Excretory System Water balance & nitrogenous waste removal

Homeostasis Keeping the balance  Animal body needs to coordinate many systems all at once Temperature Blood sugar levels Energy production Water balance & intracellular waste disposal Nutrients Ion balance Cell growth  Maintaining a “steady state” condition

Animal body systems evolved to support multicellular life intracellular waste extracellular waste O2O2 CHO aa CH CO 2 NH 3 aa O2O2 CH O2O2 aa CO 2 NH 3 O2O2 aa CH aa CHO O2O2 Diffusion too slow!

Overcoming limitations of diffusion Evolution of exchange systems for:  Distributing nutrients Circulatory system  Removing wastes Excretory system aa CO 2 NH 3 O2O2 aa CH aa CHO O2O2 systems to support multicellular organisms

Intracellular waste What waste products?  What do we digest our food into … Carbohydrates = CHO  CO 2 + H 2 O Lipids = CHO  CO 2 + H 2 O Proteins = CHON  CO 2 + H 2 O + N Nucleic acids + CHONP  CO 2 + H 2 O + P + N lots! very little CO 2 + H 2 O NH 3 = ammonia | ||| H H N C–OH O R H –C– Animals poison themselves from the inside by digesting proteins! cellular digestion… cellular waste

Nitrogenous waste disposal Ammonia (NH 3 )  Very toxic Carcinogenic  Very soluble Easily crosses membranes  Must dilute it & get rid of it … fast! How you get rid of nitrogenous waste depends on:  Who you are (evolutionary relationship)  Where you live (habitat) aquaticterrestrialterrestrial egg layer

Aquatic organisms  Can afford to lose water  Excrete ammonia Most toxic Terrestrial  Need to conserve water  Urea Less toxic Terrestrial egg layers  Need to conserve water  Need to protect embryo in egg  Excrete uric acid Least toxic Nitrogenous waste

Land animals Nitrogen waste disposal on land  Need to conserve water  Must process ammonia so less toxic Urea = larger molecule = less soluble = less toxic  2NH 2 + CO 2 = urea  Produced in liver  Kidney Filter solutes out of blood Reabsorb H 2 O (+ any useful solutes) Excrete waste  Urine = urea, salts, excess sugar & H 2 O  Concentrated NH 3 would be too toxic O C H N H H N H Urea costs energy to synthesize, but it’s worth it! mammals

Mammalian system bloodfiltrate concentrated urine Filter solutes out of blood & reabsorb H 2 O + desirable solutes Key functions:  Filtration Fluids (water & solutes) filtered out of blood  Reabsorption Selectively reabsorb (diffusion) needed water + solutes back into blood  Secretion Pump out any other unwanted solutes to urine  Excretion Expel concentrated urine (N waste + solutes + toxins) from body

Mammalian kidney kidney bladder ureter urethra renal vein & artery nephron epithelial cells adrenal gland inferior vena cava aorta

Mammalian kidney

Nephron why selective reabsorption & not selective filtration? Functional units of kidney  1 million nephrons per kidney Function  Filter out urea & other solutes (salt, sugar …)  Blood plasma filtered into nephron High pressure flow  Selective reabsorption of valuable solutes & H 2 O back into bloodstream Greater flexibility & control

Nephron Interaction of circulatory & excretory system Circulatory system  Glomerulus = ball of capillaries Excretory system  Nephron  Bowman’s capsule  Loop of Henle Proximal tubule Descending limb Ascending limb Distal tubule  Collecting duct Proximal tubule Distal tubule Glomerulus Collecting duct Loop of Henle Amino acids Glucose H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O Na + Cl - Mg ++ Ca ++ How can different sections allow the diffusion of different molecules? Bowman’s capsule Na + Cl -

Nephron Afferent arteriole: brings blood to the nephron to be filtered Efferent arteriole: removes blood from nephron (minus filtered components) Glomerulus: capillary tuft where filtration occurs Bowman’s Capsule: first part of nephron where filtrate is collected Proximal convoluted tubule: where selective reabsorption takes place

Nephron Loop of Henle: important for establishing a salt gradient in the medulla Distal convoluted tubule: final site of selective reabsorption Collecting duct: feeds into ureter and is where osmoregulation occurs Vasa recta: blood network that reabsorbs components from the filtrate

Counter-current exchange Blood flow through the vasa recta is in the opposite direction of the filtrate flow through the nephron Controls osmoregulation glomerulus vasa recta nephron

Nephron: filtrate high blood pressure in kidneys force to push (filter) H 2 O & solutes out of blood vessel BIG problems when you start out with high blood pressure in system hypertension = kidney damage At glomerulus  Filtered out of blood H 2 O Glucose Salt/ions Amino acids Urea  Not filtered out Cells Proteins

Nephron: ultrafiltration Ultrafiltration occurs between the glomerulus & the Bowman’s Capsule 2 things required to form the filtrate:  Hydrostatic pressure  Basement membrane

Nephron: ultrafiltration Hydrostatic pressure  The glomerulus increases blood pressure by forming narrow branches (which also increases surface area)  This pressure is maintained by a narrow efferent arteriole (relative to diameter of afferent arteriole) Restricts outflow of blood Keeps pressure high  The net pressure gradient in the glomerulus forces blood into the capsule space

Nephron: ultrafiltration Basement membrane  Fine mesh that restricts the passage of blood cells & proteins Sole filtration barrier  Blood can exit the glomerulus directly through pores in the capillaries Fenestrated  Filtrate can enter the Bowman’s Capsule directly because the podocytes that surround the glomerulus contain filtration slits between their pedicels  The basement membrane lies between the glomerulus & Bowman’s Capsule

Nephron: reaborption Proximal tubule  Reabsorbed back into the blood NaCl  Active transport of Na +  Cl - follows by diffusion H 2 O Glucose  Secondary active transport  Cotransport with Na + Amino acids HCO 3 -  Bicarbonate  Buffer for blood pH Descending limb Ascending limb

Nephron: reaborption Proximal tubule Actually, a Na-K pump -uses ATP -sets up gradient for secondary active transport with Na- glucose cotransporter Discovery led to the invention of Gatorade

Nephron: re-absorption Descending limb Ascending limb Loop of Henle  Descending limb High permeability to H 2 O  Many aquaporins in cell membranes Low permeability to salts  Few Na + or Cl - channels  Reabsorbed H 2 O structure fits function! Remember counter- current exchange

Nephron: re-absorption Descending limb Ascending limb structure fits function! Remember counter- current exchange Loop of Henle  Ascending limb Low permeability to H 2 O Cl - pump Na + follows by diffusion  Reabsorbed Salts  Maintains osmotic gradient

Nephron: re-absorption Distal tubule  Reabsorbed Salts Calcium H 2 O HCO 3 -  bicarbonate  Secreted Potassium H +

Nephron: re-absorption & excretion Descending limb Ascending limb Collecting duct  Reabsorbed H 2 O  Filtrate becomes increasingly concentrated as more water is lost to increasing salt gradient in medulla  Excreted Concentrated urine passed to bladder

Nephron: re-absorption & excretion Antidiuretic hormone (ADH/vasopressin)  Released from pituitary gland In response to dehydration  Increases permeability of the collecting duct to water More water is reabsorbed Urine becomes more concentrated  When the individual is rehydrated, ADH levels decrease

Osmotic control in nephron the value of a counter current exchange system How is all this re-absorption achieved?  Counter-current exchange and tight osmotic control to reduce the energy cost of excretion  Use diffusion & cotransporters instead of active transport whenever possible

Summary Not filtered out  Cells  Proteins Reabsorbed: active transport  Na +  Cl - Reabsorbed: diffusion  Na + H2OH2O Excreted  Urea  Excess H 2 O  Amino acids  Glucose  Cl -  Excess solutes (salts, glucose)  Toxins, drugs, “unknowns” Too big & remain in blood

Any Questions??