Urinary System
Two major functions:
1. Elimination
o Removal of harmful wastes.
2. Regulation
o Control and balance of several substances in the blood including
water, acids and bases (pH), ions and salts
The major organs of the system include:
- 2 kidneys: filters blood, producing urine
- 2 ureters: carry urine to urinary bladder
- 1 urinary bladder: stores urine
- 1 urethra: carries urine to the outside of the body
Associated blood vessels:
- renal
artery: carries
contaminated blood into the kidneys
- renal
vein: carries
purified blood from the kidney, placing the blood back into circulation
Kidneys
- main excretory
organs of the body
- found
on the back wall of the abdomen on either side of the spine just below the
ribs
- basic
filtering unit inside the kidney is the nephron
unit
- each
kidney contains 1 – 1.25 million nephron units
- parts
of the nephron unit are found in both the cortex and medulla of the kidney
Three major sections of the kidney:
1. Cortex
o Outer part of the kidney
o Contains the glomerulus, Bowman’s capsule and blood
supply of
the nephron unit
2. Medulla
o Middle of the kidney
o Contains the loop
of Henle and collecting duct of the nephron unit
3. Pelvis
o Inner cavity of the kidney
o Area where the collecting ducts of all the nephron units terminate
The nephron unit carries
out its filtering of the blood in two stages:
1. Filtration
§ Blood leaves the renal artery and enters the glomerulus
§ Blood pressure forces the plasma of the blood, containing both
waste material and useful material, into the Bowman’s
capsule
§ This material is called filtrate
§ Filtrate contains such things as:
|
§ water
|
§ glucose
|
§ sodium
|
|
§ urea
|
§ amino
acids
|
§ potassium
|
|
§ salt
|
§ vitamins
|
§ uric
acid
|
2. Reabsorption
§ Useful materials contained within the filtrate are reabsorbed by
the capillary network that surrounds the renal tubule
§ Reabsorption of water occurs by osmosis
§ Reabsorption of glucose, sodium, amino acids, etc occurs by active transport
§ Fluid that remains in the renal tubule after reabsorbtion is urine
§ Path of urine: collecting duct Ü pelvis Ü ureter Ü urinary
bladder Ü urethra
o The kidney maintains homeostasis by regulating the following
mechanisms:
2.
Water
balance
§ Hypothalamus of the brain directs the pituitary gland of the
endocrine system to control levels of the hormone vasopressin or andtidiuretic hormone (ADH) in the blood
§ This hormone travels through the blood to the kidneys where it
directs the rate of water reabsorption
§ Increased vasopressin Ü increased
water reabsorption
§ Decreased vasopressin Ü decreased
water reabsorption
3.
Blood
pressure
§ Blood volume is regulated by the hormone aldosterone
§ Aldosterone affects the rate of sodium ion reabsorption which in
turn affects the rate of water reabsorption
§ Increased aldosterone Ü increased
water reabsorption Ü higher
blood pressure
§ Decreased aldosterone Ü decreased
water reabsorption Ü lower
blood pressure
4.
Blood
composition
§ Useful materials contained in the filtrate (glucose, amino acids,
etc) are reabsorbed until the threshold
level of
the given material is reached
§ Defined as the maximum amount of material that can be moved across
the nephron back into the blood
§ When the threshold is reached, the remaining material is removed
as waste e.g. glucose in diabetes
5.
Blood
pH
§ Should be close to 7
§ The kidney removes excessively acidic or basic substances from the
blood
6.
Waste
removal
§ Urea is
the major waste product removed from the kidneys
§ Placed in the blood by the liver through the detoxification
process
§ Excess amino acids are broken down in the liver
§ Converted to ammonia and then to urea
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