Factors affect the rate of reabsorption of fluid

***Factors affect the rate of reabsorption of fluid
There are many factors affect the rate of reabsorption of fluid.
1. Osmotic diuresis: In diabetes mellitus the proximal tubules fail to reabsorb all the glucose, non absorbed glucose forming filtered fluid with high osmotic pressure passes through the tubules and carriers a large of water and other substances that filtered in excessive quantities from the plasma such as sucrose, manitol and urea also increase osmotic diuresis.
2. Plasma colloid osmotic pressure: sudden increase in plasma colloid osmotic pressure for any cause lead to decrease rate of fluid excretion due to decrease GFR and increase tubular reabsorption.
3. Sympathetic stimulation: causes constriction of the afferent arterioles and this lead to decrease the glomerular pressure, decrease GFR and decrease blood flow into peritubular capillaries lead to increase tubular reabsorption.
4. Arterial pressure:
a- Increase in arterial pressure lead to increase in the glomerular pressure, which turn increase the GFR leading to increased urine output.
b- Increase in arterial pressure also lead to increase the pressure in the peritubular capillaries, decreasing tubular reabsorption leading to increased urine output.
*So small increase in arterial pressure causes increase in urinary excretion of Na and water.
5. Hormonal control:
1. ADH: causes an increase in permeability of distal tubules, collecting tubules and collecting ducts to water, so leading to decrease urinary volume output.
2. Aldosterone: causes an increase in reabsorption of Na ions followed by water reabsorption and increase K ions secretion in cortical collecting tubule. So leading to decrease urinary volume output.
3. Angiotensin II:
• Stimulates aldosterone secretion which in turn increases Na and water reabsorption.
• Constricts the efferent arterioles.
• Effect directly on proximal tubule to increase Na and water reabsorption by stimulating Na-K ATPase pump at the basolateral membrane of tubular cell and Na-H exchange at luminal side of the tubular cell leading to decrease urinary volume output.
4. Atrial natriuretic peptide: is a hormone that released from cardiac atria as a result of plasma volume expansion. It inhibits the reabsorption of Na and water by renal tubules especially in collecting ducts with increase urinary output.
5. Parathyroid hormone: Increases the reabsorption of Ca and Mg ions from ascending limb of loop of Henle and distal tubule and it inhibits reabsorption of phosphate from the proximal tubule.
***Renal mechanism for excreting diluted or concentrated urine
The kidney can excrete a large volume of dilute urine or a small volume of concentrated urine without major changes in rate of excretion of solutes such as Na and K ions. This ability is necessary for survival, especially when fluid intake is limited. When the fluid become too dilute the kidney automatically excrete a great excess of water in urine causing the urine to be diluted, increasing body fluid osmolality back toward normal. Conversely when the osmolality of body fluid is too great the kidney automatically excretes an excess of solutes in urine causing the urine to be concentrated, reduce the body fluid osmolality again back toward normal and this done by following mechanism:
1. The renal mechanism for excreting a dilute urine:
In case of fluid overload, the body will get rid this excessive amount of fluid via the following mechanism:
? As fluid flow through the proximal tubule, solutes and water are reabsorbed in equal proportion, so little change in osmolarity occurs (proximal tubule fluid remains isotonic to the plasma), with an osmolarity of about 300 mOsm /l.
? As fluid passes down the descending loop of Henle, water is reabsorbed by osmosis and the tubular fluid reaches equilibrium with surrounding interstitial fluid of the renal medulla, which is very hypertonic of about 1200 mOsm /l, the tubular fluid becomes more concentrated.
? In the ascending limb of loop of Henle, especially the thick segment, Na, K, and Cl are reabsorbed and it s impermeable to water, the tubular fluid becomes more dilute as it flow up the ascending loop of Henle into the early distal tubule with osmolality decreasing progressively to about 100 mOsm /l at time the fluid enters early distal tubular segment.
? In the cortical collecting tubule and collecting duct, there is additional reabsorption of NaCl. This portion of the tubule is also impermeable to water (in the absence of ADH) and additional reabsorption of solute causes the tubular fluid to become more diluted with osmolarity about 50 mOsm/l.
2. Renal mechanism for excreting a concentrated urine:
In case of fluid depletion for any cause the kidney excrete concentrated urine.
The basic for forming concentrated urine are:
A. High level of ADH: (increases the permeability of the distal tubules and collecting duct to water), when the body fluid are too concentrated, the posterior pituitary gland secretes large amount of ADH, which causes the kidney to excrete excessive amounts of solutes and conserve water in the body. Conversely, in the absence of ADH the kidney excretes dilute urine. This removing excessive amount of water from the body fluids and causing them to become concentrated once again.