Transcript Life: The Science of Biology, 8e

David Sadava H. Craig Heller Gordon H. Orians
William K. Purves David M. Hillis
Biologia.blu
C – Il corpo umano
Excretory System and Salt
and Water Balance
Excretory System and Salt and Water Balance
• What roles do excretory organs play in
maintaining homeostasis?
• How does the human kidney produce
concentrated urine?
• How do humans maintain salt and water
balance?
• What mechanisms regulate kidney function?
• What is dialysis?
Excretory System and Salt and Water Balance - What roles do excretory organs play in
maintaining homeostasis?
Excretory organs control volume,
concentration, and composition of the
extracellular fluid.
Cell volume depends on water
movement to or from extracellular
fluid.
Water movement depends on
differences in solute concentrations.
Excretory System and Salt and Water Balance - How does the human kidney produce
concentrated urine?
Kidneys filter blood and produce urine.
Ureter: a duct from the kidney that
leads to the urinary bladder.
Urethra: a tube for urine excretion.
Excretory System and Salt and Water Balance - How does the human kidney produce
concentrated urine?
The human excretory system (part 1)
Excretory System and Salt and Water Balance - How does the human kidney produce
concentrated urine?
The ureter, renal artery, and renal
vein enter the kidney on the concave
side.
The ureter branches and envelops the
renal pyramids.
Renal pyramids make up the medulla,
or internal core.
The cortex is the outer layer of the
medulla.
Excretory System and Salt and Water Balance - How does the human kidney produce
concentrated urine?
The human excretory system (part 2)
Excretory System and Salt and Water Balance - How does the human kidney produce
concentrated urine?
Nephrons have a regular organization:
• glomeruli are located in the cortex;
• the proximal convoluted tubules—
the initial, twisted segments of the
renal tubules—are located in the
cortex.
Excretory System and Salt and Water Balance - How do humans maintain salt and
water balance?
Urine formation in vertebrate nephrons:
• filtration - blood is filtered in a
glomerulus, a ball of capillaries;
• tubular reabsorption - glomerular
filtrate flows into the renal tubule,
where it is modified by reabsorption of
specific ions, nutrients, and water;
• tubular secretion - glomerular filtrate
in the renal tubule is further modified.
Excretory System and Salt and Water Balance - How do humans maintain salt and
water balance?
Tubule cells transport substances to
be excreted into the tubular contents.
Peritubular capillaries work with the
glomerular capillaries, and transport
substances to and from the renal
tubules.
Excretory System and Salt and Water Balance - How do humans maintain salt and
water balance?
The vertebrate nephron
Excretory System and Salt and Water Balance - How do humans maintain salt and
water balance?
Blood is filtered in the glomerulus.
Blood enters through the afferent
arteriole and leaves through the
efferent arteriole.
The efferent arteriole becomes the
peritubular capillaries, which
surround the tubule and serve as
exchange sites.
Excretory System and Salt and Water Balance - How do humans maintain salt and
water balance?
The renal tubule begins with
Bowman’s capsule, which encloses
the glomerulus.
Podocytes are capsule cells that
contact the glomerular capillaries.
Podocytes have fine processes that
wrap around the capillaries.
Excretory System and Salt and Water Balance - How do humans maintain salt and
water balance?
The glomerulus produces a fluid that
lacks cells and large molecules.
The rate of filtration is high in the
glomerulus due to:
• high capillary blood pressure;
• high permeability of glomerular
capillaries and their podocytes.
Excretory System and Salt and Water Balance - How do humans maintain salt and
water balance?
Fluid passing down the renal tubule
changes composition through
reabsorption and secretion.
Tubule cells:
• reabsorb molecules from the tubule
fluid;
• secrete into the fluid substances
delivered by the peritubular
capillaries.
Excretory System and Salt and Water Balance - How do humans maintain salt and
water balance?
The renal tubule descends into the
medulla and forms the loop of Henle
which is important for urine
concentration.
After forming the loop, the tubule
returns to the cortex.
The ascending limb of the loop of
Henle becomes the distal
convoluted tubule.
Excretory System and Salt and Water Balance - How do humans maintain salt and
water balance?
The distal convoluted tubules join the
collecting duct in the cortex.
Collecting ducts descend through renal
pyramids and empty into the pelvis.
Pelvic divisions join and leave the
kidney as the ureter.
Excretory System and Salt and Water Balance - How do humans maintain salt and
water balance?
Blood vessel organization in the kidney
parallels that of the nephrons.
Afferent arterioles carry blood to the
glomeruli.
Glomeruli are drained by arterioles that
become the peritubular capillaries.
Excretory System and Salt and Water Balance - How do humans maintain salt and
water balance?
The vasa recta is a network of
peritubular capillaries parallel to the
loops of Henle and the collecting
duct.
All peritubular capillaries from a
nephron join into venules that lead to
the renal vein.
Excretory System and Salt and Water Balance - How do humans maintain salt and
water balance?
The proximal convoluted tubule (PCT)
is responsible for most reabsorption
of water and solutes.
• PCT cells have microvilli to increase
surface area, and have mitochondria.
• They actively transport Na+, glucose,
and amino acids.
• Water follows the transport of
solutes.
Excretory System and Salt and Water Balance - How do humans maintain salt and
water balance?
Concentration of urine is due to a
countercurrent multiplier mechanism in
the loops of Henle.
Tubule fluid flows in opposite directions in
the ascending and descending limbs.
The loops increase osmolarity of interstitial
fluid in a graduated way.
Excretory System and Salt and Water Balance - How do humans maintain salt and
water balance?
Concentrating the urine
Excretory System and Salt and Water Balance - How do humans maintain salt and
water balance?
Loop of Henle segments:
• thick ascending limb actively transports Na+
(Cl– follows) and raises their concentration in
the interstitial fluid;
• thin descending limb loses water to the
neighboring interstitial fluid with high Na+ and
Cl– concentration;
• thin ascending limb receives concentrated
fluid from descending limb and allows diffusion
of Na+ and Cl– into the interstitial fluid.
Excretory System and Salt and Water Balance - How do humans maintain salt and
water balance?
Kidneys also regulate blood pH.
HCO3– ions are the major buffers in
blood, formed from hydration of CO2,
followed by dissociation of carbonic
acid (H2CO3).
Excretory System and Salt and Water Balance - How do humans maintain salt and
water balance?
The buffer system can be controlled by
physiology.
Lungs control level of CO2 in blood;
called the acid portion of the reaction
as more CO2 = more H+.
Kidneys control the base portion by
removing H+ and adding HCO3–.
Excretory System and Salt and Water Balance - How do humans maintain salt and
water balance?
pH regulation
Excretory System and Salt and Water Balance - What mechanisms regulate kidney
function?
A constant glomerular filtration rate
(GFR) needs blood supplied to the
kidneys under adequate pressure.
Autoregulatory mechanisms ensure
blood supply and blood pressure:
• dilation of afferent renal arterioles—
maintains glomerular blood pressure;
• kidney releases renin if GFR still
falls, this activates angiotensin.
Excretory System and Salt and Water Balance - What mechanisms regulate kidney
function?
Angiotensin: constricts efferent renal
arterioles and peripheral blood
vessels to raise blood pressure.
Stimulates release of aldosterone to
increase Na+ uptake, and stimulates
thirst to increase water ingestion to
raise blood volume and pressure.
Excretory System and Salt and Water Balance - What mechanisms regulate kidney
function?
The hypothalamus can stimulate
release of antidiuretic hormone
(ADH), also called vasopressin.
ADH increases the permeability of
membranes to water.
Osmoreceptors that detect a rise in
blood osmolarity will stimulate ADH
release.
Excretory System and Salt and Water Balance - What mechanisms regulate kidney
function?
Antidiuretic hormone increases blood pressure and
promotes water reabsorption
Excretory System and Salt and Water Balance - What is dialysis?
Renal failure results in retention of:
• salt and water (high blood pressure);
• urea (uremic poisoning);
• metabolic acids (acidosis).
Dialysis treatment passes blood
through membrane channels bathed
in a plasma-like solution to remove
wastes.
Excretory System and Salt and Water Balance - What is dialysis?
Artificial kidneys and dialysis