THIRST
DR. Shaheenkousar Haroon
Rashid
Learning Objectives
 At the end of the session you should
be able to
 Describe the basic physiological
mechanism of thirst
 Discuss body’s response to thirst
THIRST
 A need to drink is perceived as thirst,
which causes an individual to seek
thirst quenching beverages.
 The sensation is mediated by higher
cortical areas- anterior singulate gyrus
and insular cortex.
 Thirst is sated well before osmolality
changes probably because of sensory
input from oropharyngeal and GI
osmoreceptors
Sodium balance
Most NaCl intake added during food preparation
Sweat output depends on body temperature
Urine output of NaCl is regulated by blood pressure
Water balance
Metabolically produced by oxidation of H-containing nutrients
Insensible loss: expiration of 37 saturated air, evaporation through skin
(different from sweat)
Urine output regulated by vasopressin (antidiuretic hormone ADH)
Regulation of ECF
• Sympathetic nervous system: decreased
MAP in afferent arteriole trigger sympathetic
activity
• Renin-angitensin-aldosterone system
• Antidirutic hormone
• Atrial Natriuretic peptide
Vasopressin (ADH) release & actions
Vasopressin release stimulated by:
1. slight (1%) increase in plasma osmolality
2. large (~10-15%) reduction in plasma volume
Vasopressin action:
increases permeability of collecting duct to water
Renal medulla
 ADH levels  collecting duct permeability  water reabsorption 
urine volume with  osmolality
ADH Mechanism of Action
Receptor binds, causes vesicles to fuse and insert aquaporins (water channels)
into the membrane.
Juxtaglomerular apparatus
Macula densa: specialized cells in wall of distal tubule
Juxtaglomerular cells (Granular cells): contain renin, sympathetic nerves
Water Loading
4.
Drink Water
Plasma Osmolality
Activation of osmoreceptors in
anterior hypothalamus
ADH secretion from Post. Pituitary
Water permeability in late DT and CT
Water Reabsorption
Urine Osmolality
Urine Volume
Water
Deprivation
4.
Drink Water
Plasma Osmolality
Activation of osmoreceptors in
anterior hypothalamus
ADH secretion from Post. Pituitary
Water permeability in late DT and CT
Water Reabsorption
Urine Osmolality
Urine Volume
Water transport & vasopressin actions
Juxtaglomerular apparatus
Macula densa: specialized cells in wall of distal tubule
Juxtaglomerular cells (Granular cells): contain renin, sympathetic nerves
Mesangial cells.
1. Sympathetic NS
3. Pressure (less stretch)
RENIN
2. Decreased NaCl sensing
Renin angiotensin aldosterone system
Actions of aldosterone
Aldosterone actions:
is a steroid
hormone
(mineralocorticoid
family)
produced by the outer section (zona
glomerulosa) of the adrenal cortex in
the adrenal gland It plays a central role
in the regulation of blood pressure
mainly by acting on the distal
tubules
and
collecting
ducts
of
the nephron, increasing reabsorption of
ions and water in the kidney, to cause
the conservation of sodium, secretion
of potassium, increase in water
retention, and increase in blood
pressure and blood volume.
Regulation of thirst
Sensation of thirst stimulated by:
1. 1%  osmolality
2. >10-15%  blood volume
3.  angiotensin II
Diabetes Insipidus
 Loss of ADH secretion or insensitivity
of kidneys to ADH
 Large severely dilute amounts of
urine
 Increased intake of water
 Danger lies in hyponatremia and
ultimate central nervous system
edema and death.
Isotonic saline
Extracellular volume
Blood pressure
-
Renin, angiotensin
aldosterone
Atrial Natriuretic peptide
Sympathetic NS
Sodium excretion
ADH secretion
Plasma osmolality
Water excretion
SIADH (hypotonic expansion) vs. Diabetes Insipidus (hypertonic contraction)
↑ Anti-diuretic hormone ↓
↑ water excretion
↑ water reabsorption
at principal cells
↑ thirst – activation of
hypothalamic
osmoreceptors
↓ vascular volume
↑ vascular volume
↓ MAP
↑ MAP
↑ ANP
↑ renin-angiotensin
aldosterone
↓ renin-angiotensinaldosterone
↓ Na reabsorption
↓ K excretion
hyponatremia
hypernatremia
↑ Na reabsorption
↑ K excretion
Water retention + Na excretion explains low plasma osmolality
and relatively increased urine osmolality (SIADH)
Take Home message
Always, through my whole life, I've
had a thirst for knowledge.
Emith Smith
Practice Questions
 Define Thirst
 Discuss the mechanism of Regulation
of Thirst
 Role of Kidneys in regulation of ECF
osmolality
THANK YOU