Osmoregulation and Excretion

• Osmoregulation controls solute concentrations and regulates water gain/loss, influenced by the environment.
• Excretion rids the body of excess nitrogenous wastes from protein and nucleic acid breakdown, dependent on metabolism and water availability.

Solute Concentrations and Movement

• Solutions separated by semi-permeable membranes have varying water and solute concentrations, measured in osmolarity (moles/L).
• Isosmotic solutions have the same osmolarity and exhibit no net water movement across a semi-permeable membrane.
• Hyperosmotic solutions have higher solute concentrations, while hypoosmotic solutions have lower solute concentrations.
• Water flows from hypoosmotic to hyperosmotic conditions via osmosis, diluting the solute.

Tonicity and Environment

• Tonicity considers the effect of solutions on cells.
• Osmoregulation strategies vary based on the animal's environment.

Aquatic Organisms

• Aquatic organisms face osmoregulatory challenges in hypoosmotic (freshwater) or hyperosmotic (marine) environments.
• Osmoconformers, found in marine environments, maintain cells and tissues isosmotic to their surroundings.
• Osmoregulators, including freshwater, land, and some marine organisms, control internal osmolarity independent of the environment.
• Freshwater organisms combat excess water intake and actively uptake salts, while marine organisms prevent water loss and excrete salts.

Transient Environments and Land Animals

• Anhydrobiosis allows some organisms like tardigrades to survive extreme dehydration by protecting cell membranes with trehalose.
• Land animals have adaptations to conserve water and minimize water loss.
• Maintaining osmolarity requires energy expenditure, especially for osmoregulators maintaining osmotic gradients.

Transport Epithelia

• Transport epithelia facilitate solute movement in specific directions, often arranged in tube-like structures to maximize surface area.
• Albatrosses possess specialized transport epithelium that excretes saltwater via their nostrils, enabling them to drink seawater.

Nitrogenous Wastes

• Metabolic wastes must be dissolved in water for excretion, impacting water balance.
• Nitrogen from protein and nucleic acid breakdown is released as toxic ammonia.
• Animals convert ammonia to less toxic forms to conserve water.

Types of Nitrogenous Wastes

• Ammonia: Highly toxic, requires significant water for excretion; common in freshwater aquatic animals.
• Urea: Requires energy for conversion from ammonia, less toxic than ammonia, still needs water for excretion.
• Uric Acid: Most water-conserving; non-toxic, excreted as semi-solid waste, most energy-expensive to produce.

Tubular Structures for Osmoregulation

• Tubular structures near blood vessels facilitate waste removal through four steps:

Four Steps of Waste Removal

• Filtration: Hydrostatic pressure forces blood components against a semi-permeable membrane, separating filtrate.
• Reabsorption: Recovers essential ions and solutes from the filtrate back into the bloodstream.
• Secretion: Actively transports additional solutes and materials from the blood into the tubules.
• Excretion: Eliminates the prepared filtrate from the excretory system.

Types of Tubular Systems

• Protonephridia: Found in flatworms, filter interstitial fluid through cap cells and tubules, excreting through body wall openings.
• Metanephridia: In annelids, collect fluids from the coelom, with cilia drawing fluid into tubules associated with capillaries, producing dilute urine.
• Malpighian Tubules: In insects and terrestrial arthropods, secrete wastes into tubules immersed in hemolymph, reabsorbing water and solutes in the digestive system, excreting dry uric acid with feces.
• Kidneys: Complex organs with tubules closely associated with capillaries, performing all four excretion functions; urine stored in bladder, connected by ureters and urethra.

Mammalian Excretory System

• The mammalian kidney comprises the renal cortex (filtration and secretion), medulla (reabsorption and concentration), and pelvis (urine collection).

Mammalian Nephron Function

• The nephron, the kidney's functional unit, closely interacts with capillaries.
• Filtration occurs in Bowman's capsule, where the glomerulus filters blood, producing a filtrate with similar solute concentrations to blood plasma.
• The proximal convoluted tubule reabsorbs essential solutes (ions, nutrients) and water, balancing pH.
• The descending Loop of Henle further reduces filtrate volume through aquaporins, allowing water to move into the interstitial fluid.
• The ascending Loop of Henle, impermeable to water, allows NaCl to diffuse out, maintaining interstitial fluid osmolarity.
• The distal convoluted tubule regulates potassium and NaCl levels and adjusts pH.
• The collecting duct processes filtrate into urine, with hormonal control determining concentration.

Urine Concentration in Mammals

• Mammalian kidneys conserve water by concentrating urine, with desert dwellers achieving higher concentrations.
• Osmolarity gradients in the loop of Henle facilitate water removal from tubules into interstitial fluid.

Hormonal Control

• Antidiuretic Hormone (ADH): Released by the posterior pituitary, ADH increases aquaporins in the collecting duct, enhancing water reabsorption.
• Renin-Angiotensin-Aldosterone System (RAAS): The juxtaglomerular apparatus (JGA) monitors blood pressure/volume; renin release triggers angiotensin II production, causing vasoconstriction and aldosterone release to increase Na+ and water reabsorption.
• Atrial Natriuretic Peptide (ANP): Released by the heart in response to increased blood pressure/volume, ANP inhibits renin and RAAS.