Download The Kidney

The Kidney
• The kidneys are part of the urinary system
and their job is to:
– filter and cleanse the bloodstream of
molecules like urea (waste product of used
amino acids) and other molecular wastes
– osmoregulation (maintain the water to salt
ratio within the body)
• Excretion is the removal from the body of
waste products of metabolic pathways
• Renal artery – takes blood to the kidneys
• Renal vein – drains blood from the kidneys
• Urine – fluid produced by the kidneys; it
accumulates in the area called the renal pelvis
• Renal pelvis – collects the urine and drains it
into a tube called the ureter, which then takes
the urine to the bladder
• Renal cortex – the outer part of the kidney
• Renal medulla - the inside part of the kidney
*Note: the cortex and medulla are packed with tubules and blood
vessels for filtration and each kidney contains ~80km of tubules
Nephrons
• The filtering units within the kidneys (each
kidney is made up of ~1.25 million of them)
• Each nephron consists of:
– A capillary bed called the glomerulus, which
filters substances from the blood
– A capsule surrounding the glomerulus called
Bowman’s capsule
– A small tube extending from Bowman’s capsule
with parts named in order: proximal convoluted
tubule, loop of Henle, distal convoluted tubule
– A secondary capillary bed called the peritubular
capillary bed which surrounds the tubules
mentioned above
Ultrafiltration
• Blood moving from the heart to the kidneys
splits off into capillaries inside the
glomerulus; these capillaries are fenestrated
and open when blood is flowing at high
pressure
• Ultrafiltration is the process by which various
substances are filtered out of the bloodstream
through the glomerulus due to the unusually
high blood pressure
• The fluids not filtered out by the glomerulus
including blood cells and proteins exit
Bowman’s capsule in the bloodstream
• The filtered fluid (filtrate) then passes through
the basement membrane inside the glomerulus,
then through the tubules so that the process of
reabsorption can begin
Reabsorption of Molecules
• The filtrate that leaves Bowman’s capsule
contains substances that the body needs and
cannot afford to lose in urine (H2O, salt ions,
glucose)
• Most reabsorption within the nephron occurs in
the proximal convoluted tubule; the reabsorbed
nutrients re-enter the bloodstream by way of the
surrounding peritubular capillary bed
• The inner lining of the proximal convoluted
tubule contains microvilli in order to increase
surface area for reabsoption; also cells of the
tubule contain mitochondria to aid in active
transport
Methods of Reabsorption
• Glucose – active transport moves all the
glucose from the filtrate back into the
bloodstream (in normal functioning kidney)
• Salt ions – the majority of salt ions (e.g., Na+, Cl-,
K+) must be reabsorbed; ions move via active
transport from tubules into capillaries
• Water – the movement of the salt ions causes
the movement of water molecules to follow the
same route via Osmosis (remember, water needs
to move into high concentration solutions to help bring
it back to equilibrium)
Osmoregulation
• It is the body’s response mechanisms which
attempt to maintain homeostatic levels of water
• These mechanisms occur in the kidneys and
counteract things like:
– Volume of H2O ingested through liquids and food
– Perspiration rate (exercise and temp dependent)
– Ventilation rate (exercise dependent)
– Amount of salt ingested via foods
How the following parts help to maintain the
water balance in blood:
• Loop of Henle – in descending loop water
moves back into blood via osmosis and in
ascending loop salt ions pumped out via active
transport
*Note: this loss of salt ions into the medulla aids in
keeping the concentration gradient necessary for
osmosis
• Medulla – the loop of Henle extends into the
medulla region of the kidney; medulla is filled
with ions (hypertonic compared to tubules) and
this high level of concentration is what allows
H2O molecules to continually diffuse from
tubules back into blood
• Collecting duct – the wall of the collecting duct
is permeable therefore H2O can pass from
filtrate into blood; presence or absence of ADH
(anti-diuretic hormone) determines the
permeability of the walls
• ADH – released from the pituitary gland when
the concentration of solutes in the blood is too
high; presence of ADH causes walls of
collecting duct to be permeable to H2O (water
moves into medulla tissue and then into peritubular
capillaries to help make blood less concentrated); if
no ADH is present then walls are impermeable
and water stays in tubules and helps dilute
urine
How do the kidneys change the blood?
Molecule Blood Plasma
(from renal
artery)
Glomerular
Filtrate (fluid
Urine
entering tubules
after
ultrafiltration)
Proteins
> 700
NONE
NONE
Glucose
> 90
>90
NONE
Urea
30
30
> 1800
*Note: all # are in mg/100 mL; need to understand the relationships, not the #
Diabetic’s Urine
• In healthy people, all the glucose that they
eat is reabsorbed back into the bloodstream
from the filtrate via active transport
• Active transport mechanisms have a
maximum rate at which they can move
substances and when the maximum is
exceeded (like an undiagnosed diabetic who
eats a carb-filled meal) then active transport
cannot keep up and there will be glucose in
the urine
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