Chapter 44 – Osmoregulation and Excretion

Homeostasis 2 key homeostatic processes: Osmoregulation – how animals regulate their solute concentrations and balance the gain and loss of water Excretion – how animals get rid of the nitrogen-containing waste products of metabolism

Osmotic Challenges: Aquatic Osmoconformer: no active adjustment of internal osmolarity (marine animals); isoosmotic to environment, no tendency to lose or gain water, expend little energy Osmoregulator: adjust internal osmolarity because body fluids are not isoosmotic (freshwater, marine, terrestrial); costs energy Marine fishes (hyperosmotic)- keep water, lose salt; excretes large amounts of urine salt Freshwater fishes (hypoosmotic)- lose water, keep salt; drinks large amount of water

Osmotic Challenges: Terrestrial Terrestrial organisms must also regulate their water levels Adaptations include: Waxy cuticle in plants Shells of land snails Waxy layers of insect exoskeleton Layers of dead skin Being nocturnal Most lose water from moist surface areas of gas exchange, urine, feces, and skin Must gain water from diet or through metabolism

Nitrogenous Wastes Nitrogenous wastes: breakdown products of proteins and nucleic acids; can be very toxic Ammonia (NH3): very toxic; most aquatic animals, many fish Urea: lower toxicity; mammals, most amphibians, sharks, bony fish (in liver; combo of NH3 and CO2) Uric acid: lower toxicity, paste-like, little water loss; birds, insects, many reptiles, land snails

Excretory Process Filtration – excretory tubule collects a filtrate from blood, water and solutes forced into excretory tubule Reabsorption – transport epithelium reclaims valuable substances from filtrate Secretion – waste substances, toxins and excess ions, are extracted from body fluids and are added to the filtrate Excretion – filtrate leaves the system and the body

Excretory Systems Protonephridia – flatworms (“flame-bulb” systems) Metanephridia – annelids (ciliated funnel system) Malpighian tubules – insects (tubes in digestive tract) Kidneys – vertebrates

Mammal Excretory System Renal artery/vein: kidney blood flow Ureter: urine excretory duct Urinary bladder: urine storage Urethra: urine elimination tube Renal cortex (outer region) Renal medulla (inner region) Nephron: functional unit of kidney, approx. 1 million per kidney) Cortical nephrons (confined to renal cortex; 80%) Juxtamedullary nephrons (extend into renal medulla; 20%; only in mammals and birds, important in water conservation – hyperosmotic urine)

Nephron Structure Afferent arteriole: supplies blood to nephron from renal artery Glomerulus: ball of capillaries Efferent arteriole: blood from glomerulus Bowman’s capsule: surrounds glomerulus Proximal tubule: secretion & reabsorption Peritubular capillaries: from efferent arteriole; surround proximal & distal tubules Loop of Henle: water & salt balance Distal tubule: secretion & reabsorption Collecting duct: carries filtrate to renal pelvis

Basic Nephron Function

Proximal Tubule, Loop of Henle, Distal Tubule PT – reabsorb HCO3-, NaCl, water, nutrients, and K+ ions; secrete H+ and NH3 Decending L of H – reabsorb water Ascending L of H – reabsorb NaCl DT – reabsorb NaCl, water, and HCO3-; secrete K + and H +

Collecting Duct Carries filtrate through the medulla to the renal pelvis Reabsorbs NaCl, urea, and water

Water Conservation in Kidney Creates hyperosmotic urine Conserves water through both active and passive processes Review

Kidney regulation: hormones Antidiuretic hormone (ADH) ~ secretion increases permeability of distal tubules and collecting ducts to water (H2O back to body); inhibited by alcohol and coffee Juxtaglomerular apparatus (JGA) ~ reduced salt intake--->enzyme renin initiates conversion of angiotensinogen to angiotensin II increase blood pressure and blood volume by constricting capillaries Angiotensin II also stimulates adrenal glands to secrete aldosterone; acts on distal tubules to reabsorb more sodium, thereby increasing blood pressure (renin-angiotension-aldosterone system; RAAS) Atrial natriuretic factor (ANF) ~ walls of atria; inhibits release of renin, salt reabsorption, and aldosterone release