Urinary (excretory) system of Man
It consists a
pair of bean shaped organs called kidneys, a pair of ducts called ureters which
arise from each kidney, a muscular sac like organ the urinary bladder and a
tubule called urethra which arises from bladder and empties urine outside the
body. The bladder end of urethra has a sphincter valve, which controls release
of urine from the bladder.
Structure of
kidney:
Each kidney is
bean shaped organ which is about 10 cm long. They lie in the abdominal cavity
being attached with dorsal body wall on either side of vertebral column. Each
kidney is enclosed by a thin membranous covering called peritoneum and located
on the top of each kidney is adrenal gland. The kidneys receive blood through
paired renal arteries which directly arise from dorsal aorta. The blood is
drained out from each kidney by a renal vein. Both renal arteries enter and
renal veins leave kidney at the site called hilns.
A longitudinal
section of kidney shows that it consists of two distinct regions, an outer
darker region, the cortex and an inner lighter region, the medulla. Medulla
consists of many cone shaped structures called pyramids. Urine is dropped
continuously from the tips of pyramids into funnel shaped spaces called pelvis
of the ureter.
Nephron:
Each kidney is
composed of about a million microscopic tubules or nephrons. It is regarded as
structural and functional unit of kidney which performs osmoregulation as well
as excretion.
Each nephron
begins with a spherical structure called malpighian body. It lies in the cortex
of kidney. Malpighian body is composed of cup like Bowman’s capsule. Inside
this cup lies a dense network of capillaries known as glomerulus. The Bowman’s
capsule gives out a coiled tubule known as proximal convoluted tubule. It lies
in the cortex region. A network of capillaries is also attached with proximal
convoluted tubule. Proximal convoluted tubule narrows and descends down into
the medulla, makes a u turn and comes back in the cortex. This narrow U-shape
part of tubule is known as loop of cortex. Thus it has a descending and an
ascending limb. The ascending limb of the loop of Henle becomes larger in
diameter in the cortex and forms a coil called distal convoluted tubule.
Distal
convoluted tubule finally opens into common collecting duct. The latter
receives urine from a number of nephrons and drains into the pelvis of the
kidney.
Blood supply to
the glomerulus of a nephron begins as an afferent arteriole which arises from
renal arteriole. From the glomerulus blood is carried by the efferent arteriole
to two capillary networks. One of the network called peritubular capillaries is
associated with proximal and distal convoluted tubules while the other one
called vasa recta runs straight parallel besides the limbs of the loop of
Henle.
Function:
Kidney performs
regulatory functions by different processes viz filtration, re-absorption,
tubular secretions and counter current exchange. These processes are carried on
at the level of each nephron.
Kidneys play vital
role in osmoregulation as well as excretion of nitrogenous wastes.
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