Urinary System

Urinary System

Gross Anatomy

Consist of six organs

–Kidneys (2)

–Ureters (2)

–Urinary bladder

–Urethra

•Right slightly lower than left due to space occupied by liver

Renal parenchyma is divided into two zones

–Renal cortex- about 1cm thick

–Inner medulla

Micro Anatomy

•Each kidney contains 1.2 million functional units called Nephrons.

•A nephron consist of two principal parts

–Renal corpuscle- (glomerulus) where the blood plasma is filtered

–Renal tubule- processes the filtrate into urine

•Consist of a ball of capillaries called a glomerulus enclosed in a two-layered glomerular (Bowman's) capsule

Structure of the Bowman's capsule

In the glomerulus the filtrate of plasma has to pass through three layers:

-          The fenestrated epithelium of the capillary- the filtering membrane

-          The basement membrane of the bowman's capsule- It contains connective tissue and Mesangial cells which are both phagocytic and contractile.

-          The epithelial cells of the Bowman's capsule called podocytes which are wrapped around the capillaries.

• Renal Tubule
  

•Is a duct that leads away form the glomerular capsule and ends at the tip of a medullary pyramid

•Divided into four major regions

–Proximal convoluted tubule

–loop of Henle

–Distal convoluted tubule

–Collecting duct -(not really a part of the nephron- receives urine from many Nephrons

• Proximal Convoluted Tubule

–Longest and most coiled

–Simple cuboidal epithelium with microvilli (brush border for absorption capacity) and numerous mitochondria

• Loop of Henle

•Descending limb- first portion of the loop, passes from the cortex into the medulla

•At its deep end it turns 180^o and forms an ascending limb that returns to the cortex. 

•It is divided into thick and thin segments

–Thin segments- has a simple squamous epithelium

•Cells have low metabolic rate but are very permeable to water

•Forms the lower part of the descending limb, the bend, and partway up the ascending limb

–Thick segments- have a simple cuboidal epithelium with lots of mitochondria in the cells due to the high metabolic activity of active transport.

•Form initial parts of the descending limb and part or all of the ascending limb

• Distal Convoluted Tubule

•Shorter and less coiled than the PCT.

•Has a cuboidal epithelium with smooth-surface cells nearly devoid of microvilli

• Collecting Duct

•The DCTs of several Nephrons drain into a this straight tube, which passes into the medulla

•Are both lined with simple cuboidal epithelium

There are two populations of Nephrons:

•Cortical Nephrons- Nephrons close to the kidney surface

–Have shorter nephron loops that dip only slightly into the outer medulla before turning back

•Juxtamedullary nephron- Nephrons close to the medulla

–Have very long loops that extend to the apex of the renal pyramid

–Responsible for maintaining the salinity gradient

Blood Supply

•Receives 21% of the cardiac output

•Each kidney is supplied by a renal artery (sometimes 2 or more)

•The interlobular artery branches off afferent arterioles to the glomerulus of each nephron

•The glomerulus is drained by an efferent arteriole

Nerve Supply

•Renal nerves arise from the superior mesenteric ganglion and enter the kidney at the hilum.

•They follow the branches of the renal artery to innervate each nephron

•Consist mostly of sympathetic fibers that regulate blood flow into and out of each nephron thus controlling the filtration rate and urine formation

Functions of the kidney

-The main function is to regulate the volume and composition of the ECF.

-Excrete metabolic wastes e.g. Urea

-Excrete foreign substances and their derivatives .g. Drugs and food additives

-synthesize Prostaglandins and kinins which act within the kidney

-Function as an endocrine organ; produces EPO, renin, Calcitrol (The active form of Vit D)