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Urinary System

The purpose of the urinary system is to rid the body of waste products, and excess nutrients. 

The urinary system consists of the kidneys, ureters, urinary bladder (usually called "the bladder") and urethra.  Males have longer urethras than females do.

It serves as a flushing system for the body tissues through the vehicle of the blood.  In addition to excreting waste, the kidneys also serve regulating functions within the body to help maintain homeostasis, including:
  1. maintaining blood volume (and blood pressure), and maintaining tissue fluid volume
  2. regulating electrolyte concentration of blood, and of tissue,
  3. regulating the acid-base balance of blood, and of tissue,
  4. kidney endocrine function:  secretion of erythropoietin, a hormone that stimulates increased production of red blood cells by the bone marrow,
  5. Vitamin D activation.  Vitamin D (the sunshine- ultraviolet light vitamin) is absorbed by the skin.  It is activated in the kidney.  Vitamin D controls intestinal absorption of calcium and phosphorus.
Kidneys:  usually there are two, a right and a left; although some people are born with one, shaped like and therefore called a horseshoe kidney. The kidney is located posterior to the peritoneum and is therefore said to be retroperitoneal.  Most kidneys are about 4" in length.  They are shaped like large lima beans, with their indentation, called a hilus, located medially.  The superior portion of the kidneys are protected by ribs 10 though 12.  The inferior portion of the kidneys are protected by skin and muscle only.  It is possible for a kidney to be knocked out of position by a hard and specifically placed impact.  People who have floating kidneys suffer complications.  It is possible to live with only one well functioning kidney, however, people who have only one well functioning kidney do have restrictions on their activities.

In gross anatomy, the kidneys have an outer cortex, an inner medulla, and a pelvis which is the medial portion with the large blood vessels.

Adipose (aka Fatty) Tissue:  the kidneys are surrounded with an external pad of adipose tissue.

Renal Cortex:  outer (lateral) area of kidneys.  Place to find nephrons.

Renal Medulla:  inner area of kidneys.  Place to find collecting tubules and renal pyramids.

Renal Pelvis:  medial area of kidneys.  Place to find entrance of blood vessels and ureter.

Hilus:  medial indentation of kidney.  Where renal artery enters, renal vein and ureter emerge.

Kidneys:  Understanding the microanatomy anatomy of the kidneys is essential to understanding how the kidneys work and the destruction that can be done to them by disease.

Nephron:  Functional Unit of the Kidney.  Consists of:  afferent arteriole (which takes blood into the nephron), efferent arteriole (which brings blood out of the nephron), peritubular capillaries, renal corpuscle (glomerulus plus Bowman's capsule), renal tubule (proximal convoluted tubule, loop of Henle, distal convoluted tubule).

Glomerulus:  The minute convoluted capillary network in between the afferent arteriole and the efferent arteriole.  It is contained within the Bowman's capsule

Bowman's Capsule (aka Glomerular Capsule):  an expanded end of the renal tubule.  It contains the glomerulus.  The inner layer is very porous to extract and absorb plasma from the glomerulus.  The outer layer is impermeable to contain the plasma that has been extracted.

Renal Corpuscle:  Glomerulus plus Bowman's Capsule.  The purpose of the renal corpuscle is to extract the renal filtrate (it is not yet to be called urine) from blood.

Renal Tubule:  proximal convoluted tubule, loop of Henle, distal convoluted tubule, collecting tubule.

Collecting tubule:  The proximal convoluted tubules of nephrons communicate urine into the collecting tubule.  One collecting tubule collects urine from several nephron.  Several collecting tubules unite to form papillary ducts.

Pyramid:  Renal pyramids are striations that appear to create sections out of the renal medulla.  There are between 10 and 15 of them in each kidney.  The renal pyramids contain the loops of Henle and the collecting tubules.

Papillary ducts:  Located at the most medial portion of the renal pyramids (at the intersection of the renal medulla and renal pelvis).  Papillary ducts communicate urine from collecting tubules into renal calyces.

Calyx:  Located within the renal pelvis.  Renal calyces receive urine from the papillary ducts and communicate it into the ureter.

Cellular Waste:  Nitrogenous waste:  urea, creatinine, ammonia.

Blood Pathway:  abdominal aorta, renal artery, renal arterioles, afferent arteriole, glomerulus, efferent arteriole, peritubular capillaries, renal venule, renal vein, inferior vena cava.

Afferent arteriole:  The arteriole that brings blood into the glomerulus.  It has a larger lumen than the efferent arteriole.

Efferent arteriole:  The arteriole that carries the concentrated blood (more cells, less plasma) away from the glomerulus and to the peritubular capillaries.  It has a smaller lumen than the afferent arteriole.

Peritubular Capillaries:  The network of capillaries surrounding the proximal convoluted tubule, the loop of Henle and the distal convoluted tubule.  The function of the peritubular capillaries is to reabsorb some of the nutrients and plasma that were extracted in the Bowman's capsule.

Smooth muscle cell:  the cells of the kidney.

Ureters:  (2 of them, right and left) tubes that communicate urine from the kidneys to the urinary bladder.  Its walls contain smooth muscle.

Urinary bladder (one of them):  A collecting and holding sac for urine within the body.  Its walls contain smooth muscle called the detrussor muscle.  It contracts to eliminate urine.

Internal Urinary sphincter:  (one of them) an involuntary muscle at the junction of the urinary bladder and the urethra.

Urethra (one of them):  communicates urine from the urinary bladder to the urethral orifice and out of the body.

External Urinary sphincter:  voluntary urinary sphincter located within the urethra.

Urine pathway:  starting inside kidney:  Bowman's capsule, renal tubule (proximal convoluted tubule, loop of Henle, distal convoluted tubule, collecting tubule), renal pyramid, papillary duct, renal calyces, ureter, urinary bladder, internal sphincter, urethra, external sphincter, urethral orifice.

Filtration of Urine out of the blood is a 3 step process:
  1. extraction of plasma from the glomerulus in the Bowman's capsule (aka glomerular filtration)
  2. absorption of some plasma and nutrients from the renal filtrate by the peritubular capillaries from the proximal and distal convoluted tubules and the loop of Henle (aka tubular reabsorption).
  3. tubular secretion also occurs between the peritubular capillaries and the renal tubule.
    In tubular secretion, the peritubular capillaries secrete:  ammonia, creatinine, hydrogen ions, and the metabolic products of medications into the peritubular capillaries, to join the renal filtrate.
Glomerular Filtration Rate (aka GFR):  The amount of renal filtrate formed by both kidneys in one minute.  If the blood flow increases, GRF also increases.  If the blood flow decreases, GFR decreases.  The average GFR is 100 to 125 ml per minute.

Urinary Output:  within 24 hours, the normal urinary output is 1 to 2 liters.  This can vary considerably dependent upon not only many internal controls but also how much fluid is consumed in those 24 hours.

Urination (aka micturition, aka voiding):  the process of expelling urine.

Normal Constituents:  Urine should contain water, salt, nitrogenous wastes (urea, creatinine, ammonia, uric acid).

Abnormal Constituents:  Urine does not normally contain:  sugar (glucose), protein, red blood cells, occult blood, ketones, bacteria.

Normal Values for Urine
  • Water:  1 to 2 liters daily.
  • Color:  straw to amber.  Bright yellow can be due to high intake of Vitamin B (riboflavin?)
  • Specific Gravity:  1.010 to 1.025 (distilled water is 1.000).  Lower specific gravity would be found in Diabetes Insipidus, higher specific gravity would be found in chronic renal failure.
  • pH:  can vary between 4.6 and 8.0.  Normal values usually being 5.5 to 7.4.  Higher pH (more alkaline urine) is associated with a vegetarian diet.  Lower pH (more acid urine) is associated with a high protein diet.
For information about Urinary Tract Infections and Kidney Stones, click here.