ANHB1102 Study Guide - Final Guide: Collecting Duct System, Descending Limb Of Loop Of Henle, Peritubular Capillaries
3 Jun 2018
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TOPIC SEVEN: The Urinary System
Function of the Urinary System:
The main function of the urinary system is to maintain the volume and
composition of body fluids within normal limits (osmoregulation), as well as
ridding the body of waste products of cellular metabolism. Through this, the
blood pH of the body is balanced. The urinary system also plays a role in
regulating blood pressure and red blood cell production.
Excretion of Nitrogenous Wastes:
There are three different types of nitrogenous wastes – ammonia, urea and uric
acid.
Ammonia is too toxic to be stored in the body as it does not diffuse readily into
the air. However, it is highly soluble in water, and for an animal surrounded by
water, ammonia readily diffuses out of its cells. Due to the toxicity of ammonia, it
must be transported and excreted in large volumes of very dilute solutions. This
form of excretion is found in most aquatic animals, including fish.
Urea is highly soluble in water and is 100 000 times less toxic than ammonia. It
can be stored in a concentrated solution however water is required for its
disposal. This form of excretion is found in mammals, amphibians, sharks and
some bony fish.
Uric acid is a more complex molecule than urea that is relatively nontoxic. It is
largely insoluble in water so excreting uric acid minimizes water loss. However,
due to the structure of the molecule, more energy is required for it to be
excreted, but the savings in body water balances this energy cost. This form of
excretion is found in birds, insects, many reptiles and land snails.
Structure of the Urinary System:
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The organs of the urinary system are said to be retroperitoneal, located behind
the peritoneum, against the posterior body wall.
The Kidneys
In all vertebrates, the basic
functional unit of the kidney is the
nephron. The nephron consists of a
folded tubule and associated blood
vessels that extract a filtrate from
the blood, and refine that filtrate into
a much smaller amount of urine.
The Ureters
Urine enters the ureters from the renal pelvis, resulting in the walls of the ureter
to be stretched. A peristaltic wave of contractions of the ureter muscle wall will
draw urine from the renal pelvis down to the bladder.
The Urinary Bladder
The bladder is a muscular sac located on
the floor of the pelvic cavity. It has a
capacity of 500mL, but can be stretched
to a maximum of 700-800mL. The
muscularis (muscle wall) is made up of
3 layers of smooth muscle. Long folds
called rugae are found to enable the
bladder to stretch. The mucosa is lined
with transitional epithelium and is
highly distensible. As the bladder fills
with urine, it expands superiorly, flattening the rugae and epithelium from 5-6
layers to only 2 or 3.
The Urethra
The urethra conveys urine out of the body. In females, the urethra is 3-4 cm long
while in males it is 18cm long. The urethra contains 2 sphincters – an internal
and external urethral sphincter. The internal urethral sphincter is a thickening of
the smooth muscle of the bladder and is under involuntary control. Its main
function is to compress the urethra and retain urine in the bladder. The external
urethral sphincter is made up of skeletal muscle of the pelvic floor and provides
voluntary control over the urethra.
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Blood Vessels of the Kidney
Each kidney is supplied by a
renal artery branching off the
aorta. Blood will leave the
kidney via the renal vein and
drains into the inferior vena
cava. Kidneys comprise <1% of
total body weight, however it
receives 20-25% of total
cardiac output (or 5L of blood).
1 100 – 2 000 L of blood passes
through the capillaries of the
kidneys per day, with the kidney extracting ~ 180 L of filtrate per day. However,
the kidneys refine the filtrate and concentrate the urine, returning most water
and solutes to the blood and will generally excrete 1.5L of urine per day.
Neural Control of Micturition:
Micturition is the act of urinating.
As the bladder fills stretch receptors in the bladder wall send signals to the sacral
spinal cord and the brain, where motor nerves will contract the muscle of the
bladder and move urine out of it. If it is timely to urinate, motor signals will be
sent to the muscle of the bladder to contract, resulting in the relaxation of the
internal urethral sphincter. The emptying of the bladder is under voluntary
control, controlled by motor signals from the brain.
The Nephron:
Each kidney has ~ 1.2 million
nephrons.
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Document Summary
The main function of the urinary system is to maintain the volume and composition of body fluids within normal limits (osmoregulation), as well as ridding the body of waste products of cellular metabolism. Through this, the blood ph of the body is balanced. The urinary system also plays a role in regulating blood pressure and red blood cell production. There are three different types of nitrogenous wastes ammonia, urea and uric acid. Ammonia is too toxic to be stored in the body as it does not diffuse readily into the air. However, it is highly soluble in water, and for an animal surrounded by water, ammonia readily diffuses out of its cells. Due to the toxicity of ammonia, it must be transported and excreted in large volumes of very dilute solutions. This form of excretion is found in most aquatic animals, including fish. Urea is highly soluble in water and is 100 000 times less toxic than ammonia.
