David Sadava H. Craig Heller Gordon H. Orians William K. Purves David M. Hillis

Biologia.bluC – Il corpo umano

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

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 - What roles do excretory organs play in maintaining homeostasis?

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?

Excretory System and Salt and Water Balance - How does the human kidney produce concentrated urine?

The human excretory system (part 1)

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?

Excretory System and Salt and Water Balance - How does the human kidney produce concentrated urine?

The human excretory system (part 2)

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 does the human kidney produce concentrated urine?

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?

Excretory System and Salt and Water Balance - How do humans maintain salt and water balance?

The vertebrate nephron

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?

Excretory System and Salt and Water Balance - How do humans maintain salt and water balance?

Concentrating the urine

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?

Excretory System and Salt and Water Balance - How do humans maintain salt and water balance?

pH regulation

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?

Excretory System and Salt and Water Balance - What mechanisms regulate kidney function?

Antidiuretic hormone increases blood pressure and promotes water reabsorption

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?

Excretory System and Salt and Water Balance - What is dialysis?

Artificial kidneys and dialysis