2. La nuova biologia.blu Anatomia e fisiologia dei viventi S
David Sadava, David M. Hillis,
H. Craig Heller, May R. Berenbaum
La nuova biologia.blu
Anatomia e fisiologia dei viventi S

3. Salt and Water Balance and Nitrogen Excretion

4. How Does the Mammalian Excretory System Work?
Maintaining homeostasis of the extracellular fluid is the job of the excretory system.
Excretory systems control volume, concentration, and composition of the extracellular fluid, and excrete wastes.
Kidney—the main excretory organ
Nephron—the main functional unit of the kidney.

5. How Is the Mammalian Kidney Made?
Kidneys filter blood and produce urine.
Ureter—a duct from the kidney that leads to the urinary bladder.
Urethra—a tube for urine excretion leading from the urinary bladder where urine is stored to the outside of the body.
The ureter, renal artery, and renal vein enter the kidney on the concave side.
The ureter branches and envelops the renal pyramids.

6. The Human Excretory System
Renal pyramids make up the medulla, or internal core.
The cortex is the outer layer of the kidney.
Figure The Human Excretory System (A) The human kidneys lie against the back wall of the abdominal cavity, in the region of the middle back. (B) A highly organized internal structure is the basis for kidney function. Certain parts of the nephrons are in the organ’s outer region, called the cortex; other parts are in the internal region, called the medulla. (C) The glomeruli and the proximal and distal convoluted tubules are located in the cortex of the kidney. The loops of Henle run in parallel as straight sections down into the renal medulla and back up to the cortex. Collecting ducts run from the cortex to the inner surface of the medulla, where they open into the ureter. The vasa recta are peritubular capillaries that parallel the loops of Henle.

7. How Is the Mammalian Kidney Made?
Organization of nephrons:
Glomeruli and Bowman’s capsules are located in the cortex
Proximal convoluted tubules—the initial, twisted segments of the renal tubules, are in the cortex
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.

8. How Is the Mammalian Kidney Made?
The Bowman’s capsule encloses the glomerulus.
Podocytes are capsule cells that contact the glomerular capillaries.
Podocytes have fine processes that wrap around and interdigitate with the capillaries.

9. How Is the Mammalian Kidney Made?
The distal convoluted tubules join the collecting duct in the cortex.
Collecting ducts descend through renal pyramids and empty into the funnel-shaped pelvis.
These join together and leave the kidney as the ureter.

10. How Is the Mammalian Kidney Made?
Blood vessel organization in the kidney parallels that of the nephrons.
Blood enters the glomerular capillaries via an afferent arteriole and leaves in an efferent arteriole.
Blood then enters another capillary bed, the peritubular capillaries surrounding the renal tubules.
All peritubular capillaries from a nephron join into venules that lead to the renal vein.

11. How Does the Mammalian Kidney Produce Concentrated Urine?
Urine formation in vertebrate nephrons involves three processes:
Filtration—blood is filtered in the glomerulus
Tubular reabsorption—glomerular filtrate flows into the renal tubule, where it is modified by reabsorption of specific ions, nutrients, and water.
Tubular secretion—tubule cells transport substances to be excreted into the glomerular filtrate.

12. The Vertebrate Nephron
Figure The Vertebrate Nephron The vertebrate nephron consists of a renal tubule closely associated with two capillary beds, the glomerulus and the peritubular capillaries.

13. How Does the Mammalian Kidney Produce Concentrated Urine?
Mammals product urine that is more concentrated than the extracellular fluid.
This is due to a countercurrent multiplier mechanism in the loops of Henle.
Tubule fluid flows in opposite directions in the ascending and descending limbs.
A solute concentration gradient is created in the renal medulla.

14. Concentrating the Urine
Figure Concentrating the Urine A countercurrent multiplier mechanism enables the mammalian kidney to produce urine that is far more concentrated than the blood plasma. The composition—but not the concentration—of the filtrate is changed by the proximal convoluted tubule, which reabsorbs valuable molecules (including NaCl). Bulk reabsorption of water follows osmotically. The urine concentration process begins in the thick ascending limb of the loop of Henle, which reabsorbs NaCl but is impermeable to H2O. Some of the reabsorbed NaCl enters the descending limb and is thereby trapped in the renal medulla, creating a concentration gradient in the interstitial fluid. As urine in the collecting duct passes through this concentration gradient, it can lose water osmotically and become highly concentrated.

15. How Does the Mammalian Kidney Produce Concentrated Urine?
The vasa recta is a network of peritubular capillaries parallel to the loops of Henle and the collecting duct.
Fluid passing down the renal tubule changes composition through reabsorption and secretion by tubule cells.
Because of the actions of the renal tubules, the excreted urine is very different from the original filtrate.

16. How Does the Mammalian Kidney Produce Concentrated Urine?
Kidneys also regulate pH of extracellular fluids.
HCO3– ions are the major buffers in blood.

17. How Are Kidney Functions Regulated?
A constant glomerular filtration rate (GFR) requires that blood is supplied to the kidneys under adequate pressure.
If GFR falls, kidney releases the enzyme renin, which activates angiotensin.
Angiotensin constricts efferent renal arterioles and peripheral blood vessels all over body to raise blood pressure.

18. How Are Kidney Functions Regulated?
Angiotensin stimulates adrenal cortex to release aldosterone.

19. How Are Kidney Functions Regulated?
The hypothalamus can stimulate release of antidiuretic hormone (ADH), or vasopressin, from the posterior pituitary.
ADH acts on collecting duct cells to insert aquaporins and increase permeability to water.
Osmoreceptors and stretch receptors can stimulate ADH release.

20. What Are the Major Diseases?
Renal failure results in:
Salt and water retention (high blood pressure)
Urea retention (uremic poisoning)
Decreasing pH (acidosis)
Dialysis treatment passes blood through membrane channels bathed in a plasma-like solution to remove wastes.

21. How Do Animals Excrete Nitrogen?
Animals must eliminate metabolic waste products.
Ammonia (NH3) is the most common nitrogenous waste. It is toxic and must be excreted continuously or converted.

22. How Do Invertebrate Excretory Systems Work?
Invertebrates have a variety of mechanisms to maintain salt and water balance and excrete nitrogen.
Protonephridia (Planaria)
In annelids, each segment contains a coelom (body cavity) and a pair of metanephridia.

23. How Do Vertebrates Excretory Systems Work?
Insect excretory systems consist of Malpighian tubules—blind-ended tubules that open into the gut.
Amphibians in dry environments reduce permeability of their skin to water.
Estivation is a state of low metabolic activity and low water turnover.
Marine fishes must conserve water in their high osmolarity environment.

24. How Do Vertebrates Excretory Systems Work?
Reptiles have three major adaptations that allow them to exist in dry regions:
Internal fertilization and shelled eggs
Dry epidermis that retards water loss
Excretion of nitrogenous wastes as uric acid, with little water loss.

25. Inc. All rights reserved
Adapted from
Life: The Science of Biology, Tenth Edition, Sinauer Associates, Sunderland, MA, 2014
Inc. All rights reserved