100%(1)1 out of 1 people found this document helpful
This preview shows page 23 - 24 out of 46 pages.
The enzyme renin is secreted by the kidney from specialized cells called granular cells of the juxtaglomerular apparatus in response to three stimuli:A decrease in arterial blood pressure (that could be related to a decrease in blood volume) as detected by baroreceptors (pressure-sensitive cells). This is the most direct causal link between blood pressure and renin secretion (the other two methods operate via longer pathways).A decrease in NaCl levels in the ultrafiltrate of the nephron. This flow is measured by the macula densa of the juxtaglomerular apparatus.Sympathetic activity, which also controls blood pressure, acting through the beta1adrenergic receptors.Summary of juxtaglomerular apparatusFUNCTION: Maintenance of blood pressure by regulating renal blood flow. Responsible for regulatingboth intrarenal (intrinsic myogenic autoregulation &tubuloglomerular feedback) and extrarenal (renin-angiotensin-aldosterone) mechanisms necessary to maintain both renal and entire body volume status.LOCATION: The juxtaglomerular apparatus (JGA) is located at a point where the distal tubule passes through the fork formed by the afferent & efferent arterioles of the same nephron. ANATOMICAL PARTS: (1) granular (juxtaglomerular cells), (2) macula densa & mesangial cells o(1) the granular (juxtaglomerular cells) of the afferent arteriole are intrarenal baroreceptors that synthesize and store renin, which is secreted in response to specific stimuli (e.g., low blood flow, decreased NaCl delivery)o(2) the macula densa, a region of the distal convoluted tubule characterized by tubular epithelial cells which are more densely-packed than in other regions of the nephron. If senses decreased NaCl delivery (lower B.P.): signals granular cells to release renin; if senses increased NaCl (higher B.P.): secretes ATP & adenosine to constrict afferent arteriole.o(3) mesangial cells, which form connections via actin and microtubules which allow for selective vasoconstriction/vasodilation of the renal afferent and efferent arterioles with mesangial cell contraction. [Also don’t forget the podocytes which are not part of the JA which also have actin-like filaments that control the filtration slits of capillaries within Bowman’s capsule].VasopressinAlso known as antidiuretic hormone (ADH), comes from the hypothalamus in brain.In response to a water deficit, vasopressin increases permeability of the distal and collecting tubules to H2O by promotinginsertion of aquaporins into theluminal membrane.These cells are impermeable to H2Oin the absence of vasopressin.Alcohol: Inhibits vasopressinsecretion; urine becomeshypoosmotic-body dehydrates(reason why one is so thirsty afterdrinking alcoholic drinks).Mechanism of vasopressinBlood-borne vasopressin binds withits receptor sites on the basolateralmembrane of a cell in the distal orcollecting tubule.This binding activates the cyclicAMP (cAMP) second-messengersystem within the cell.