page_title: Thirst
section_title: Renin-angiotensin system
hierarchical_section_title: Thirst / Detection / Decreased volume / Renin-angiotensin system
caption_attribution_description: English: The renin-angiotensin system (RAS) or the renin-angiotensin-aldosterone system (RAAS). Start reading this schematic from the left, where it says "Decrease in renal perfusion (juxtaglomerular apparatus)". Alternatively, the RAAS can also be activated by a low NaCl concentration in the macula densa or by sympathetic activation. Legend info: Blue and red dashed arrows indicate stimulatory or inhibitory signals, which is also indicated by the +/-. In the tubule and collecting duct graphics, the grey dashed arrows indicate passive transport processes, contrary to the active transport processes which are indicated by the solid grey arrows. The other solid arrows either indicate a secretion from an organ (blue, with a starting Spot) or a reaction (black). These 2 processes can be stimulated or inhibited by other factors.
context_page_description: Thirst is the craving for potable fluids, resulting in the basic instinct of animals to drink. It is an essential mechanism involved in fluid balance. It arises from a lack of fluids or an increase in the concentration of certain osmolites, such as sodium. If the water volume of the body falls below a certain threshold or the osmolite concentration becomes too high, structures in the brain detect changes in blood constituents and signal thirst.
  Continuous dehydration can cause acute and chronic diseases, but is most often associated with renal and neurological disorders. Excessive thirst, called polydipsia, along with excessive urination, known as polyuria, may be an indication of diabetes mellitus or diabetes insipidus.
  There are receptors and other systems in the body that detect a decreased volume or an increased osmolite concentration. Some sources distinguish "extracellular thirst" from "intracellular thirst", where extracellular thirst is thirst generated by decreased volume and intracellular thirst is thirst generated by increased osmolite concentration.
context_section_description: Hypovolemia leads to activation of the renin angiotensin system (RAS) and is detected by cells in the kidney. When these cells detect decreased blood flow due to the low volume they secrete an enzyme called renin. Renin then enters the blood where it catalyzes a protein called angiotensinogen to angiotensin I. Angiotensin I is then almost immediately converted by an enzyme already present in the blood to the active form of the protein, angiotensin II. Angiotensin II then travels in the blood until it reaches the posterior pituitary gland and the adrenal cortex, where it causes a cascade effect of hormones that cause the kidneys to retain water and sodium, increasing blood pressure. It is also responsible for the initiation of drinking behavior and salt appetite via the subfornical organ.
caption: Renin-angiotensin-aldosterone system
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