Osmoregulation and Excretion

Osmoregulation

The balance of uptake and loss of water and dissolved solutes.

Osmosis

The movement of water down its concentration gradient across a selectively permeable membrane.

Osmolarity

The solute concentration of a solution.

Isoosmotic

Two solutions with the same osmolarity.

Hyperosmotic

A solution with higher solute concentration and lower free water concentration.

Hypoosmotic

A solution with lower solute concentration and higher free water concentration.

Osmoconformers

Organisms that are isoosmotic with their surroundings and do not regulate osmolarity.

Osmoregulators

Organisms that expend energy to control water uptake and loss in varying environments.

Stenohaline

An organism that can only tolerate a narrow range of salinity.

Euryhaline

An organism that can tolerate a wide range of salinity.

Freshwater fish (Osmoregulation)

Do not drink water, actively pump in solutes, and excrete dilute urine.

Marine bony fish (Osmoregulation)

Drink seawater, actively pump solutes out, and excrete tiny volumes of urine.

Terrestrial animals (Water Conservation)

Conserve water gained from food and metabolism by minimizing excretion loss.

Ammonia excretion (Fish & Crawfish)

Utilize a flushing strategy to dump toxic ammonia directly via the gills.

Urea excretion (Mammals)

Convert toxic ammonia into less toxic urea, transported to the kidneys.

Uric acid excretion (Birds & Reptiles)

Convert ammonia to uric/guanic acid; requires energy but conserves the most water.

Filtration

The passage of body fluids through capillaries to remove unwanted material.

Reabsorption

The process of reclaiming valuable solutes from the filtrate.

Secretion

The process of adding nonessential solutes and wastes into the filtrate.

Excretion

The final release of processed filtrate containing nitrogenous wastes from the body.

Protonephridia

Excretory systems utilizing flame bulbs to remove waste.

Metanephridia

Excretory systems utilizing a collecting tubule and bladder to process waste.

Malpighian tubules

Excretory organs in insects that deposit waste directly into the digestive tract.

Kidney Structure (Two regions)

The outer renal cortex and the inner renal medulla.

Blood Flow Pathway in Kidney

Renal artery → afferent arteriole → glomerulus → efferent arteriole → peritubular capillaries → renal vein.

Urine Flow Pathway in Nephron

Bowman's capsule → proximal tubule → descending loop → ascending loop → distal tubule → collecting duct.

Bowman's capsule

Collects water, ions, glucose, and urea filtered from the glomerulus.

Proximal tubule

Reabsorbs ~65% of water/Na⁺ and all glucose; secretes drugs and toxins.

Descending loop of Henle

Permeable to water but strictly impermeable to salts; concentrates the filtrate.

Ascending loop of Henle

Permeable to salts but strictly impermeable to water; dilutes the filtrate.

Distal tubule

Fine-tunes electrolytes; site of aldosterone (Na⁺ reabsorption) and PTH (Ca²⁺ reabsorption).

Collecting duct

Final site of water reabsorption regulated by antidiuretic hormone (ADH).

Antidiuretic Hormone (ADH) function

Increases water permeability of the collecting duct to conserve water.

Dehydration (High Osmolarity)

Triggers ADH release, increasing water reabsorption and producing concentrated urine.

Urea Pathway (Proximal Tubule)

About 50% of filtered urea is passively reabsorbed here to maintain osmotic balance.

Urea Recycling (Collecting Duct)

Under ADH, urea diffuses into the medulla, increasing osmolarity to help concentrate urine.

Renal artery (Function & Pathway)

Delivers oxygenated, unfiltered blood from aorta to kidneys. Pathway: Renal artery → segmental → interlobar → arcuate → interlobular → afferent.

Afferent arteriole (Function & Pathway)

Supplies blood into glomerulus; regulates pressure. Pathway: Interlobular artery → afferent arteriole → glomerulus.

Glomerulus (Function & Pathway)

Capillary tuft performing filtration of water, ions, small solutes. Pathway: Afferent arteriole → glomerular capillaries → efferent.

Efferent arteriole (Function & Pathway)

Carries filtered blood out of glomerulus; maintains high pressure. Pathway: Glomerulus → efferent → peritubular/vasa recta.

Peritubular capillaries (Function & Pathway)

Surround tubules; perform reabsorption and secretion. Pathway: Efferent arteriole → peritubular capillaries → venules → renal vein.

Renal vein (Function & Pathway)

Returns filtered, deoxygenated blood to circulation. Pathway: Peritubular capillaries → venules → renal vein → inferior vena cava.

Inferior vena cava (Function & Pathway)

Returns blood to right atrium. Pathway: Renal vein → inferior vena cava → heart.

Bowman's capsule (Function & Pathway)

Collects filtrate from glomerulus. Pathway: Bowman's capsule → proximal tubule.

Proximal tubule (Function & Pathway)

Reabsorbs ≈65% water, Na⁺, all glucose/amino acids; secretes toxins. Pathway: Bowman's → proximal → descending loop.

Descending loop of Henle (Function & Pathway)

Water leaves (permeable to water, not solutes); concentrates filtrate. Pathway: Proximal → descending → ascending.

Ascending loop of Henle (Function & Pathway)

Reabsorbs Na⁺, K⁺, Cl⁻ (impermeable to water); dilutes filtrate. Pathway: Descending → ascending → distal.

Distal tubule (Function & Pathway)

Fine-tunes electrolytes (aldosterone reabsorbs Na⁺, PTH reabsorbs Ca²⁺). Pathway: Ascending → distal → collecting duct.

Collecting duct (Function & Pathway)

Final water reabsorption under ADH. Pathway: Distal → collecting duct → renal papilla → renal pelvis.

Renal pelvis (Function & Pathway)

Funnel collecting urine from collecting ducts. Pathway: Collecting ducts → renal pelvis → ureter.

Ureter (Function & Pathway)

Muscular tube using peristalsis to move urine. Pathway: Renal pelvis → ureter → bladder.

Urinary bladder (Function & Pathway)

Stores urine; detrusor muscle contracts during urination. Pathway: Ureter → bladder → urethra.

Urethra (Function & Pathway)

Conducts urine out of body; controlled by sphincters. Pathway: Bladder → urethra → external environment.