Urine Formation Introduction .
[1]. Urine formation is a blood cleansing function.
[2]. Normally, about 1,300 mL of blood (26% of cardiac output) enters the kidneys.
[3]. Kidneys excrete the unwanted substances along with water from the blood as urine.
[4]. Normal urinary output is 1 L/day to 1.5 L/day.
Processes of Urine Formation .
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| Events of urine formation . |
[1]. When blood passes through glomerular capillaries, the plasma is filtered into the Bowman capsule. This process is called glomerular filtration.
[2]. Filtrate from Bowman capsule passes through the tubular portion of the nephron. While passing through the tubule, the filtrate undergoes various changes both in quality and in quantity.
[3]. Many wanted substances like glucose, amino acids, water and electrolytes are reabsorbed from the tubules. This process is called tubular reabsorption.
[4]. And, some unwanted substances are secreted into the tubule from peritubular blood vessels. This process is called tubular secretion or excretion .
The urine formation includes three processes:
A. Glomerular filtration
B. Tubular reabsorption
C. Tubular secretion.
Among these three processes filtration is the function of the glomerulus.
Reabsorption and secretion are the functions of tubular portion of the nephron.
Glomerular Filtration .
[1]. Glomerular filtration is the process by which the blood is filtered while passing through the glomerular capillaries by filtration membrane.
[2]. It is the first process of urine formation. The structure of filtration membrane is well suited for filtration.
Filtration Membrane .
Filtration membrane is formed by three layers:
1. Glomerular capillary membrane
2. Basement membrane
3. Visceral layer of Bowman capsule.
1. Glomerular capillary membrane .
[1]. Glomerular capillary membrane is formed by single layer of endothelial cells, which are attached to the basement membrane.
[2]. The capillary membrane has many pores called fenestrae or filtration pores with a diameter of 0.1 µ.
2. Basement membrane .
[1]. Basement membrane of glomerular capillaries and the basement membrane of visceral layer of Bowman capsule fuse together.
[2]. The fused basement membrane separates the endothelium of glomerular capillary and the epithelium of visceral layer of Bowman capsule.
3. Visceral layer of Bowman capsule .
[1]. This layer is formed by a single layer of flattened epithelial cells resting on a basement membrane .
[2]. Each cell is connected with the basement membrane by cytoplasmic extensions called pedicles or feet. Epithelial cells with pedicles are called podocytes .
[3]. Pedicles interdigitate leaving small cleft like spaces in between.
[4]. The cleft like space is called slit pore or filtration slit. Filtration takes place through these slit pores.
Process of Glomerular Filtration .
[1]. When blood passes through glomerular capillaries, the plasma is filtered into the Bowman capsule .
[2]. All the substances of plasma are filtered except the plasma proteins. The filtered fluid is called glomerular filtrate.
Ultrafiltration .
[1]. Glomerular filtration is called ultrafiltration because even the minute particles are filtered. But, the plasma proteins are not filtered due to their large molecular size.
[2]. The protein molecules are larger than the slit pores present in the endothelium of capillaries. Thus, the glomerular filtrate contains all the substances present in plasma except the plasma proteins.
Method of Collection of Glomerular Filtrate .
[1]. Glomerular filtrate is collected in experimental animals by micro-puncture technique.
[2]. This technique involves insertion of a micropipette into the Bowman capsule and aspiration of filtrate.
Glomerular filtration rate .
[1]. Glomerular filtration rate (GFR) is defined as the total quantity of filtrate formed in all the nephrons of both the kidneys in the given unit of time.
[2]. Normal GFR is 125 mL/minute or about 180 L/day.
Filtration fraction .
[1]. Filtration fraction is the fraction (portion) of the renal plasma, which becomes the filtrate.
[2]. It is the ratio between renal plasma flow and glomerular filtration rate. It is expressed in percentage.
[3]. Normal filtration fraction varies from 15% to 20%.
Pressures Determining Filtration .
Pressures, which determine the GFR are:
[1] . Glomerular capillary pressure
[2] . Colloidal osmotic pressure in the glomeruli
[3] . Hydrostatic pressure in the Bowman capsule.
These pressures determine the GFR by either favoring or opposing the filtration.
1. Glomerular Capillary Pressure .
[1]. Glomerular capillary pressure is the pressure exerted by the blood in glomerular capillaries.
[2]. It is about 60 mm Hg and, varies between 45 and 70 mm Hg.
[3]. Glomerular capillary pressure is the highest capillary pressure in the body. This pressure favors glomerular filtration.
2. Colloidal Osmotic Pressure .
[1]. It is the pressure exerted by plasma proteins in the glomeruli.
[2]. The plasma proteins are not filtered through the glomerular capillaries and remain in the glomerular capillaries.
[3]. These proteins develop the colloidal osmotic pressure, which is about 25 mm Hg. It opposes glomerular filtration.
3. Hydrostatic Pressure in Bowman Capsule .
[1]. It is the pressure exerted by the filtrate in Bowman capsule. It is also called capsular pressure.
[2]. It is about 15 mm Hg. It also opposes glomerular filtration.
Net Filtration Pressure .
[1]. Net filtration pressure is the balance between pressure favoring filtration and pressures opposing filtration. It is otherwise known as effective filtration pressure or essential filtration pressure.
[2]. Net filtration pressure is about 20 mm Hg and, it varies between 15 and 20 mm Hg.
Starling Hypothesis and Starling Forces .
[1]. Determination of net filtration pressure is based on Starling hypothesis.
[2]. Starling hypothesis states that the net filtration through capillary membrane is proportional to hydrostatic pressure difference across the membrane minus oncotic pressure difference.
[3]. Hydrostatic pressure within the glomerular capillaries is the glomerular capillary pressure. All the pressures involved in determination of filtration are called Starling forces.
Filtration coefficient .
[1]. Filtration coefficient is the GFR in terms of net filtration pressure.
[2]. It is the GFR per mm Hg of net filtration pressure.
[3]. For example, when GFR is 125 mL/min and net filtration pressure is 20 mm Hg.
Factor Regulating (Affecting ) GFR .
1. Renal Blood Flow .
[1]. It is the most important factor that is necessary for glomerular filtration. GFR is directly proportional to renal blood flow.
[2]. Normal blood flow to both the kidneys is 1,300 mL/minute.
[3]. The renal blood flow itself is controlled by autoregulation.
2. Tubuloglomerular Feedback .
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| Tubuloglomerular feedback . |
[1]. Tubuloglomerular feedback is the mechanism that regulates GFR through renal tubule and macula densa .
[2]. Macula densa of juxtaglomerular apparatus in the terminal portion of thick ascending limb is sensitive to the sodium chloride in the tubular fluid.
[3]. When the glomerular filtrate passes through the terminal portion of thick ascending segment, macula densa acts like a sensor.
[4]. It detects the concentration of sodium chloride in the tubular fluid and accordingly alters the glomerular blood flow and GFR.
[5]. Macula densa detects the sodium chloride concentration via Na+ K+ 2Cl– cotransporter (NKCC2).
When the concentration of sodium chloride increases in the filtrate .
[1]. When GFR increases, concentration of sodium chloride increases in the filtrate.
[2]. Macula densa releases adenosine from ATP. Adenosine causes constriction of afferent arteriole. So the blood flow through glomerulus decreases leading to decrease in GFR.
[3]. Adenosine acts on afferent arteriole via adenosine A1 receptors.
[4]. There are several other factors, which increase or decrease the sensitivity of tubuloglomerular feedback.
Factors increasing the sensitivity of tubuloglomerular feedback .
1. Adenosine .
2. Thromboxane .
3. Prostaglandin E2 .
4. Hydroxyeicosatetranoic acid.
Factors decreasing the sensitivity of tubuloglomerular feedback .
1. Atrial natriuretic peptide .
2. Prostaglandin I2 .
3. Cyclic AMP (cAMP) .
4.. Nitrous oxide.
When the concentration of sodium chloride decreases in the filtrate .
[1]. When GFR decreases, concentration of sodium chloride decreases in the filtrate.
[2]. Macula densa secretes prostaglandin (PGE2 ), bradykinin and renin. PGE2 and bradykinin cause dilatation of afferent arteriole.
[3]. Renin induces the formation of angiotensin II, which causes constriction of efferent arteriole.
[4]. The dilatation of afferent arteriole and constriction of efferent arteriole leads to increase in glomerular blood flow and GFR.
3. Glomerular Capillary Pressure .
[1]. Glomerular filtration rate is directly proportional to glomerular capillary pressure.
[2]. Normal glomerular capillary pressure is 60 mm Hg.
[3]. When glomerular capillary pressure increases, the GFR also increases.
[4]. Capillary pressure, in turn depends upon the renal blood flow and arterial blood pressure.
4. Colloidal Osmotic Pressure .
[1]. Glomerular filtration rate is inversely proportional to colloidal osmotic pressure, which is exerted by plasma proteins in the glomerular capillary blood.
[2]. Normal colloidal osmotic pressure is 25 mm Hg.
[3]. When colloidal osmotic pressure increases as in the case of dehydration or increased plasma protein level GFR decreases.
[4]. When colloidal osmotic pressure is low as in hypoproteinemia, GFR increases.
5. Hydrostatic Pressure in Bowman Capsule .
[1]. GFR is inversely proportional to this. Normally, it is 15 mm Hg.
[2]. When the hydrostatic pressure increases in the Bowman capsule, it decreases GFR.
[3]. Hydrostatic pressure in Bowman capsule increases in conditions like obstruction of urethra and edema of kidney beneath renal capsule.
6. Constriction of Afferent Arteriole .
Constriction of afferent arteriole reduces the blood flow to the glomerular capillaries, which in turn reduces GFR.
7. Constriction of Efferent Arteriole .
[1]. If efferent arteriole is constricted, initially the GFR increases because of stagnation of blood in the capillaries. Later when all the substances are filtered from this blood, further filtration does not occur .
[2]. It is because, the efferent arteriolar constriction prevents outflow of blood from glomerulus and no fresh blood enters the glomerulus for filtration.
8. Systemic Arterial Pressure .
[1]. Renal blood flow and GFR are not affected as long as the mean arterial blood pressure is in between 60 and 180 mm Hg due to the autoregulatory mechanism .
[2]. Variation in pressure above 180 mm Hg or below 60 mm Hg affects the renal blood flow and GFR accordingly, because the autoregulatory mechanism fails beyond this range.
9. Sympathetic Stimulation .
[1]. Afferent and efferent arterioles are supplied by sympathetic nerves.
[2]. The mild or moderate stimulation of sympathetic nerves does not cause any significant change either in renal blood flow or GFR.
[3]. Strong sympathetic stimulation causes severe constriction of the blood vessels by releasing the neurotransmitter substance, noradrenaline.
[4]. The effect is more severe on the efferent arterioles than on the afferent arterioles. So, initially there is increase in filtration but later it decreases.
[5]. However, if the stimulation is continued for more than 30 minutes, there is recovery of both renal blood flow and GFR. It is because of reduction in sympathetic neurotransmitter.
10. Surface Area of Capillary Membrane .
[1]. GFR is directly proportional to the surface area of the capillary membrane.
[2]. If the glomerular capillary membrane is affected as in the cases of some renal diseases, the surface area for filtration decreases. So there is reduction in GFR.
11. Permeability of Capillary Membrane .
[1]. GFR is directly proportional to the permeability of glomerular capillary membrane.
[2]. In many abnormal conditions like hypoxia, lack of blood supply, presence of toxic agents, etc. the permeability of the capillary membrane increases.
[3]. In such conditions, even plasma proteins are filtered and excreted in urine.
12. Contraction of Glomerular Mesangial Cells .
[1]. Glomerular mesangial cells are situated in between the glomerular capillaries.
[2]. Contraction of these cells decreases surface area of capillaries resulting in reduction in GFR .
13. Hormonal and Other Factors .
Many hormones and other secretory factors alter GFR by affecting the blood flow through glomerulus.
Factors increasing GFR by vasodilatation .
1. Atrial natriuretic peptide .
2. Brain natriuretic peptide .
3. cAMP .
4. Dopamine .
5. Endothelial-derived nitric oxide .
6. Prostaglandin (PGE2) .
Factors decreasing GFR by vasoconstriction .
1. Angiotensin II .
2. Endothelins .
3. Noradrenaline .
4. Platelet-activating factor .
5. Platelet-derived growth factor .
6. Prostaglandin (PGF2 ).
Tubular Reabsorption .
[1]. Tubular reabsorption is the process by which water and other substances are transported from renal tubules back to the blood.
[2]. When the glomerular filtrate flows through the tubular portion of nephron, both quantitative and qualitative changes occur.
[3]. Large quantity of water (more than 99%), electrolytes and other substances are reabsorbed by the tubular epithelial cells.
[4]. The reabsorbed substances move into the interstitial fluid of renal medulla. And, from here, the substances move into the blood in peritubular capillaries.
[5]. Since the substances are taken back into the blood from the glomerular filtrate, the entire process is called tubular reabsorption.
Method of Collection of Tubular Fluid .
There are two methods to collect the tubular fluid for analysis.
1. Micro-puncture Technique .
[1]. A micropipette is inserted into the Bowman capsule and different parts of tubular portion in the nephrons of experimental animals to collect the fluid.
[2]. The fluid samples are analyzed and compared with each other to assess the changes in different parts of nephron.
2. Stop-flow Method .
[1]. Ureter is obstructed so that the back pressure rises and stops the glomerular filtration.
[2]. The obstruction is continued for 8 minutes.
[3]. It causes some changes in the fluid present in different parts of the tubular portion.
[4]. Later, the obstruction is released and about 30 samples of 0.5 mL of urine are collected separately at regular intervals of 30 seconds.
[5]. The first sample contains the fluid from collecting duct.
[6]. Successive samples contain the fluid from distal convoluted tubule, loops of Henle and proximal convoluted tubule respectively. All the samples are analyzed.
Selective Reabsorption .
[1]. Tubular reabsorption is known as selective reabsorption because the tubular cells reabsorb only the substances necessary for the body.
[2]. Essential substances such as glucose, amino acids and vitamins are completely reabsorbed from renal tubule. Whereas the unwanted substances like metabolic waste products are not reabsorbed and excreted through urine.
Mechanism of Reabsorption .
Basic transport mechanisms involved in tubular reabsorption are of two types:
1. Active reabsorption .
2. Passive reabsorption.
1. Active Reabsorption .
[1]. Active reabsorption is the movement of molecules against the electrochemical (uphill) gradient.
[2]. It needs liberation of energy, which is derived from ATP.
[3]. Substances reabsorbed actively from the renal tubule are sodium, calcium, potassium, phosphates, sulfates, bicarbonates, glucose, amino acids, ascorbic acid, uric acid and ketone bodies.
2. Passive Reabsorption .
[1]. Passive reabsorption is the movement of molecules along the electrochemical (downhill) gradient. This process does not need energy.
[2]. Substances reabsorbed passively are chloride, urea and water.
Routes of Reabsorption .
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| Routes of Reabsorption . |
Reabsorption of substances from tubular lumen into the peritubular capillary occurs by two routes:
1. Transcellular route
2. Paracellular route.
1. Transcellular Route .
[1]. In this route the substances move through the cell.
[2]. It includes transport of substances from:
a. Tubular lumen into tubular cell through apical (luminal) surface of the cell membrane .
b. Tubular cell into interstitial fluid .
c. Interstitial fluid into capillary.
2. Paracellular Route .
[1]. In this route, the substances move through the intercellular space.
[2]. It includes transport of substances from:
a. Tubular lumen into interstitial fluid present in lateral intercellular space through the tight junction between the cells .
b. Interstitial fluid into capillary .
Site of Reabsorption .
Reabsorption of the substances occurs in almost all the segments of tubular portion of nephron.
1. Substances Reabsorbed from Proximal Convoluted Tubule .
[1]. About 7/8 of the filtrate (about 88%) is reabsorbed in proximal convoluted tubule.
[2]. The brush border of epithelial cells in proximal convoluted tubule increases the surface area and facilitates the reabsorption.
[3]. Substances reabsorbed from proximal convoluted tubule are glucose, amino acids, sodium, potassium, calcium, bicarbonates, chlorides, phosphates, urea, uric acid and water.
2. Substances Reabsorbed from Loop of Henle .
Substances reabsorbed from loop of Henle are sodium and chloride.
3. Substances Reabsorbed from Distal Convoluted Tubule .
Sodium, calcium, bicarbonate and water are reabsorbed from distal convoluted tubule.
Regulation of Tubular Reabsorption .
Tubular reabsorption is regulated by three factors:
1. Glomerulotubular balance .
2. Hormonal factors .
3. Nervous factors.
1. Glomerulotubular Balance .
[1]. Glomerulotubular balance is the balance between the filtration and reabsorption of solutes and water in kidney.
[2]. When GFR increases, the tubular load of solutes and water in the proximal convoluted tubule is increased.
[3]. It is followed by increase in the reabsorption of solutes and water.
[4]. This process helps in the constant reabsorption of solute particularly sodium and water from renal tubule.
Mechanism of Glomerulotubular balance .
[1]. Glomerulotubular balance occurs because of osmotic pressure in the peritubular capillaries.
[2]. When GFR increases, more amount of plasma proteins accumulate in the glomerulus.
[3]. Consequently, the osmotic pressure increases in the blood by the time it reaches efferent arteriole and peritubular capillaries.
[4]. The elevated osmotic pressure in the peritubular capillaries increases reabsorption of sodium and water from the tubule into the capillary blood.
2. Hormonal Factors .
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| Hormones regulating tubular reabsorption . |
Hormones, which regulate GFR are given in above Table .
3. Nervous Factor .
[1]. Activation of sympathetic nervous system increases the tubular reabsorption (particularly of sodium) from renal tubules.
[2]. It also increases the tubular reabsorption indirectly by stimulating secretion of renin from juxtaglomerular cells.
[3]. Renin causes formation of angiotensin II, which increases the sodium reabsorption .
Threshold Substances .
Depending upon the degree of reabsorption, various substances are classified into three categories
1. High-threshold substances
2. Low-threshold substances
3. Non-threshold substances.
1. High-threshold Substances .
[1]. High-threshold substances are those substances, which do not appear in urine under normal conditions.
[2]. The food substances like glucose, amino acids, acetoacetate ions and vitamins are completely reabsorbed from renal tubules and do not appear in urine under normal conditions.
[3]. These substances can appear in urine, only if their concentration in plasma is abnormally high or in renal diseases when reabsorption is affected. So, these substances are called high-threshold substances.
2. Low-threshold Substances .
[1]. Low-threshold substances are the substances, which appear in urine even under normal conditions .
[2]. The substances such as urea, uric acid and phosphate are reabsorbed to a little extend. So, these substances appear in urine even under normal conditions.
3. Non-threshold Substances .
[1]. Non-threshold substances are those substances, which are not at all reabsorbed and are excreted in urine irrespective of their plasma level.
[2]. The metabolic end products such as creatinine are the non-threshold substances.
Transport Maximum – Tm Value .
[1]. Tubular transport maximum or Tm is the rate at which the maximum amount of a substance is reabsorbed from the renal tubule. So, for every actively reabsorbed substance, there is a maximum rate at which it could be reabsorbed.
[2]. For example, the transport maximum for glucose (Tm-G) is 375 mg/minute in adult males and about 300 mg/minute in adult females.
Threshold Level in Plasma for Substances having Tm Value .
[1]. Renal threshold is the plasma concentration at which a substance appears in urine.
[2]. Every substance having Tm value has also a threshold level in plasma or blood. Below that threshold level, the substance is completely reabsorbed and does not appear in urine.
[3]. When the concentration of that substance reaches the threshold, the excess amount is not reabsorbed and, so it appears in urine. This level is called the renal threshold of that substance.
[4]. For example, the renal threshold for glucose is 180 mg/dL. That is, glucose is completely reabsorbed from tubular fluid if its concentration in blood is below 180 mg/dL. So, the glucose does not appear in urine.
[5]. When the blood level of glucose reaches 180 mg/dL it is not reabsorbed completely; hence it appears in urine.
Reabsorption of Important Substances .
Reabsorption of Sodium .
[1]. From the glomerular filtrate, 99% of sodium is reabsorbed.
[2]. Two thirds of sodium is reabsorbed in proximal convoluted tubule and remaining one third in other segments (except descending limb) and collecting duct.
[3]. Sodium reabsorption occurs in three steps:
1. Transport from lumen of renal tubules into the tubular epithelial cells .
2. Transport from tubular cells into the interstitial fluid .
3. Transport from interstitial fluid to the blood.
1. Transport from Lumen of Renal Tubules into the Tubular Epithelial Cells .
[1]. Active reabsorption of sodium ions from lumen into the tubular cells occurs by two ways:
1. In exchange for hydrogen ion by antiport (sodium counterport protein) – in proximal convoluted tubules .
2. Along with other substances like glucose and amino acids by symport (sodium cotransport protein) – in other segments and collecting duct.
[2]. It is believed that some amount of sodium diffuses along the electrochemical gradient from lumen into tubular cell across the laminar membrane.
[3]. The electrochemical gradient is developed by sodium-potassium pump.
2. Transport from Tubular Cells into the Interstitial Fluid .
[1]. Sodium is pumped outside the cells by sodium-potassium pump. This pump moves three sodium ions from the cell into interstitium and two potassium ions from interstitium into the cell.
[2]. Tubular epithelial cells are connected with their neighboring cells by tight junctions at their apical luminal edges.
[3]. But, beyond the tight junction, a small space is left between the adjoining cells along their lateral borders. This space is called lateral intercellular space. The interstitium extends into this space.
[4]. Most of the sodium ions are pumped into the lateral intercellular space by sodium-potassium pump. [5]. The rest of the sodium ions are pumped into the interstitium by the sodium-potassium pump situated at the basal part of the cell membrane.
[6]. Transport of sodium out of the tubular cell by sodium-potassium pump, decreases the sodium concentration within the cell. This develops an electrochemical gradient between the lumen and tubular cell resulting in diffusion of sodium into the cell .
3. Transport from Interstitial Fluid to the Blood .
[1]. From the interstitial fluid, sodium ions enter the peritubular capillaries by concentration gradient .
[2]. In the distal convoluted tubule, the sodium reabsorption is stimulated by the hormone aldosterone secreted by adrenal cortex.
Reabsorption of Water .
Reabsorption of water occurs from proximal and distal convoluted tubules and in collecting duct.
Reabsorption of water from proximal convoluted tubule – obligatory water reabsorption .
[1]. Obligatory reabsorption is the type of water reabsorption in proximal convoluted tubule, which is secondary (obligatory) to sodium reabsorption.
[2]. When sodium is reabsorbed from the tubule, the osmotic pressure decreases. It causes osmosis of water from renal tubule.
Reabsorption of water from distal convoluted tubule and collecting duct – facultative water reabsorption .
[1]. Facultative reabsorption is the type of water reabsorption in distal convoluted tubule and collecting duct that occurs by the activity of antidiuretic hormone (ADH).
[2]. Normally, the distal convoluted tubule and the collecting duct are not permeable to water. But in the presence of ADH, these segments become permeable to water, so it is reabsorbed.
Mechanism of action of ADH – Aquaporins .
[1]. Antidiuretic hormone increases water reabsorption in distal convoluted tubules and collecting ducts by stimulating the water channels called aquaporins.
[2]. ADH combines with vasopressin (V2) receptors in the tubular epithelial membrane and activates adenyl cyclase, to form cyclic AMP. This cyclic AMP activates the aquaporins, which increase the water reabsorption.
[3]. Aquaporins (AQP) are the membrane proteins, which function as water channels.
[4]. Though about 10 aquaporins are identified in mammals only 5 are found in humans.
[5]. Aquaporin1, 2 and 3 are present in renal tubules.
[6]. Aquaporin4 is present in brain and aquaporin5 is found in salivary glands.
[7]. Aquaporin2 forms the water channels in renal tubules.
Reabsorption of Glucose .
[1]. Glucose is completely reabsorbed in the proximal convoluted tubule. It is transported by secondary active transport (sodium cotransport) mechanism.
[2]. Glucose and sodium bind to a common carrier protein in the luminal membrane of tubular epithelium and enter the cell.
[3]. The carrier protein is called sodium-dependent glucose cotransporter 2 (SGLT2).
[4]. From tubular cell glucose is transported into medullary interstitium by another carrier protein called glucose transporter 2 (GLUT2).
Tubular maximum for glucose (Tm-G) .
In adult male, Tm-G is 375 mg/minute and in adult females it about 300 mg/minute.
Renal threshold for glucose .
[1]. Renal threshold for glucose is 180 mg/dL in venous blood.
[2]. When the blood level reaches 180 mg/dL glucose is not reabsorbed completely and appears in urine.
Splay .
[1]. Splay means deviation. With normal GFR of 125 mL/ minute and Tm-G of 375 mg/minute in an adult male the predicted (expected) renal threshold for glucose should be 300 mg/dL. But actually it is only 180 mg/dL.
[2]. When the renal threshold curves are drawn by using these values, the actual curve deviates from the ‘should be’ or predicted or ideal curve . This type of deviation is called splay.
[3]. Splay is because of the fact that all the nephrons do not have the same filtering and reabsorbing capacities.
Reabsorption of Amino Acids .
[1]. Amino acids are also reabsorbed completely in proximal convoluted tubule.
[2]. Amino acids are reabsorbed actively by the secondary active transport mechanism along with sodium.
Reabsorption of Bicarbonates .
[1]. Bicarbonate is reabsorbed actively, mostly in proximal tubule . It is reabsorbed in the form of carbon dioxide.
[2]. Bicarbonate is mostly present as sodium bicarbonate in the filtrate.
[3]. Sodium bicarbonate dissociates into sodium and bicarbonate ions in the tubular lumen.
[4]. Sodium diffuses into tubular cell in exchange of hydrogen.
[5]. Bicarbonate combines with hydrogen to form carbonic acid.
[6]. Carbonic acid dissociates into carbon dioxide and water in the presence of carbonic anhydrase. Carbon dioxide and water enter the tubular cell.
[7]. In the tubular cells, carbon dioxide combines with water to form carbonic acid. It immediately dissociates into hydrogen and bicarbonate.
[8]. Bicarbonate from the tubular cell enters the interstitium. There it combines with sodium to form sodium bicarbonate .
Tubular secretion
[1]. Tubular secretion is the process by which the substances are transported from blood into renal tubules. It is also called tubular excretion.
[2]. In addition to reabsorption from renal tubules, some substances are also secreted into the lumen from the peritubular capillaries through the tubular epithelial cells.
[3]. Dye phenol red was the first substance found to be secreted in renal tubules in experimental conditions. Later many other substances were found to be secreted .
[4]. Such substances are:
1. Para-aminohippuric acid (PAH) .
2. Diodrast .
3. 5hydroxyindoleacetic acid (5HIAA) .
4. Amino derivatives 5. Penicillin.
Substances Secreted in different Segments of Renal Tubules .
[1] . Potassium is secreted actively by sodium-potassium pump in proximal and distal convoluted tubules and collecting ducts .
[2] . Ammonia is secreted in the proximal convoluted tubule .
[3] . Hydrogen ions are secreted in the proximal and distal convoluted tubules. Maximum hydrogen ion secretion occurs in proximal tubule
[4] . Urea is secreted in loop of Henle.
Thus, urine is formed in nephron by the processes of glomerular filtration, selective reabsorption and tubular secretion.
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