The kidney
Topic 11.3
Assessment Statements
11.3.1 Define excretion.
11.3.2 Draw and label a diagram of the kidney.
11.3.3 Annotate a diagram of a glomerulus and associated nephron to show the
function of each part.
11.3.4 Explain the process of ultrafiltration, including blood pressure, fenestrated blood
capillaries and basement membrane.
11.3.5 Define osmoregulation.
11.3.6 Explain the reabsorption of glucose, water and salts in the proximal convoluted
tubule, including the roles of microvilli, osmosis and active transport.
11.3.7 Explain the roles of the loop of Henle, medulla, collecting duct and ADH
(vasopressin) in maintaining the water balance of the blood.
11.3.8 Explain the differences in the concentration of proteins, glucose and urea
between blood plasma, glomerular filtrate and urine.
11.3.9 Explain the presence of glucose in the urine of untreated diabetic patients.
Excretion
• The removal from the body of the waste
products of metabolic pathways
– Example: Urea is a waste product from the
metabolism of amino acids
Anatomy of a kidney
Renal
Renal
Function of kidney
• Renal artery takes blood into kidney
• Blood drains away from each kidney by
the renal vein
• Urine is the fluid produced by kidneys and
consists of water and dissolved waste
products which have been removed from
the bloodstream
• Urine collects within each renal pelvis
• Renal pelvis drains urine into each ureter,
which then takes the urine to the urinary
bladder
Glomerulus and associated
nephron
• Each kidney has 1.25 million filtering units
known as nephrons
• Each nephron consists of:
– Capillary bed, called a glomerulus, which filters
various substances from the blood
– Capsule surrounding the glomerulus called
Bowman’s capsule
– A small tube (tubule) that extends from
Bowman’s capsule and has parts named
• Proximal convoluted tubule
• Loop of Henle
• Distal convoluted tubule
– Second capillary bed called the peritubular
capillary bed which surrounds the three-part
tubule mentioned above
Ultrafiltration
• Term used to describe the process by which various
substances are filtered through the glomerulus under the
unusually high blood pressure in this capillary bed
• Walls of the capillaries have very small slits called
fenestrations which open when blood pressure is increased
(due to smaller diameter of efferent arteriole)
• Fluid which is ultrafiltered then passes through the basement
membrane which helps prevent large molecules like proteins
from becoming a part of the filtrate
• The filtrate next enters the proximal convoluted turuble
• Blood cells, proteins, and other molecules which did not
become a part of the filtrate exit Bowman’s capsule by way
of the efferent tubule
Reabsorption
• Filtrate that leaves Bowman’s capsule contains many
substances that the body cannot afford to lose in the urine,
such as water, salt ions, and glucose
• Substances need to be reabsorbed into the bloodstream
• Most reabsorption takes place from the proximal convoluted
tubule
• Substances leave the tubule filtrate and are taken back into
the bloodstream by way of the peritubular capillary bed
Proximal convoluted tubule
• Wall is a single cell thick
• Composed of a ring of
cells
• Interior of tube is called
the lumen
• Filtrate flows through the
lumen
• Inner portion of the tubule
cells has microvilli which
increase the surface area
for reabsorption
Transport mechanisms of
reabsorption
• Salt ions: (Na+, Cl-, K+) actively transported into the tubule
cells and then into the intercellular fluid outside the tubule;
then taken into the peritubular capillary bed
• Water: water follows salt ions by osmosis
• Glucose: in a properly functioning nephron, all the glucose that
is in the glomerular filtrate is reabsorbed in the bloodstream by
active transport; otherwise, only 50% is reabsorbed
Osmoregulation
• Body’s response mechanisms which
attempt to maintain homeostatic levels of
water
• Total volume of water eliminated depends
on:
– Total volume of water ingested recently (as
liquid and in foods)
– Perspiration rate (influenced by exercise and
environmental temperature)
– Ventilation rate (loss of water through
exhalation)
Loop of Henle
• Much of the water in the original filtrate remains even after the
filtrate has left the proximal convoluted tubule
• Water and dissolved solutes enter the descending loop of
Henle which is permeable to water, but relatively impermeable
to salt ions
• The filtrate enters the ascending portion of the loop of Henle
where the tubule is relatively impermeable to water, but
permeable to salt ions
• Loop of Henle extends down into the medulla region and
creates a hypertonic environment in the medulla
• The filtrate that moves up the ascending loop and into the
distal convoluted tubule is relatively hypotonic
ADH and the collecting duct
• Collecting duct is differentially permeable to water
• Its permeability depends on the presence or absence of
antidiuretic hormone (ADH)
• ADH is secreted by posterior pituitary gland, circulates in
bloodstream, and targets the kidney collecting ducts
• If ADH is present, collecting duct becomes more permeable
water and water moves out of duct and into the medulla
intercellular fluid then into peritubular capillary bed
• If ADH is not present, collecting duct becomes impermeable to
water and water stays in the collecting duct and the urine is
more dilute
Concentrations of molecules
Molecule
Amount in blood
plasma in mg 100
ml-1
Amount in
glomerular filtrate
in mg 100 ml-1
Amount in urine
in mg 100 ml-1
Proteins
> 700
0
0
Glucose
> 90
> 90
0
Urea
30
30
> 1800
Logic behind each change
• Proteins are too large to fit through the
basement membrane within the
glomerulus. Thus proteins do not become
a part of the filtrate or urine.
• Glucose does become a part of the filtrate,
but active transport takes 100% of the
glucose back into the peritubular capillary
bed. In a healthy person, no glucose
appears in the urine.
• Urea is not toxic unless its concentration is
too high in the blood plasma. The very
high concentration of urea in urine is due
to the reabsorption of water.
Glucose in urine of untreated
diabetics
• Active transport mechanisms have a
maximum rate at which they can move
substances
• When the maximum threshold
concentration of glucose is exceeded, the
active transport cannot move all the
glucose back into the bloodstream
• Some glucose, therefore, remains in the
urine