Describe : Human excretory system.

In humans, the excretory system consists of a pair of kidneys, a urinary bladder and a urethra.

Kidney : Reddish brown, bean shaped structure.

$(i)$ Location : It is situated between the levels of last thoracic and third lumbar vertebra close to the dorsal inner wall of the abdominal cavity.

$(ii)$ Size : Each kidney of an adult human measures $10-12 \mathrm{~cm}$ in length, $5-7\; cm$ in width, $2-3 \mathrm{~cm}$ in thickness.

$(iii)$ Weight : Average weight is $120-170 \mathrm{gm}$.

Human Urinary system

Both kidneys are not situated on same plane, right kidney is slightly on lower plane than left because liver is situated above it in the right side of thoracic cavity.

External surface of the kidney is convex and internal surface of the kidney is concave.

Internal structure of the kidney : L.s. of kidney shows, at the concave surface, large opening called Hilum.

Through the hilum, ureter, blood vessels and nerves enter.

Inner to the hilum is a broad funnel shaped space called renal pelvis is present.

$=$ Projection of renal pelvis are called calyces.

- The outer layer of kidney is a tough capsule.

Inside the kidney, there are two zones, an outer cortex and an inner medulla.

$\Rightarrow$ The medulla is divided into a few conical masses (medullary pyramids) projecting into the calyces.

The cortex extends in between the medullary pyramids as renal columns called columns of bertini.

$-$ Each kidney has nearly one million complex tubular structures called nephrons.