Human Physiology

Lesson 23 : Urinary System

Structure of Kidneys and Urinary Tract

Kidneys are bean shaped solid organs, about 4 inches long and one inch in thickness lying in the abdominal cavity. Each kidney is a bean shaped organ covered with a fibrous capsule. Blood vessels and ureter enters or leave the kidney through its hilus portion situated on the medial side. In horizontal section, the kidney is divided into outer cortex area which is reddish in colour and highly vascularized and inner medulla that is pale in colour and less vascularized. Each kidney contains millions of microscopic Nephrons (renal tubules) which are regarded as functional unit of kidney. Nephrons and blood vessels of kidney are arranged in systematic manner due to which the organ is able is to perform its excretory function efficiently.

Nephron structure: Each nephron is a very minute tube like structure originating as Bowman's capsule in the cortex area. It is cup like in shape and its bottom continues as proximal convoluted tubule (PCT). PCT is continued as loop of Henle (LH) as a thin tubule embedded in medulla part of kidney. From the deepest portion it takes U turn and run upwards toward cortex forming descending (DLH) and ascending (ALH) loop of Henle, respectively. Inside renal cortex, ALH continue as distal convoluted tubule (DCT) and then as collecting tubule (CT). CT again enters medulla and open into pelvis, a common space for opening of all nephrons, which later continue as ureter and leave the kidney.

A tuft of blood capillaries called 'Glomerulus' is very closely associated with Bowman's capsule of nephrons and it is the site for filtration of blood plasma, the first step in formation of urine.

Urinary Tract: Ureters are small muscular tubes draining urine from each of kidney to the urinary bladder. Urinary bladder is pear shaped organ situated in the pelvic cavity for the storage of urine. Bladder opens into urethra for final expulsion of urine. The opening into urethra is guarded by two sphincters known as internal and external urethral sphincter. The former is involuntary in action and later one is under voluntary control. Expulsion of urine through urethra occurs following relaxation of both sphincters.