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3−steps are involved in the formation of urine
A) Ultra ﬁltration
- It is ﬁltration under pressure.
- Glomerular capillary pressure (45 mm of Hg) favours ﬁltration.
- The Colloidal osmotic pressure (due to plasma proteins, particularly albumin) acts against ﬁlteration. Its value is ~20 mm of Hg.
- The Capsular ﬁltrate pressure, due to the glomerular ﬁltrate in the Bowman’s capsule, also acts against ﬁltration. Its value is ~10 mm of Hg. Net ﬁltration pressure = 45 − (20+10) mm of Hg = 15 mm of Hg or 10 − 20 mm of Hg.
- Only 1/5 of plasma (20%) gets ﬁltered from glomerulus per unit time. It is about 125 ml per minute.
Amount of glomerular ﬁltrate = 125 ml / min = 7.5 litre/hr = 180 litre/day The amount of urine formed per day is ~1.8 litre, i.e. 1% of 180 litre per day, which means that 99% of glomerular ﬁltrate is reabsorbed.
Autoregulation of glomerular ﬁltration rate
2-mechanism of autoregulation
1) Myogenic System
When BP is high the stretching of afferent arteriole is increased. This stimulates ‘stretch receptors’ of the arteriole, and the diameter of afferent arteriole is reduced Atrial natriuretic factor (ANF), a hormone secreted from the heart (atrium), promotes the loss of sodium in the urine and also reduces B.P.
2) Renin-angiotensin-aldosterone system (RAAS)
The wall of afferent arteriole contains the renin-secreting juxta-glomerular cells (JGC). At this point, the epithelium of distal tubule is histologically modiﬁ ed to form Macula densa (MD). The JGC, MD and adjoining granulated cells collectively form Juxtaglomerular apparatus (JGA).
When BP is low, the JG cells secrete renin. This converts Angiotensinogen protein of blood plasma, produced from the liver, into Angiotensin I and then Angiotensin II. Angiotensin II is vasoconstrictor and increases BP. Angiotensin II also stimulates Adrenal Cortex for the secretion of Aldosterone, the latter absorbs Sodium from glomerular ﬁltrate and raises BP.
Aldosterone is antagonistic to Atrial Natriuretic factor-ANF.
Composition of glomerular ﬁlterate
The glomerular ﬁlterate is Plasma minus proteins.
It consists of
- Amino acids
- Uric Acid
- Hippuric Acid
- Phosphates etc.
The concentration of glucose in glomerular ﬁltrate is equal to its concentration in blood plasma.
B) Selective reabsorption
a) In PCT− The maximum reabsorption from glomerular ﬁltrate occurs in this part.
- Glucose and amino acids are absorbed (100%) by active transportation.
- [Active transportation requires energy (ATPs) and is against concentration gradient.] • Absorption of water is 60−70% (obligatory absorption).
- Absorption of salts (sodium) also ranges from 60−70%. The absorption of chloride is however, passive.
- Absorption of urea is ~50% while absorption of uric acid is ~95% (later added into glomerular ﬁ ltrate by tubular secretion).
- Creatinine and sulphates are not at all reabsorbed.
- The absorption of bicarbonates and phosphates is more than 90%.
High Threshold Substances
eg. Glucose and amino acids These substances are completely reabsorbed from the glomerular ﬁltrate and do not appear in the urine in normal conditions. Such substances appear in urine only when their concentration reaches beyond renal threshold. (For glucose threshold value is 170-180 mg/100 ml of blood plasma).
Low Threshold Substances
eg. Urea and Uric acid These substances are not completely reabsorbed from the glomerular ﬁltrate and, therefore, appear in the urine in normal conditions.
B) In Henle’s Loop− The glomerular ﬁltrate in Henle’s loop ﬂows in the opposite direction and parallel, in descending and ascending limbs. This mechanism is called Countercurrent mechanism/system which makes the ﬂuid more concentrated.
In fact, there are 2−counter current mechanisms
(i) Counter current multiplier (in Henle’s loop).
(ii) Countercurrent exchanger (in Vasa recta)
Counter Current Mechanism
The Counter Current Mechanism is the process in which inﬂ ow runs parallel and opposite but in the close proximity to the outﬂ ow for some distance.
The osmolarity of plasma or the interstitial ﬂuid in body parts is about 300m Osm/lit whereas the osmolarity in the interstial ﬂuid in the medulla (Medullary interstitium) of the kidney is (progressively) much higher, i.e. about 1200m Osm/lit. This means that renal Medullary interstitium has accumulated solutes in great excess of water. Once this high solute concentration is achieved in medulla it is maintained by balanced inﬂ ow and outﬂ ow of solutes and water.
- The Loop of Henle acts as ‘counter current multiplier’ by producing a concentration gradient of high osmolarity in medullary interstitium.
- The Vasa Recta acts as ‘counter-current exchanger’ by maintaining or preserving this high solute concentration in the medulla.
The main cause of high medullary osmolarity is the active transport of sodium (and co-transport of potassium, chloride and other ions) from the thick ascending loop of Henle into the interstitium. Since this ascending limb is impermeable to water, the solutes pumped out is not followed by osmotic ﬂow of water into the interstitium. This pump establishes a concentration gradient of about 200m Osm/lit between tubular lumen and interstitial ﬂuid. The descending limb of Henle’s loop, in comparison to ascending limb, is highly permeable to water, hence the water diffuses out into interstitium and the osmolarity of tubular ﬂuid (of descending limb) quickly becomes equal to the medullary osmolarity. These steps are repeated over and over, and the net effect is the addition of more and more solute to the medulla in excess of water. Thus the trapping of solute due to active pumping out of ions out of thick ascending limb of Henle gradually multiplies the concentration gradient, which eventually raises the osmolarity at the tip of the loop to ~1200m Osm/lit.
(This repetitive re-absorption of sodium from the thick ascending limb of loop of Henle and the continuous inﬂow of new sodium chloride from PCT is ‘counter current multiplication’).
The Vasa Recta, a capillary network around the individual limb of Henle’s loop, acts as counter-current exchanger and preserves the high solute concentration in medullary interstitium. As blood (in Vasta Recta) descends into medullary region it progressively becomes more concentrated, partly by loss of water into interstitium and party by solute entry from the medullary interstitium into Vasa Recta. By the time the blood reaches the tip of Vasa Recta it has a concentration of about 1200m Osm/lit, the same as in medullary interstitium.
Now, as the blood ascends towards the cortex it becomes less concentrated because the solute diffuses out into interstitium and the water moves into this part of Vasa Recta. Thus, there is a large amount of ﬂuid-and-solute exchange across Vasa Recta; but there is almost no dilution of interstitial ﬂuid in the renal medulla. So, the vasa Recta does not create medullary hyperosmolarity but does prevent it from being diluted.
The absorption of water in the descending limb of Henle’s loop is ~15%. This absorption depends upon the length of Henle’s loop. In birds where the excretory matter is solid (uric acid), the Henle’s loops are very long.
The absorption of sodium occurs in ascending limb only.
C) In DCT − In this region, the absorption of water is 4−5%. But, in the presence of Vasopressin hormone (ADH), the absorption of water from glomerular ﬁltrate increases and may reach up to 10%. It is due to change of permeability of DCT for water.
Aldosterone hormone also changes the permeability of DCT, but for sodium, to increase its absorption.
The absorption of water, as well as sodium in DCT, is facultative.
In the presence of ADH, due to more water absorption, the urine becomes concentrated. (This also allows the urea to pass out of the tubule in medullary region. In the absence of ADH the urea from the medullary tissue may enter into the tubules.)
D) In collecting tubule and collecting duct− The absorption of water (5−10%) in this part is also ADH dependent. The osmolarity of the ﬂuid ultimately reaches to ~1200 m osm/litre.
The term ‘urine’ can be used for the ﬁltrate in collecting tubules. From Bowman’s capsule to DCT the ﬂuid is, however, called glomerular ﬁltrate.
C. Tubular Secretion
Certain secretions, produced from the lining of uriniferous tubule, are added into glomerular ﬁltrate. They include:
(1) Creatinine, (2) ammonia, (3) hydrogen ions, (4) potassium ions and (5) uric acid
The maximum tubular secretion is added from the lining of PCT and DCT.
Composition of Urine
Amount − 1−2 litre/day
pH − ~6 (4.5−7), generally acidic
Speciﬁc gravity − 1.01−1.03
Urea − 2% (2 g/100 ml of urine)
Salts − 2% (Mainly NaCl)
Diuretics increase the volume of urine and amount of electrolyte excretion.
The urine of a normal person may contain water-soluble vitamins but does not contain proteins and blood elements. The urinary output is increased by the excessive intake of water, proteins, tea, coffee, salts and alcohol etc.
The micturition or urination is primarily a spinal reﬂ ex, regulated by higher voluntary centre in brain. During micturition, the smooth muscles of urinary bladder, called Detrusor muscle, contract and the sphincter of urethra relax to void the urine.