Describe the human excretory system.

(N/A) In humans,the excretory system consists of a pair of kidneys,a pair of ureters,a urinary bladder,and a urethra.
Kidney: It is a reddish-brown,bean-shaped structure.
$(i)$ Location: It is situated between the levels of the 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 $10-12 \ cm$ in length,$5-7 \ cm$ in width,and $2-3 \ cm$ in thickness.
(iii) Weight: The average weight is $120-170 \ g$.
Both kidneys are not situated on the same plane; the right kidney is slightly lower than the left because the liver is situated above it on the right side of the thoracic cavity.
The external surface of the kidney is convex,and the internal surface is concave.
Internal structure of the kidney: $A$ longitudinal section ($L$.$S$.) of the kidney shows a large opening at the concave surface called the hilum.
Through the hilum,the ureter,blood vessels,and nerves enter.
Inner to the hilum is a broad,funnel-shaped space called the renal pelvis.
Projections of the renal pelvis are called calyces.
The outer layer of the kidney is a tough capsule.
Inside the kidney,there are two zones: an outer cortex and an inner medulla.
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.