Excretion 2 Flashcards
what are the steps involved in urine formation
Urine formation involves three main processes namely, glomerular
filtration, reabsorption and secretion, that takes place in different parts of
the nephron.
explain the first step of urine formation- glomerular filtration
The first step in urine formation is the filtration of blood, which is carried
out by the glomerulus and is called glomerular filtration.
The glomerular capillary blood pressure causes filtration of blood
through 3 layers, i.e., the endothelium of glomerular blood vessels, the
epithelium of Bowman’s capsule and a basement membrane between these
two layers. The epithelial cells of Bowman’s capsule called podocytes are
arranged in an intricate manner so as to leave some minute spaces called
filtration slits or slit pores. Blood is filtered so finely through these
membranes, that almost all the constituents of the plasma except the
proteins pass onto the lumen of the Bowman’s capsule. Therefore, it is
considered as a process of ultra filtration.
what is glomerular filtration rate
The amount of the filtrate formed by the kidneys per minute is called
glomerular filtration rate (GFR). GFR in a healthy individual is
approximately 125 ml/minute, i.e., 180 litres per day.
On an average,
1100-1200 ml of blood is filtered by the kidneys per minute which constitute
roughly 1/5th of the blood pumped out by each ventricle of the heart in a
minute.
define reabsorption
A comparison of the volume of the filtrate formed per day (180 litres
per day) with that of the urine released (1.5 litres), suggest that nearly 99
per cent of the filtrate has to be reabsorbed by the renal tubules. This
process is called reabsorption. The tubular epithelial cells in different
segments of nephron perform this either by active or passive mechanisms.
For example, substances like glucose, amino acids, Na+
, etc., in the filtrate
are reabsorbed actively whereas the nitrogenous wastes are absorbed by
passive transport. Reabsorption of water also occurs passively in the initial
segments of the nephron
reabsorption or tubular reabsorption is the process by which the nephron removes water and solutes from the tubular fluid and returns them to the circulating blood.
what is tubular secretion
During urine formation, the tubular cells secrete substances like H+
,K+ and ammonia into the filtrate. Tubular secretion is also an important step in urine formation as it helps in the maintenance of ionic and acid base balance of body fluids.
describe proximal convuluted tubule?
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, ammonia and potassium ions into the filtrate and by
absorption of HCO3– from it.
describe 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.
describe distal convoluted tubule
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.
describe 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
why is it called counter current mechanism
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.
how is gradient established
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
what does the counter current mechanism help to achive
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.
how can kidney function be regulated
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.
how does hypothalamus help in monitoring the functioning of the kidneys
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.
explain the effect and function osf JGA
JGA is a special sensitive region formed by
cellular modifications in the distal convoluted tubule and the afferent
arteriole at the location of their contact.
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.
It is called as RAS mechanism or RAAS mechanism
