Renal function

Reabsorption and Secretion Along Different Parts of the Nephron

Proximal Tubular Reabsorption Normally, about 65 % of the filtered load ofsodium , water and a slightly lower percentage of filtered chloride are reabsorbed.

Proximal Tubules

The proximal tubule epithelial cells are highly metabolic large numbers of mitochondria have an extensive brush border extensive labyrinth of intercellular and basal channels which provides extensive membrane surface area of the epithelium have protein carrier molecules that transport a large fraction of the sodium ions across the luminal membrane counter-transport mechanisms

Secretion by the Proximal Tubule.

The proximal tubule secretes * organic acids and bases ( bile salts, oxalate, urate, and catecholamines). * harmful drugs or toxins (penicillin and salicylates). *secretion of hydrogen ions (an important step in the removal of bicarbonate ions from the tubule (by combining H+ with the HCO3- to form H2CO3, which then dissociates into H2O and CO2). *para-aminohippuric acid (PAH). PAH :is secreted so rapidly that the average person can clear about 90 % of the PAH from the plasma flowing through the kidneys and excrete it in the urine. The rate of PAH clearance can be used to estimate the renal plasma flow.


Loop of Henle The loop of Henle consists of three functionally distinct segments: 1-descending segment 2-thin ascending segment, 3-and thick ascending segment.


The descending and thin ascending segments have -thin epith membranes no brush borders, few mitochondria, minimal levels of metabolic activity. The descending thin segment is highly permeable to water and moderately permeable to most solutes, including urea and sodium.


About 20 % of the filtered water is reabsorbed in the loop of Henle, and almost all of this occurs in the thin descending limb&does not reabsorb significant amounts of any of solutes. The ascending limb, including both the thin and the thick portions, is virtually impermeable to water, this is important for concentrating the urine.

The thin ascending limb has a much lower reabsorptive capacity than the thick segment. About 25 % of the filtered loads of sodium, chloride, and potassium are reabsorbed in the loop of Henle, mostly in the thick ascending limb. The thick ascending loop of Henle is impermeable to water.( most of the water remains in the tubule). The tubular fluid in the ascending limb becomes very dilute as it flows toward the distal tubule,& this is important in allowing the kidneys to dilute or concentrate the urine under different conditions.


The sodium-potassium ATPase pump in the basolateral cell membrane maintains a low intracellular Na +and a negative electrical potential in the cell. The thick ascending limb also has a sodium hydrogen counter-transport mechanism in its luminal cell membrane that mediates sodium reabsorption and hydrogen secretion in this segment.

Distal Tubule

Consist of : *Early distal tubule) forms part of the juxtaglomerular complex that provides feedback control of GFR and blood flow in this same nephron). *Late distal tubule(reabsorbs most of the ions, including sodium, potassium, and chloride, but is virtually impermeable to water and urea).

Approximately 5 % of the filtered load of sodium chloride is reabsorbed in the early distal tubule.

Late Distal Tubule and CorticalCollecting Tubule

Late Distal Tubule and CorticalCollecting Tubule

Two cell types: *principal cells *the intercalated cells The principal cells reabsorb sodium and water from the lumen and secrete potassium The intercalated cells reabsorb potassium ions and secrete hydrogen ions into the tubular lumen

-Aldosterone antagonists compete with aldosterone. -Sodium channel blockers directly inhibit the entry of sodium into the sodium channels


Intercalated Cells Hydrogen ion secretion by the intercalated cells is mediated by a hydrogen-ATPase transport mechanism. Hydrogen is generated in this cell by the action of carbonic anhydrase on water and carbon dioxide to form carbonic acid.


The functional characteristics of the late distal tubule and cortical collecting tubule: *almost completely impermeable to urea *Both the late distal tubule and the cortical collecting tubule segments reabsorb sodium controlled by (aldosterone) *the intercalated cells play a key role in acid-base regulation of the body fluids ( through secretion of hydrogen ions by an active hydrogen-ATPase mechanism) *The permeability of the late distal tubule and cortical collecting duct to water is controlled by the concentration of ADH With high levels of ADH, these tubular segments are permeable to water and vice versa.

Medullary Collecting Duct

Medullary collecting ducts reabsorb less than 10 % of the filtered water and sodium.



Special characteristics of medullary collecting Segment: The permeability of the medullary collecting duct to water is controlled by the level of ADH. permeable to urea capable of secreting hydrogen ions against a large concentration gradient plays a key role in regulating acid-base balance.

Regulation of Tubular Reabsorption

Glomerulotubular Balance: the intrinsic ability of the tubules to increase their reabsorption rate in response to increased tubular load (increased tubular inflow). For e.g.:, if GFR is increased from 125 ml/min to 150 ml/min, the absolute rate of proximal tubular reabsorption also increases from about 81 ml/min (65 % of GFR) to about 97.5 ml/min (65 % of GFR).


The importance of glomerulotubular balance is that it helps to prevent overloading of the distal tubular segments when GFR increases. Glomerulotubular balance acts as a second line of defense to buffer the effects of spontaneous changes in GFR on urine output( the 1st line is renal autoregulatory mechanisms).

Peritubular Capillary and Renal Interstitial Fluid Physical Forces

The rate of reabsorption affected by 1. hydrostatic pressure 2. colloid osmotic pressure As the glomerular filtrate passes through the renal tubules, more than 99 % of the water and most of the solutes are normally reabsorbed. The normal rate of peritubular capillary reabsorption is about 124 ml/min.

The net reabsorptive pressure

The net reabsorptive pressure is normally about 10 mmHg, causing fluid and solutes to be reabsorbed into the peritubular capillaries.



The peritubular capillary hydrostatic pressure is influenced by: --the arterial pressure and --resistances of the afferent and efferent arterioles.The major determinant of peritubular capillary Reabsorption: (1) Increases in arterial pressure tend to ↑↑ peritubular capillary hydrostatic pressure and ↓↓ reabsorption rate. (2) Increase in resistance of either the afferent or the efferent arterioles ↓↓ peritubular capillary hydrostatic pressure and ↑↑ reabsorption rate.

The colloid osmotic pressure of peritubular capillaries is determined by: the systemic plasma colloid osmotic pressure the filtration fraction; the higher the filtration fraction, the greater the fraction of plasma filtered through the glomerulus, the more protein becomes in the plasma that remains behind. Note :The reabsorption rate is normally about 124 ml/min and net reabsorption pressure is10 mmHg, Kf normally is about 12.4ml/min/mm Hg.


Filtration fraction(Ff)= GFR/renal plasma flow * If GFR ↑↑ the Ff will ↑↑ * ↓↓ renal plasma flow will ↑↑ Ff * Some renal vasoconstrictors, (angiotensin II), increase peritubular capillary reabsorption by decreasing renal plasma flow and increasing (Ff ).


Proximal tubular and peritubular capillary reabsorption under normal conditions and during decreased peritubular capillary reabsorption caused by either increasing peritubular capillary hydrostatic pressure (Pc) or decreasing peritubular capillary colloid osmotic pressure (pc). Reduced peritubular capillary reabsorption,, decreases the net reabsorption of solutes and water by increasing the amounts of solutes and water that leak back into the tubular lumen through the tight junctions of the tubular epithelial cells, especially in the proximal tubule.


Effect of Arterial Pressure on Urine Output Pressure natriuresis and pressure diuresis: Small increases in arterial pressure cause marked increases in urinary excretion of sodium and water, but because of autoregulatory mechanisms ↑↑ the arterial pressure between 75-160 mm Hg usually has only a small effect on renal blood flow and GFR.


Factors affect the rate of tubular reabsorption of fluid Osmotic diuresis Plasma colloid osmotic pressure Sympathetic stimulation Arterial pressure (without autoregulation of the GFR in diseased kidney) Hormonal control


Hormonal control 1- ADH  ↓↓ urine output 2.Aldosterone  ↑↑ Na reabsorption and ↑↑ K & H ions secretion  ↓↓ urine output. 3.Angiotensin II The effects (functions) of angiotensin II:--stimulates aldosterone secretion  ↑↑ Na reabsorption and ↑↑ K & H ions secretion  ↓↓ urine output. -- stimulates directly (proximal tubules) Na-K ATPase pump and Na-H exchange  ↑↑ in Na reabsorption and K & H ions secretion  ↓↓ urine output



--constricts the efferent arterioles  ↓↓ urine output ↓↓ the blood flow into the peritubular capillaries  the capillary pressure is ↓↓  ↑↑ tubular reabsorption.--  decreases the blood flow through glomeruli  very high proportion of plasma fluid to filter through the glomerular membrane into tubules  ↑↑ plasma proteins in the blood leaving the glomeruli  ↑↑ colloid osmotic pressure of the peritubular capillary  ↑↑ tubular reabsorption. -- constricts Mesangial cells  reduces the capillary filtration coefficient  ↓↓ GFR  ↓↓ urine output.


Hormonal control (continue)4.Atrial natriuretic peptide inhibits the reabsorption of Na and water by the renal tubules in the collecting ducts lead to↑↑ urine output. 5. Parathyroid hormone ↑↑ the reabsorption of Ca and Mg ions from the ascending limb of loop of Henle and distal tubule and inhibits the reabsorption of phosphate from the proximal tubule).