Excretion and the Kidney
HL (Paper 1 and 2)
Excretion
• What is excretion?
– Elimination of waste from the metabolic processes, to
maintain homeostasis
• What are the products we excrete?
–
–
–
–
Undigested material (egested – feces)
Metabolites
CO2
Urea
• Why do we eliminate waste?
– Stop toxins from circulating in the body
– Maintain homeostasis
The Kidney
• Pair of reddish organs,
smaller than your fist, just
under the last rib
• Blood flow
– Each has a renal vein and
renal artery, which branches
to a network of capillaries,
forming the pyramid
– To empty urine, each has a
ureter emptying into the
bladder
The Kidney
• Parts of the Kidney
– Cortex – outer portion
– Medulla – inner portion
– Pyramid – collection of
capillaries
– Pelvis – urine empties from
the collecting ducts, and
collects before emptying
into ureter to bladder
– Functional unit, called the
nephron, microscopic
structure
Functions of the Kidney
• The main functions of the kidney are:
– To maintain water balance in the body.
– To remove nitrogenous metabolic wastes and
salts. You call this ‘pee’, we call it Urea. Urine is
the whole thing together.
– To allow the body to reabsorb substances it needs.
Urea
• Made from the breakdown of
proteins, from the more toxic
ammonia (NH3)
• Broken down in the liver
• Transported from liver, in
blood to kidneys to be filtered
and eliminated
Nephron – functional unit of the
Kidney
• Principles of the functioning of the nephron:
– Ultra-filtration
– Reabsorption
– Secretion
Ultrafiltration
• Achieved by pressure built up in the
glomerular vessels, due to the efferent vessel
being more narrow exiting the Bowman’s
capsule than the afferent vessel entering the
Bowman’s capsule
• Fluid pushed into the Bowman’s capsule, is
called the filtrate, consisting of ions, glucose
and urea
Ultrafiltration
• Accomplished by:
– Wall of the glomerulus – fenestrations
– Basement membrane of the glomerulus
– Outer wall of the glomerulus - Podocytes
Glomerulus and Bowman’s Capsule
Osmoregulation and Reabsorption
• Osmoregulalation – the control of water
balance of the blood, tissue and cytoplasm of
a living organism.
• Achieved by the Proximal Convoluted Tubule
and Loop of Henle
– Using osmosis, diffusion and active transport
Osmoregulation and Reabsorption
• Since the blood leaving the Bowman’s capsule
is more concentrated (low water), as the
capillary comes close to the PCT:
– osmosis drives the reabsorption of water
– Sodium, glucose and other ions, are move by
active transport
– Once the salts move into, more water moves by
osmosis
• Large surface area is achieved by microvilli
and large amounts of mitochondria in
microvilli cells, for active transport
Osmoregulation and Reabsorption
• Filtrate leaving the PCT, is mostly water,
sodium, and urea, and goes into the Loop of
Henle
• As filtrate goes into the descending loop, it
goes into the medulla, which is concentrated
with sodium
• More water is absorbed, into the capillaries
along side the loop, along with some sodium,
which stays in the tissue of the medulla
Osmoregulation and Reabsorption
• Ascending Loop is impermeable to water, and
salts are removed by active transport
– The sodium remains in the medulla tissue, to maintain
a concentration gradient
• The concentration is maintained by the vasa
recta, vessels that run along the loop, exchanging
salts
• This process is called the vasa recta counter
current exchange
• Filtrate leaving has only water and urea,
metabolites and some sodium
Secretion
• Filtrate goes into the distal convoluted tubule
and collecting duct.
• DCT and CT are permeable to water, only if
ADH is present
• If water is needed in body (ie. Low blood
volume, High Osmotic Pressure)
– ADH is secreted and water goes from the filtrate
to the blood
– Concentrated Urine
Secretion
• If there is excess water (ie. High blood volume,
low Osmotic Pressure)
– no ADH will be secreted
– Dilute Urine
• If the urine is too concentrated, this can lead
to kidney stones