HUMAN KIDNEY
In
human beings, a pair of kidneys, a pair of urethras and a urinary bladder
represent the excretory system. The nitrogenous end product in man is urea.
Kidney
is a bean shaped, red coloured organ. Kidneys are attached to the dorsal body
wall, on either side of vertebral column in the abdominal cavity. Its outer
margin is convex and inner margin in concave.
The
notch present on the inner margin is called hilus. Renal artery, which brings
blood to the kidney from heart, enters the kidney through hilus.
Renal vein, which carries blood from the
kidney to the heart, and ureter come out of the kidney through hilus. Each
ureter is a thin walled tube. The anterior end of ureter is wide and called pelvis.
The
ureters open into the urinary bladder. Urine is stored in the bladder till it
is filled and then is sent out through a canal called urethra. On the top of
each kidney, an adrenal gland is present. It is an endocrine gland and has
nothing to do with excretion.
To
understand the structure of the kidney, we will study the longitudinal section
of the kidney. Outer region of kidney is
deep red in colour and is called as cortex. The inner region is light red in
colour and is called as medulla.
In
the medulla are 9-12 inverted pyramid like structures with their broad base
towards the cortex and narrow end towards the pelvis. These extend into pelvis.
The
broad end of each pyramid is branched and gives the appears of sepals of a
flower. Hence, these are called calyces. Through calyces, urine reaches funnel
like ducts and from there into pelvis. From pelvis urine enters ureters.
The
structural units of kidney are long tubules called nephrons. These are responsible
for urine formation. In human kidney, there are about 2 lakhs of nephrons.
Each
nephron has two portions – a cup like structure called bowman’s capsule and a
tubular portion. Bowman’s capsule is a two layered cup with space in between
the two layers.
Inside
the cup of the Bowman’s capsule, there is a network of large number of
capillaries which is called as glomerulus. After entering the kidney, the renal artery divides into a number of
arterioles in the kidney.
Each
renal arterioles enters a Bowman’s capsule as afferent arteriole, forms a
capillary network inside the cup like structure and leaves the Bowman’s capsule
as efferent arteriole.
The
tubular part of nephron has three parts – proximal convoluted tubule, loop of
Henle and distal convoluted tubule. The distal convoluted tubules of nephrons
open intocollecting tubues.
Urine
is formed in the Bowman’s capsule. The capillaries in the glomerulus have small
microscopic pores in their walls. When the blood flows through the capillaries
in the glomerulus, it gets filtered through the pores in the walls of the
capillaries. Blood pressure provides the required pressure for this filtration.
All
the water soluble, small molecular weight substances such as salts, glucose,
amino acids along with nitrogenous wastes are filtered from blood. Blood cells
and proteins are not filtered through the pores present in the capillary walls.
This filtration is called ultrafiltration.
Approximately
120 ml of urine is formed in the kidney per minute. In 24 hours, about 175 liters
of urine is formed. However, all this is not excreted – only one to two liters of urine is excreted and the rest of it is
reabsorbed into the body.
The
reabsorption of the urine produced by Bowman’s capsule takes place in the
tubular portion. When the filtrate reaches the proximal convoluted tubule, the
epithelial cells of the tubule reabsorb several substances in a selective
manner and transport them back to blood.
Reabsorption
of water and salts also occurs in the loop of henle and in distal convoluted
tubule. Water, salts and other substances reabsorbed by the tubule are
transported into the blood.
Unabsorbed
substances along with water enter the collecting duct which discharges urine
into pelvis and then into ureter. Urine is produced continuously and is
concentrated continuously.
This
reaches Smooth muscles (involuntary muscles) in the wall of ureter produce
peristaltic movements which prople urine into the urinary bladder.
The urinary
bladder is a muscular sac and opens to the outside through urethra.At
the Junction of bladder and urethra is a sphincter. Urination is a reflex
action.
But in elders, it is voluntary. In children, urination is an
involuntary action. Normally, when about 200 to 300 ml of urine reaches the
bladder, the muscles in its wall contract and relax expelling urine.
DIALYSIS
AND ITS PRINCIPLES
In
the previous chapter you have learnt that a mumber of wastes are produced in
the body which are carried by the blood to the kidneys. In the kidneys, these
wastes are separated from the blood and excreted as urine. In certain disease
conditions, the nephrons in the kidney die.
n
such persons, blood is not filtered and urine is not produced. As a result,
waste products accumulate in the blood and they become toxic to the body. Such
people may loose consciousness or may even die.
Doctors
treat such patients by artificially removing the waste material from the blood.
The separation of wastes from blood by artificial method is called dialysis.
Dialysis machine is used for this purpose.
Blood from the artery is sent into
the dialysis machine where it is filtered and the filtered blood is sent into
the body through a vein. Persons whose kidneys do not function, have to undergo
dialysis once in three or four days throughout their life or get a new kidney
transplanted.
THANKYOU,
