Reabsorption of salt and water & Renal plasma clearance

Reabsorption of salt, water and glucose

The kidneys filter about 180 l per day, but the urine excreted is about 1 – 2 l (1%)Per day.

This means that 99% of the ultrafiltrate is reabsorbed and 1% excreted.

The minimum volume of urine required per day to eliminate the metabolic wastesis 400 ml (obligatory water loss).

Water reabsorption in the proximal tubules is by osmosis.

Salt and water reabsorption

Reabsorption of glucose

Glucose is easily filtered by the glumeruli into the renal tubules.

375 mg/min is reabsorbed and the rest is excreted (Glycosuria).

This occur in the proximal tubule by secondary active transport which transport glucose with sodium.

65% water reabsorbed

20% water reabsorbed

Reabsorption of salt and water• ~ 65 % of salt and water which were

filtered at the glomerulus are reabsorbed back into blood in the proximal tubule.

• Another 20% of water is reabsorbed in the descending loop of the Henle.

• The total 85% salt and water which are reabsorbed occur constantly and are not under any hormonal regulations.

• Still a large volume (~ 180 L x 15% = 27 L) is remained to be reabsorbed in the rest of the nephron tubules (distal convoluted tubules and collecting ducts).

Counter current multiplier system

Descending limb of loop of Henle

Ascending limb of loop of Henle

NaCl

< 300

Capillary

> 300 mOsm/L> 300> 300

~ 100 mOsmgoes to distalconvoluted tubules and collecting duct.

NaClWater

Distal CT

Collecting Duct

100 mOsm

ADH

NaCl

300

400

600

800

300

400

600

800

wat

er

Water reabsorption in distal convoluted tubules and collecting duct

Vasa recta and countercurrent exchange

The role of urea in the concentration of the urine

Plasma concentration and antidiuretic hormone ADH

Solute

Urea

Glucose

Amino acids

Total inorganic ions*

Protein

Organic toxins

Plasma Filtrate Final urine % reclaimed

+

-

+

+

+

+

+

+

+

+

+

+

+

+

+

++

-

-

50%

100%

100%

95.5%

-

0%

The final products of the process

Composition of the plasma, glomerular filtrate, and the urine:

* mainly Na+ and Cl-

Renal clearance of inulin

Renal plasma clearance

Renal clearance of Inulin measurement of GFR

• Inulin is not produced by our cells and it is not metabolized. Also it is fully filtered and neither secreted nor reabsorbed.

Rate of urine formation Inulin concentration in urine

Inulin excreted = V (ml/min) x U (mg/ml)

Inulin Filtered = GFR (ml/min) x P (mg/ml)

Glomerular Filteration Rate Plasma concentration of Inulin

GFR x P = V x U

GFR (ml/min) = V (ml/min) x U (mg/ml)

P (mg/ml)

- Inulin clearance depends on GFR.

Inulin filtered = inulin excrected

Renal Plasma Clearance of Solutes

Clearance of inulin (ml/min) = GFR (ml/min)

- A substance that is filtered and reabsorbed has clearance < GFR or inulin

- A substance that is filtered and secreted has clearance > GFR or inulin

Clearance (ml/min) = V (ml/min) x U (mg/ml)

P (mg/ml)

Clearance of a solute is the volume of plasma that is completely cleared from that solute in one minute.

-Urea is filtered like inulin but also partiallyreabsorbed.

-So how clearance of urea is compared to GFR?

UREA CLEARNCE (ml/min) GFR (ml/min)< ?

Clearance of Urea

PAH is used to measure renal plasma flow.

PAH is filtered like inulin but in addition it is alsosecreted.

-So how clearance of PAH is compared to GFR?

PAH (ml/min) GFR (ml/min)> ?

Clearance of Para-aminohippuric Acid (PAH)

In the region of the late distal tubuleand the cortical collecting duct, reabsorbtion of Na+ results in secretion of K+ and H+.

In case of hyperacidity H+ is secreted at the expense of K+ this is why hyperacidity is associated with increase in blood K+.

On the other hand hyperkalemia results in secretion of K+ at the expense of H+ causing hyperacidity in the blood.

Reabsorption of Na+ and secretion of K+ are regulated by aldosterone.

Renal control of electrolyte and acid-base balance

Homeostasis of plasma Na+

Urine formationFiltration is the process of flow of water and dissolved solutes from the blood plasma to the capsule.

Reabsorption is the back uptake of water and NaCl, glucose, and amino acids from the tubular fluid to the blood.

Secretion is the addition of some substances from the blood capillaries directly into the tubules.

Excretion is the elimination of water and some solutes in form of urine.

Kidney Disease

• Glomerulonephritis• It is the inflammation of the glomeruli, or small blood vessels in the

kidneys. It may present with isolated hematuria and/or proteinuria (blood or protein in the urine).

• Acute Renal Failure• Acute: Sudden onset. Rapid reduction in urine output-usually

reversible.

• This may be due to infection, drugs, traumatic injury, major surgery, nephrotoxic poisons.

• Emergency dialysis may be needed until the situation resolves and the kidneys begin functioning again.

• Chronic Renal Insufficiency (CRI)• Slow distruction of the filtering capacity of the kidney.

• It is irreversible.

• 75% of function can be lost before it is noticeable.