Water + salt reabsorption

Substances secreted from plasma to filtrate

K+

Organic ions  —> bodily derived: e.g. prostaglandins + epinephrine —> xenobiotics: foregin molecules

H+ or HCO3-

Purpose of secretion of K+

Maintain ECF concentration 

→ Low conc in ECF → affects excitability of membrane

Location of secretion of K+

Na+-K+ pump in  distal tubule + collecting duct  Regulation: Aldosterone ( hormone )

Locations + features of location of secretion of organic ions

Organic anions + cation transporter in proximal tubule  → features: non selective → many substances transported → competition for binding to same transporter → slow rate of excretion

Composition of urine

	Important molecules ( glucose, amino acids )	Waste products ( urea + drugs )	Ions + water
Concentration	0	High	Variable ( based on blood concentration )

Purpose of reabsorption of Na+ and water

Reabsorption of Na+	maintain ECF volume + blood pressure
Water reabsorption	maintenance of ECF osmolarity  → selective reabsorption of water in nephrons

Process of water reabsorption

1. In proximal tubules: Na+ is reabsorbed + water reabsorption follows by being transported through the aquaporins permanently inserted in  the tubular cell membrane by osmosis 

2. Water permeability of distal tubule + collecting duct is controlled by vasopressin → determines the insertion of of aquaporins in luminal membrane 

Systemic action to maintain homeostasis of water content in body: detection of water balance changes in body

1. Osmolarity is too high/ ECF volume too low → arterial blood pressure too low 

2. The high osmolarity detected by hypothalamic osmoreceptors/Low arterial blood pressure is detected by left atrial volume receptors → activate hypothalamic neurons

Action of hypothalamic neurons in maintaining water balance + homeostasis

1. Sense of thirst increases→ increase H2O intake → plasma osmolarity decreases + plasma volume increases 

2. Release of vasopressin increases

   → H2O permeability of distal + collecting tubules increases

   → H2O reabsorption increases + urine output decreases

   → plasma osmolarity decreases + plasma volume increases

   → ECF volume increases / vasopressin also causes arteriolar vasoconstriction

   → arterial blood pressure increases 

Process of concentrated urine production

Interstitial fluid in medulla becomes more concentrated towards renal pelvis → selective reabsorption of water in distal tubule + collecting duct when filtrate moves towards renal pelvis

Features of loop of Henle that helps produce osmotic gradient

1. Descending limb of loop of Henle → highly permeable to water + X reabsorb Na+ 

2. Ascending limb: actively reabsorb NaCl + impermeable 

Process:

1. Filtrate equilibrates w/ medullary interstitial fluid in descending loop of Henle → water leaves through aquaporins 

2. Filtrate conc decreases in ascending limb of loop of Henle as NaCl is pumped out 

→ filtrate conc leaving loop of Henle has lower conc than interstitial fluid 

Site of reabsorption of ions + role

Site of reabsorption	Role
Proximal tubule	Aid reabsorption of glucose, amino acids, water, Cl- and urea
Loop of Henle	Allows production of urine of varying concentration
Distal tubule + collecting duct	Regulates ECF volume

Hormonal system that aids reabsorption of Na+ + stimulation of system

Renin-angiotensin-aldosterone system —> RAAS

Stimulation of renin release:

1. Decrease in blood pressure of afferent arteriole → decrease in stretch of granular cells 

2. Reduction of NaCl in distal tubule detected by macula densa cells 

3. Stimulation of sympathetic nervous system

Process of RAAS action on reabsorption of Na+

1. Granular cells release renin 

2. Renin activates angiotensinogen → angiotensin I 

3. Angiotensin I is converted into angiotensin II by angiotensin converting enzyme 

4. Angiotensin II triggers release of aldosterone from adrenal cortex 

5. Aldosterone stimulates Na+ absorption from  distal tubule + collecting duct by promoting insertion of additional Na+ channels + pumps in tubular cell membrane 

Function of Natriuretic peptides + stimulants

Function: inhibits Na+ reabsorption → lowers blood volume + pressure 

→ stimulate Na+ + water excretion to lower blood volume + pressure

Stimulants: release by stretch of heart chamber ( increase in blood volume + pressure ) 

Action of atrial natriuretic peptides + effects on blood pressure

Inhibitory action of ANP	Effects
Na+ reabsorption by kidney tubules	Na+ excretion increases → H2O excretion increases → Reduce increase in NaCl/ ECF volume/ arterial blood pressure
Salt conserving RAAS
Smooth muscles of afferent arterioles	Afferent arteriolar vasodilation → glomerular filtration rate → Na+ and H2O filtered increases → blood volume decreases
Sympathetic nervous system	Cardiac output decreases + total peripheral resistance → reduction in arterial BP