Lesson Review
• The Role of the Kidney
• It plays a central role in homeostasis,
• Function: The primary role of the Kidneys is
osmoregulation
• Role 2: Excretion of all nitrogenous wastes
Fresh Water fish
• Fish (bony) living in freshwater are hypertonic
to their surroundings.
• Hypertonic – they maintain a higher
concentration of solutes in their body
• Water therefore tends to diffuse into their
bodies - need to continuously get rid of the
excess water.
• Their kidneys produce copious amounts of
very dilute urine
Salt Water Fish
• Their internal body fluids are less concentrated
(more dilute) than the surrounding water.
• They absorb the water and salts.
• The water is retained and the salts are actively
excreted, some via the gills and some via the
kidneys.
• Saltwater bony fish excrete very little urine.
• Marine cartilaginous fish (sharks and rays) have
their tissues isotonic with the seawater so that
there is no net movement of water in or out.
Mammalian Excretory System
• Main Function: the kidneys of mammals
regulate the internal water and salt
concentrations in the body and excrete urea,
the nitrogenous waste produced by
mammals.
Kidney Structure
Nephrons
• Each kidney is made up of about one million
small filtering units called nephrons.
• It is in these structures that urine is produced.
• Each nephron is a convoluted tubule
• The nephrons are surrounded by a dense
network of capillaries.
The Formation of Urine
• The kidneys continuously process a large
volume of blood to form a small volume of
urine.
• This involves three processes: filtration,
reabsorption and secretion.
Filtration
• Blood is brought to the kidney by the renal
artery. This divides into smaller vessels which
form a network of capillaries called a
glamorous outside the Bowman’s capsule.
• The pressure is very high in the glomerulus
and this causes some fluid to be forced out
through the walls of the blood vessels into the
Bowman’s capsule.
• This liquid consists of urea, glucose, amino
acids, some hormones, vitamins, salts and
water (no plasma proteins or blood cells).
• This liquid is known as glomerular filtrate.
• Filtration is a non- selective process.
• Reabsorption:
• Surrounding each nephron is a large capillary
network.
• As the filtrate travels down the capsule the
materials that the body can reuse are
reabsorbed into the blood.
• Reabsorption is an active process that
requires energy.
• Secretion:
• Secretion is a selective process where by the
body actively transports substances from the
blood into the nephron.
Loop of Henle
• Inthe descending part, the walls are permeable to
water but not to salt.
• Water passes across by osmosis.
• In the ascending part the walls are permeable to salt
not to water.
• Salt passes out passively across a thin walled section
and then actively across a thick walled section.
• The salt passing out makes the interstitial fluid of the
medulla area of the kidney quite concentrated. (This
hypertonic medulla helps remove water by osmosis
from the descending part and collecting duct).
• In the distal tubule – selective reabsorption and
secretion again occur to adjust pH of the blood
and level of salts particularly sodium and
potassium.
• The walls of the collecting ducts are permeable
to water but not to salt. Water passes out by
osmosis and the final filtrate or urine is formed.
(due to high salt concentration in the medulla)
• Diffusion and osmosis are passive forms of transport
that do not require the expenditure of energy.
• Diffusion and osmosis involve the movement of
substances with the concentration gradient – that is
from where there are many particles to where there
are few.
• Movement of substances against a concentration
gradient requires energy. This is called active transport.
• In the kidneys both forms of transport are use in
the regulation of the body fluid composition.
• Passive transport occurs in filtration and the
osmosis of water back into the blood.
• Active transport occurs in the secretion of
substances into the nephron, the active transport
of nutrients back into the blood and the selective
reabsorption of salts required by the body.
• Diffusion and osmosis are both examples of
passive transport, relying on the random
movement of molecules. Diffusion is too slow
for the normal functioning of the body and
does not select for useful solutes.
• Osmosis only deals with the movement of
water and thus would only allow water to
move out of the body not nitrogenous waste.
– Aldosterone is a steroid hormone secreted by the adrenal gland.
– Function: Its function is to regulate the transfer of sodium and
potassium ions in the kidney.
– When sodium levels are low, aldosterone is released into the
blood causing more sodium ions to pass from the nephron to
the blood.
– Water then flows from the nephron into the blood by osmosis.
This results in the homeostatic balance of blood pressure.
– If there is an increase in blood volume and pressure (resulting
from high salt concentrations, which causes water retention),
the out put of aldosterone is reduced. Less salt and water is
reabsorbed by the nephron tubules and increased amounts of
water and salts are lost in the urine.
•
– Antidiuretic hormone (ADH or vasopressin) controls water
reabsorption in the nephron. Made in the hypothalamus.
– When levels of fluid in the blood drop, the hypothalamus causes
the pituitary gland to release ADH.
– ADH increases the permeability of the collecting ducts and distal
tubules allowing more water to be reabsorbed from the urine
into the blood.
– The resulting urine is more concentrated.
– When there is too much fluid in the blood, sensors in the heart
cause the hypothalamus to reduce the production of ADH in the
pituitary, decreasing the amount of water reabsorbed in the
kidney.
– This results in a lower blood volume and larger quantities of
more dilute urine.
Blood water
concentration falls
Less water reabsorbed
from kidney tubules.
Large volume of dilute
urine.
Pituitary gland
releases more ADH
Pituitary gland
releases less ADH
More water
reabsorbed from
kidney tubules
Blood water
concentration rises