Gurukula JL/DL Special | Filtration.. Formation.. Micturition

July 17, 2023 / 03:49 PM IST

EXCRETORY SYSTEM Continuation.. 10th july

FUNCTION OF THE TUBULES

Proximal Convoluted Tubule (PCT):
PCT is lined by simple cuboidal brush border epithelium which increases the surface area for reabsorption.
Nearly all of the essential nutrients, and 70-80 per cent of electrolytes and water are reabsorbed by this segment.
PCT also helps to maintain the pH and ionic balance of the body fluids by selective secretion of hydrogen ions and ammonia into the filtrate and by absorption of HCO3 – from it.
Henle’s Loop:
Reabsorption is minimum in its ascending limb.
However, this region plays a significant role in the maintenance of high osmolarity of medullary interstitial fluid.
The descending limb of loop of Henle is permeable to water but almost impermeable to electrolytes. This concentrates the filtrate as it moves down.
The ascending limb is impermeable to water but allows transport of electrolytes actively or passively.
Therefore, as the concentrated filtrate pass upward, it gets diluted due to the passage of electrolytes to the medullary fluid.
Distal Convoluted Tubule (DCT):
Conditional reabsorption of Na+ and water takes place in this segment.
DCT is also capable of reabsorption of HCO3 – and selective secretion of hydrogen and potassium ions and NH3 to maintain the pH and sodium-potassium balance in blood.
Collecting Duct:
This long duct extends from the cortex of the kidney to the inner parts of the medulla.
Large amounts of water could be reabsorbed from this region to produce a concentrated urine.
This segment allows passage of small amounts of urea into the medullary interstitium to keep up the osmolarity.
It also plays a role in the maintenance of pH and ionic balance of blood by the selective secretion of H+ and K+ ions.

MECHANISM OF CONCENT RATION OF THE FILTRATE

Mammals have the ability to produce a concentrated urine.
The Henle’s loop and vasa recta play a significant role in this.
The flow of filtrate in the two limbs of Henle’s loop is in opposite directions and thus forms a counter current.
The flow of blood through the two limbs of vasa recta is also in a counter current pattern.
The proximity between the Henle’s loop and vasa recta, as well as the counter current in them help in maintaining an increasing osmolarity towards the inner medullary interstitium, i.e., from 300 mOsmolL–1 in the cortex to about 1200 mOsmolL–1 in the inner medulla.
This gradient is mainly caused by NaCl and urea.
NaCl is transported by the ascending limb of Henle’s loop which is exchanged with the descending limb of vasa recta.
NaCl is returned to the interstitium by the ascending portion of vasa recta.
Similarly, small amounts of urea enter the thin segment of the ascending limb of Henle’s loop which is transported back to the interstitium by the collecting tubule.
The above-described transport of substances facilitated by the special arrangement of Henle’s loop and vasa recta is called the counter current mechanism.
This mechanism helps to maintain a concentration gradient in the medullary interstitium.
Presence of such interstitial gradient helps in an easy passage of water from the collecting tubule thereby concentrating the filtrate (urine).
Human kidneys can produce urine nearly four times concentrated than the initial filtrate formed.
The functioning of the kidneys is efficiently monitored and regulated by hormonal feedback mechanisms involving the hypothalamus, JGA and to a certain extent, the heart.
Osmoreceptors in the body are activated by changes in blood volume, body fluid volume and ionic concentration.
An excessive loss of fluid from the body can activate these receptors which stimulate the hypothalamus to release antidiuretic hormone (ADH) or vasopressin from the neurohypophysis.
ADH facilitates water reabsorption from latter parts of the tubule, thereby preventing diuresis.
An increase in body fluid volume can switch off the osmoreceptors and suppress the ADH release to complete the feedback.
ADH can also affect the kidney function by its constrictory effects on blood vessels.
This causes an increase in blood pressure. An increase in blood pressure can increase the glomerular blood flow and thereby the GFR.
The JGA plays a complex regulatory role.
A fall in glomerular blood flow/glomerular blood pressure/GFR can activate the JG cells to release renin which converts angiotensinogen in blood to angiotensin I and further to angiotensin II.
Angiotensin II, being a powerful vasoconstrictor, increases the glomerular blood pressure and thereby GFR.
Angiotensin II also activates the adrenal cortex to release Aldosterone.
Aldosterone causes reabsorption of Na+ and water from the distal parts of the tubule.
This also leads to an increase in blood pressure and GFR.
This complex mechanism is generally known as the Renin-Angiotensin mechanism.
An increase in blood flow to the atria of the heart can cause the release of Atrial Natriuretic Factor (ANF).
ANF can cause vasodilation (dilation of blood vessels) and thereby decrease the blood pressure.
ANF mechanism, therefore, acts as a check on the renin-angiotensin mechanism.

MICTURITION

Urine formed by the nephrons is ultimately carried to the urinary bladder where it is stored till a voluntary signal is given by the central nervous system (CNS).
This signal is initiated by the stretching of the urinary bladder as it gets filled with urine.
In response, the stretch receptors on the walls of the bladder send signals to the CNS.
The CNS passes on motor messages to initiate the contraction of smooth muscles of the bladder and simultaneous relaxation of the urethral sphincter causing the release of urine.
The process of release of urine is called micturition and the neural mechanisms causing it is called the micturition reflex.
An adult human excretes, on an average, 1 to 1.5 litres of urine per day.
The urine formed is a light yellow coloured watery fluid which is slightly acidic (pH-6.0) and has a characteristicodour.
On an average, 25-30 gm of urea is excreted out per day.
Various conditions can affect the characteristics of urine.
Analysis of urine helps in clinical diagnosis of many metabolic discorders as well as malfunctioning of the kidney. For example, presence of glucose (Glycosuria) and ketone bodies (Ketonuria) in urine are indicative of diabetes mellitus.

Dr.Modala Mallesh
subject expert
palem, Nakrekal,
Nalgonda
Ph.9989535675