Chapter 40: Water and Electrolyte Balance in Animals
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.