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					Principles of Biology
           By
 Frank H. Osborne, Ph. D.


       Excretion
              Excretion
Excretion means eliminating or getting rid of
wastes. The body produces a variety of
different wastes and has a variety of means of
excretion. These are summarized in the
table.
Organs that perform excretion
 Organ               Product
Skin               Water, Salt
Liver              Bile
Lungs              CO2, Water
Colon              Undigested food waste
Kidneys            Urine*
          Excretion
*Urine contains the waste products
creatinine, urea and uric acid.
Creatinine and urea come from
protein metabolism while uric acid
comes from nucleic acid
metabolism
 The Human Urinary System
•The human urinary system consists of two
kidneys. They are located in the rear of the
abdominal cavity up against the inner wall of
the rear of the cavity.
 The Human Urinary System
•Each kidney has a blood supply. The left
renal artery supplies the left kidney (renal is
an adjective referring to the kidney) and the
left renal vein returns blood from the left
kidney to the inferior vena cava. Similarly
for the right kidney.
Kidney
 The Human Urinary System
Regions of the Kidney
•The kidney has two portions. The outer
portion is known as the cortex (cortex means
outer portion) and the inner portion is called
the medulla (medulla means inner portion).
 The Human Urinary System
Regions of the Kidney
•On the inner curve of the kidney is a space
called the renal pelvis. Urine collects in the
renal pelvis and exits the kidney via the
ureter. The ureter from each kidney drains
into the urinary bladder located in the pelvis.
The bladder drains via the urethra.
                The Nephron
•The nephron is the functional unit of the kidney.
There are about 1 million nephrons in each
kidney. The nephron has a long collecting tube
that terminates in a hollow, cup-like structure
called Bowman's capsule. Inside Bowman's
capsule is a network of very fine arteries called
the glomerulus.
            The Nephron
•Filtration of the blood occurs in the
glomerulus. The key is that the diameter of
the arteriole entering the glomerulus is larger
than the diameter of the arteriole leaving the
glomerulus. An arteriole is a small artery.
            The Nephron
•This difference in diameter causes a pressure
in the glomerulus that forces water, salt,
nitrogenous wastes (creatinine, urea, uric
acid), sugars, amino acids, ions and other
materials out of the bloodstream.
            The Nephron
•After this material enters Bowman's capsule,
it travels through the collecting tubule. The
collecting tubule is in intimate contact with
the capillaries of the bloodstream.
            The Nephron
•The materials that the body wants to keep
such as glucose, amino acids, and other
beneficial molecules are reabsorbed by the
collecting tubule cells and sent back into the
bloodstream.
           The Nephron
•The waste materials, some water and excess
salts remain in the collecting tubule and
travel via the ureter to the bladder.
            The Nephron
•Sometimes materials are not excreted
properly. Gout is a disease
characterized by the retention of
uric acid in the blood. The kidney
does not excrete it properly. So the
body stores it in the connective tissue, often
behind the big toe. This makes it very
difficult and painful to walk if not treated.
   The End


Principles of Biology
     Excretion

				
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posted:8/21/2011
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