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.

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.

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.

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.

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.

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.

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.