrated capillaries within Bowman's capsule of the nephron where blood plasma is filtered under hydrostatic pressure to form the primary filtrate.
Each human kidney contains approximately one million glomeruli, each supplied by an afferent arteriole and drained by a narrower efferent arteriole whose relative tone controls filtration pressure. Mesangial cells embedded in the glomerular tuft regulate capillary surface area and remove trapped macromolecules. Glomerulonephritis, an inflammatory condition affecting the glomerular filtration barrier, can destroy filtration units and cause protein and red blood cells to appear in the urine, potentially progressing to chronic kidney disease.
The glomerulus sits between two arterioles rather than between an arteriole and a venule, as most capillary beds do. This arrangement maintains unusually high hydrostatic pressure, around 55 mmHg, which drives filtration at a rate no venous-end capillary bed could sustain.
Urinary System Fun Facts →The glomerulus is a simple hollow tube. It is a tangled ball of capillaries sitting inside Bowman's capsule, not a hollow channel.
Circulatory System Fun Facts →In the freshwater rainbow trout (Oncorhynchus mykiss), glomeruli are large relative to body size, reflecting the need to filter large volumes of dilute urine to compensate for water constantly entering the body by osmosis. Each glomerulus filters plasma across the same three-layer barrier found in mammals, demonstrating how conserved this structure is across vertebrates.
Glucagon
/ GLOO-kah-gon / · Greek glykys, sweet; agon, leading forth
Glucagon is a peptide hormone secreted by alpha cells of the pancreatic islets of Langerhans in response to low blood glucose that stimulates hepatic glycogenolysis and gluconeogenesis to raise blood sugar.
Glucagon binds G-protein coupled receptors on hepatocytes, activating adenylyl cyclase, raising cyclic AMP, and activating glycogen phosphorylase to release glucose from glycogen stores. During prolonged fasting, it also promotes fatty acid oxidation in the liver, generating ketone bodies that peripheral tissues can use as fuel. Glucagon and insulin form a reciprocal hormonal pair that together maintain blood glucose within a narrow range; their balance is disrupted in both type 1 and type 2 diabetes, where glucagon secretion is often dysregulated alongside insulin deficiency or resistance.
Synthetic glucagon is used as an emergency treatment for severe hypoglycemia in diabetic patients who are unconscious and cannot swallow glucose. A single intramuscular injection of 1 mg typically raises blood glucose enough to restore consciousness within 10 to 15 minutes.
Endocrine System Fun Facts →Insulin is the only hormone that controls blood sugar. Glucagon works in the opposite direction by signaling the liver to release stored glucose and synthesize new glucose from non-carbohydrate precursors.
In fasting humans, glucagon secretion rises between meals and overnight, sustaining blood glucose above roughly 70 mg/dL. Hepatocytes respond by breaking down glycogen and ramping up gluconeogenesis from amino acids and glycerol, ensuring the brain receives a continuous glucose supply.
Kupffer cells →