Osmoregulation and Excretion CHAPTER 44

WATER BALANCE Osmolarity - total solute concentration (M) = moles of solute per liter Osmolarity - total solute concentration (M) = moles of solute per liter Osmosis – water moves from hypoosmotic (less solute) to hyperosmotic (greater solute) Osmosis – water moves from hypoosmotic (less solute) to hyperosmotic (greater solute)

Figure Salt-excreting glands in birds

Figure 44.14a Osmoregulation in a saltwater fish

Figure 44.14b Osmoregulation in a freshwater fish

Osmoconformer - does not actively adjust internal osmolarity Osmoconformer - does not actively adjust internal osmolarity  Some saltwater fish, most marine invertebrates

Osmoregulator – adjusts internal osmolarity Osmoregulator – adjusts internal osmolarity  All freshwater fish, some mammals, most marine vertebrates  Cost of regulation depends on environment

Anhydrobiosis – a few animals can lose almost all of their body water and survive in a dormant state Anhydrobiosis – a few animals can lose almost all of their body water and survive in a dormant state  Tardigrades (water bears)  Some nematodes Desiccation Desiccation  Humans die if lose 12% body water

Figure Anhydrobiosis: Hydrated tardigrade (left), dehydrated tardigrade (right)

EXCRETORY SYSTEMS Balancing water loss and gains and disposing of wastes Balancing water loss and gains and disposing of wastes Transport epithelium – layer(s) of epithelial cells the regulate solute movement (selectively permeable membranes) Transport epithelium – layer(s) of epithelial cells the regulate solute movement (selectively permeable membranes) Most harmful wastes are nitrogen products from breakdown of proteins and nucleic acids Most harmful wastes are nitrogen products from breakdown of proteins and nucleic acids

Figure Water balance in two terrestrial mammals

Figure Nitrogenous wastes

AMMONIA Most waste is NH 3 Most waste is NH 3 Most aquatic animals secrete nitrogenous wastes as ammonia or ammonium (NH 4 + ) Most aquatic animals secrete nitrogenous wastes as ammonia or ammonium (NH 4 + ) Many animals convert NH 3 to urea or uric acid Many animals convert NH 3 to urea or uric acid

UREA Mammals, most amphibians, sharks, some fishes Mammals, most amphibians, sharks, some fishes 100,000 times less toxic than NH 3 100,000 times less toxic than NH 3 Produced in vertebrate liver by combining CO 2 with NH 3 Produced in vertebrate liver by combining CO 2 with NH 3 Urea is carried to kidneys Urea is carried to kidneys

URIC ACID Land snails, birds, insects, many reptiles Land snails, birds, insects, many reptiles Least soluble in water Least soluble in water Excreted in paste-like form so little water loss Excreted in paste-like form so little water loss Can be stored within shelled egg without harming embryo Can be stored within shelled egg without harming embryo

Dispose wastes Dispose wastes Overview Overview  Filtration – fluids filtered through transport epithelia  Reabsorption – some filtered compounds are actively transported back to body fluids  Secretion – removal of wastes from body

Figure Key functions of excretory systems: an overview

PLATYHELMINTHES (FLATWORMS) Protonephridium – network of closed tubules throughout body; branches end in flame bulbs Protonephridium – network of closed tubules throughout body; branches end in flame bulbs Cilia provide movement of fluid within tubules Cilia provide movement of fluid within tubules

Functions mostly in osmoregulation Functions mostly in osmoregulation Most metabolic waste diffuse out Most metabolic waste diffuse out

Figure Protonephridia: the flame-bulb system of a planarian

ANNELIDS Metanephridium – tubular system with internal openings located in each segment Metanephridium – tubular system with internal openings located in each segment Nephrostome – internal opening where fluid enters Nephrostome – internal opening where fluid enters

Most solutes reabsorbed into blood capillaries Most solutes reabsorbed into blood capillaries Nitrogenous wastes exit via nephridiopores Nitrogenous wastes exit via nephridiopores

Figure Metanephridia of an earthworm

INSECTS AND TERRESTRIAL ARTHROPODS Malpighian tubules – remove nitrogenous wastes from hemolymph Malpighian tubules – remove nitrogenous wastes from hemolymph Empties into digestive tract and is eliminated in feces Empties into digestive tract and is eliminated in feces

Figure Malpighian tubules of insects

MAMMALIAN KIDNEY Renal artery – blood enters kidney Renal artery – blood enters kidney Renal vein – blood leave kidney Renal vein – blood leave kidney Ureter – tube that takes urine from kidney to bladder Ureter – tube that takes urine from kidney to bladder

Urinary bladder – urine (from ureters) is stored Urinary bladder – urine (from ureters) is stored Urethra – tube in which urine leaves bladder and exits body Urethra – tube in which urine leaves bladder and exits body

Renal cortex – outer region of kidney Renal cortex – outer region of kidney Renal medulla – inner region of kidney Renal medulla – inner region of kidney Renal pelvis – area in center of kidney where urine collects Renal pelvis – area in center of kidney where urine collects Nephron – functional unit of kidney Nephron – functional unit of kidney Glomerulus – ball of capillaries of each nephron Glomerulus – ball of capillaries of each nephron Bowman’s Capsule – surrounds glomerulus Bowman’s Capsule – surrounds glomerulus

Figure The human excretory system at four size scales

Figure The nephron and collecting duct: regional functions of the transport epithelium

FILTRATION OF BLOOD Blood pressure sends anything small enough into Bowman’s capsule Blood pressure sends anything small enough into Bowman’s capsule  Filtrate includes glucose, salts, nitrogenous wastes, vitamins, but not cells

 Filtrate passes through 3 regions of nephron:  Proximal tubule  Loop of Henle  Distal tubule  Filtrate (urine) Empties in collecting duct  Urine then empties into renal pelvis  Urine leaves kidneys via ureters

Blood vessels along nephrons Blood vessels along nephrons  Afferent arteriole – blood to glomerulus  Efferent arteriole – blood away from glomerulus  Divides in capillaries that supply proximal and distal tubule and loop of Henle

Secretion – substances put into proximal and distal tubules from blood (selective process) Secretion – substances put into proximal and distal tubules from blood (selective process) Reabsorption – substances put back into blood Reabsorption – substances put back into blood  Nearly all water, sugar, vitamins, and nutrients are reabsorbed

Figure How the human kidney concentrates urine: the two-solute model (Layer 1)

Figure How the human kidney concentrates urine: the two-solute model (Layer 2)

Figure How the human kidney concentrates urine: the two-solute model (Layer 3)

Filtration Proximal tubule Proximal tubule  HCO 3 -, NaCl, H 2 O, nutrients (ex. glucose and amino acids), K + reabsorbed (some actively and some passively)  H + and NH 3 secreted Descending loop of Henle Descending loop of Henle  Cells permeable to water, but not NaCl; water reabsorbed by osmosis  Filtrate becomes more concentrated

Ascending loop of Henle Ascending loop of Henle  Cells permeable to NaCl, but not water; NaCl reabsorbed by passive and then active transport  Filtrate becomes more diluted Distal Tubule Distal Tubule  HCO 3 -, NaCl, and H 2 O reabsorbed  H + and K + secreted

Collecting duct Collecting duct  Water is reabsorbed passively  NaCl is actively reabsorbed (in outer medulla)  Urea and H 2 O are passively reabsorbed (in inner medulla)  Urine becomes more concentrated

Making urine in humans Approximately 1600 L of blood flows through a pair of kidneys and we make approximately 1.5 L of urine per day Approximately 1600 L of blood flows through a pair of kidneys and we make approximately 1.5 L of urine per day Osmolarity of blood is about 300 mOsm/L and osmolarity of urine is about 1200 mOsm/L Osmolarity of blood is about 300 mOsm/L and osmolarity of urine is about 1200 mOsm/L Kidneys are water-conserving (involves lots of energy to move solutes actively) Kidneys are water-conserving (involves lots of energy to move solutes actively)

HORMONAL FEEDBACK Hormones link kidney function, water balance, and blood pressure Hormones link kidney function, water balance, and blood pressure Antidiuretic hormone (ADH) Antidiuretic hormone (ADH)  Made in hypothalamus and stored in pituitary gland  Secreted when blood osmolarity rises  May be caused by excessive sweating, diarrhea, or eating salty foods

 Increases water reabsorption in distal tubule and collecting duct (makes membranes more permeable to water)  Alcohol can inhibit ADH so excessive drinking causes dehydration (due to excessive urination)

RAAS system of regulation RAAS system of regulation Juxtaglomerular apparatus Juxtaglomerular apparatus  Specialized tissue located near afferent arteriole which supplies blood to glomerulus  Responds to low blood pressure by secreting renin  Renin activates angiotensinogen which becomes angiotension II

 Angiotension II increase blood pressure by:  Constricting arteries  Signals proximal tube to reabsorb more NaCl and water  Stimulates adrenal gland to secrete aldosterone  Aldosterone increases the reabsorption of sodium and water in distal tubes, which increases blood volume and pressure

Hypertension and Kidneys Hypertension is treated with drugs that inhibit angiotensin II. Hypertension is treated with drugs that inhibit angiotensin II. Many hypertension drugs inhibit the ACE enzyme that takes Angiotensin I and cleaves it to make Angiotensin II Many hypertension drugs inhibit the ACE enzyme that takes Angiotensin I and cleaves it to make Angiotensin II

DIVERSE ADAPTATIONS Birds: shorter loops of Henle so urine not as concentrated Birds: shorter loops of Henle so urine not as concentrated Freshwater fish: must conserve salts so more reabsorption of ions Freshwater fish: must conserve salts so more reabsorption of ions

Marine fish – lack glomeruli and Bowman’s capsules so excrete mostly divalent ions while gills excrete monovalent ions Marine fish – lack glomeruli and Bowman’s capsules so excrete mostly divalent ions while gills excrete monovalent ions Frogs: can reabsorb water from bladder Frogs: can reabsorb water from bladder

Figure A vampire bat (Desmodus rotundas), a mammal with a unique excretory situation. While the bat feeds on a blood meal, its kidneys make dilute urine copiously. Thus shedding weight for the flight home. While the bat roosts during the day, the kidneys secrete small amounts of concentrated urine.