40.1 Osmoregulation and Excretion

• The movement of water is a special case of diffusion called osmosis.

  • Osmosis occurs only when solutions are separated by a membrane that permits water to cross but selectively holds back some or all of the solutes.

• The concentration of solutes in a solution, measured in osmoles 1 per liter, is the solution's osmolarity.

• Osmoregulation is the process by which organisms control the concentrations of water and solutes in their bodies.

• Seawater is a fairly constant ionic and osmotic environment, and the concentrations of electrolytes and other solutes found in these animals nearly match those of the sea.

  • Such animals are osmoconformers.

• Seawater is hyperosmotic to the tissues of marine bony fishes-the solution outside the body has a higher solute concentration than the solution inside.

• The freshwater is hyperosmotic to the fish’s tissues.

• Ammonia and these other compounds are referred to as nitrogenous wastes.

40.2 Water and Electrolyte Balance in Marine and Freshwater Fishes

• Research on salt excretion in sharks focused on an organ called the rectal gland, which secretes a concentrated salt solution into its rectum, where it is then excreted into the environment.

• To test the hypothesis that Na+/K+-ATPase is involved in salt excretion by shark rectal glands, biologists used a plant defense compound called ouabain.

40.3 Water and Electrolyte Balance in Terrestrial Insects

• In terrestrial insects, gas exchange occurs across the membranes of epithelial cells that line the tracheae, an extensive system of tubes.

  • The insect tracheal system connects with the atmosphere at openings called spiracles.

• To maintain homeostasis, insects must also carefully regulate the composition of a blood-like fluid called hemolymph.

  • Hemolymph is pumped by the heart and transports electrolytes, nutrients, and waste products, and is modified in a regulated process to produce urine.

• To maintain water and elec balance, insects rely on excretory organs called Malpighian tubules and on the hindgut posterior portion of their digestive act.

  • The Malpighian tubules are responsible for forming a filtrate, a filtered liquid, from the hemolymph.

40.4 Water and Electrolyte Balance in Terrestrial Vertebrates

• In land-dwelling vertebrates, osmoregulation occurs primarily through events that take place in the key organ of the urinary system, the kidney.

  • The kidney is responsible for water and electrolyte balance as well as the excretion of nitrogenous wastes.

• The urine at forms in the kidney is transported via a long tube called the ureter to a storage organ, the bladder.

  • From the bladder, urine is transported to the body surface through the urethra and then excreted.

• The nephron is the basic functional unit of the kidney.

  • The work involved in maintaining water and electrolyte balance occurs in the nephron.

• Most of the approximately 1 million nephrons in a human kidney are located almost entirely in the outer region of the organ, or cortex.

  • But some nephrons extend from the cortex into the kidney’s inner region or medulla.

• In terrestrial vertebrates, urine formation begins in the renal corpuscle.

• The region of the nephron that surrounds the glomerulus is named Bowman’s capsule, or the glomerular cαpsule.

• Filtrate leaves Bowman’s capsule and enters a convoluted structure called the proximal tubule.