EXCRETION
HANDOUT
EXCRETION is the removal of organisms of toxic waste materials and substances in excess of
requirements which include
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The waste products of its chemical reactions
The excess water and salts taken in the diet.
Spent hormones.
EXCRETORY ORGANS:
LIVER
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Breaks down excess amino acids to urea
Breakdown product of haemoglobin is bilirubin which is yellow/green pigment
Bilirubin is excreted with the bile into Small intestine and expelled with the faeces
LUNGS
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Lungs supply the body with oxygen and get rid of CO2 hence it is also an excretory organ.
They also lose a great deal of water vapour.
KIDNEYS
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Kidneys remove urea and other nitrogenous waste from the blood.
They expel excess of water, salts and hormones.
SKIN
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Sweat contains lots of water, with NaCl and traces of urea dissolved in it.
When you sweat, we expel these substances from our body. So, in one sense, they are being
excreted.
THE KIDNEYS
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Two kidneys are oval structures.
They are red-brown enclosed in transparent membrane. Attached to back of the abdominal
cavity
RENAL ARTERY branches off from aorta brings oxygenated blood to them
RENAL VEIN takes deoxygenated blood away from kidney to vena cava
Tube called URETER, runs from each kidney to bladder in the lower part of the abdomen
NEED FOR EXCRETION
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Compounds made in the reactions in the body are toxic if their concentrations build up
CO2 dissolves in the tissue fluids and blood plasma to form carbonic acid.
H2CO3 increase the acidity can affect the enzyme action and can be fatal
Ammonia is made in the liver and excess amino acids are broken down
However, ammonia is alkaline and toxic hence converted to urea which is less poisonous.
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MICROSCOPIC STRUCTURE OF TUBULES
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The kidney tissue consists of many capillaries and tubes called RENAL TUBULES held together
with connective tissue
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If the kidney is cut down its length, it is seen to have outer dark region called CORTEX, lighter
inner zone, the MEDULLA.
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There is a space where the ureter joins the kidney called PELVIS
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Renal artery divides into arteriole and capillaries, mostly in cortex.
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Each arteriole leads to a GLOMERULUS
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It is a capillary repeatedly divided and coiled, making knot of vessels
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Each glomerulus is surrounded by cup shaped organ called BOWMAN’S CAPSULE/ RENAL
CAPSULE.
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B. Capsule leads to RENAL TUBULE.
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This tubule after series of coils and loops, joins a COLLECTING DUCT which passes through the
medulla to open into pelvis
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A NEPHRON is a single glomerulus with its renal capsule, renal tubule and blood capillaries.
FUNCTION OF KIDNEY/ URINE FORMATION
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The blood pressure in glomerulus causes part of the blood plasma to leak through the capillary
walls.
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RBC and plasma proteins are too big to pass out of the capillary.
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Fluid that does filter through is plasma without protein. i.e fluid contains mainly water with
dissolved salts, glucose, urea and uric acid.
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Process in which fluid is filtered out of the blood by the glomerulus is called
ULTRAFILTRATION
SELECTIVE REABSORPTION
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Filtrate from glomerulus collects in renal capsule trickles down to renal tubule
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As it does, capillaries that surround the tubule will absorb back glucose with much of water.
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Then some salts are taken back to keep the correct concentration in the blood.
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Process of absorbing the substances back needed by the body is called SELECTIVE
ABSORPTION
 Salts that are not needed by the body are left to pass on down the kidney tubule along with urea
and uric acid.
 These nitrogenous waste products, excess salts, water move down to renal tube into the pelvis
of the kidney.
 From here, fluid called URINE passes down the ureter to the bladder.