Page 7 - LN- EXCRETION

In majority of nephrons, the loop of Henle is too short and extends only very little into the
medulla i.e., lie in the renal cortex. Such two nephrons are called cortical nephrons.
Juxtamedullary Nephrons
In some of the nephrons, the loop of Henle is very long and runs deep into the medulla.
These nephrons are called juxtamedullary nephrons.
The cortical nephron forms about 80% of the total nephron count while rest 20% are the
juxtamedullary nephron.
Functions of Kidney
Following functions are served by kidney
(i) Regulation of water and electrolyte balance.
(ii) Regulation of arterial pressure.
(iii) Excretion of metabolic waste and foreign chemicals.
(iv) Secretion of hormones like renin.
2. Ureters
The pelvis of each kidney is continued as a ureter and emerges out at hilus. Ureter is a long
and muscular tube. Ureters of both sides extend posteriorly and open into the urinary
bladder.
3. Urinary Bladder
It is a thin-walled, pear-shaped, white transparent sac present in the pelvic cavity. It
temporarily stores the urine.
4. Urethra
It is a membranous tube, which conduct urine to the exterior. The urethral sphincters keep
the urethra closed except during voiding of urine.
The formation of urine is the result of the following processes
1. Glomerular Filtration
The first step of urine formation is the filtration of blood, which is carried out by the
glomerulus. That’s why this step is called glomerular filtration.
Kidneys filter about 1100-1200 mL of blood per minute, which constitute roughly l/5th of
the blood pumped out by each ventricle of the heart in a minute.
The glomerular capillary bloodpressure causes filtration of blood through three layers, i.e.,
(i) the endothelium of glomerular blood vessels.
(ii) the epithelium of Bowmans capsule.
(iii) a basement membrane (present between the above mentioned two layers).
The podocytes (epithelial cells of Bowman’s capsule) are arranged in such a manner so, as to
leave some minute spaces called filtration slits or slit pores.
On account of the high pressure in the glomerular capillaries, the substances are filtered
through these pores into the lumen of the Bowman’s capsule (but the RBC, WBC and plasma
proteins having high molecular weight are unable to pass out).
That’s why this process of filtration through glomerular capillaries in the Bowman’s capsule
is known as ultra filtration and the filtrate is called glomerular filtrate or primary urine.
It is hypotonic to urine that is actually excreted. Basic function of nephron is to clear out the
plasma from unwanted substrates and also maintain the osmotic concentration of the blood
plasma. Thus, the fluid coming out is known as urine, whose formation occurs inside the
kidney.
Glomerular Filtration Rate The amount of the filtrate formed by the kidneys per minute is
called Glomerular Filtration Rate (GFR). In a healthy person it was found approximately 125
mL/min, i.e., 180 L/day.