Osmotic Regulation and the Urinary System

49.1 Osmolarity and Osmotic Balance

Water in a multicellular body is distributed between:

• Intracellular compartment

• Extracellular compartment

Most vertebrates maintain homeostasis for:

• Total solute concentration of their extracellular fluids

• Concentration of specific inorganic ions

Important ions:

• Sodium $(Na^+)$: major cation in extracellular fluids

• Chloride $(Cl^-)$: major anion

• Divalent cations: calcium $(Ca^{+2})$ and magnesium $(Mg^{+2})$

• Monovalent cation: potassium $(K^+)$

Water exchange between the body and the environment involves intake through food, water, and metabolic water production, and output through urine, feces, evaporation, and respiration.

Osmotic Pressure

Osmolarity:

• Number of osmotically active moles of solute per liter of solution

• A solution with a higher osmolarity exerts more osmotic pressure than one with a lower osmolarity

Osmotic pressure:

• Measure of a solution’s tendency to take in water by osmosis (that is, a measure of concentration difference)

• Osmotic movement of water always occurs from a more dilute to a less dilute solution

• Measure of a solution’s ability to change the volume of a cell by osmosis

• Relative to intracellular fluid, a solution may be:

  • Hypertonic: higher osmotic pressure

  • Hypotonic: lower osmotic pressure

  • Isotonic: equitable osmotic pressure

Tonicity

Osmolarity and Osmotic Balance: Aquatic Vertebrates

Freshwater vertebrates:

• Hypertonic to their environment

• Adapted to prevent water from entering their bodies and to actively transport ions back into their bodies

Marine vertebrates:

• Hypotonic to their environment

• Adapted to retain water by drinking seawater and eliminating the excess ions through kidneys and gills

Osmolarity and Osmotic Balance: Terrestrial Vertebrates

Terrestrial vertebrates:

• Body fluids have a higher concentration of water than the surrounding air

• Tend to lose water by evaporation from skin and lungs

• Urinary/osmoregulatory systems help them retain water

49.2 Nitrogenous Wastes: Ammonia, Urea, and Uric Acid

• Amino acids and nucleic acids are catabolized into nitrogenous wastes which must be eliminated from the body.

• Ammonia, Urea, and Uric Acid

First step is deamination:

• Removal of the amino $(−NH<em>2)$ group combined with $H^+$ to form ammonia $(NH</em>3)$ in the liver.

• Toxic to cells, and thus it is only safe in dilute concentrations.

Elimination of Ammonia

• Bony fishes and amphibian tadpoles eliminate most of the ammonia by diffusion via gills.

• Elasmobranchs, adult amphibians, and mammals convert ammonia into urea, which is soluble in water.

• Birds, reptiles, and insects convert ammonia into the water-insoluble uric acid.

  • Costs most energy, but saves most water

49.3 Osmoregulatory Organs

Invertebrates

• Flatworms: use protonephridia which branch into bulblike flame cells; open to the outside of the body, but not to the inside

• Earthworms: use nephridia; open both to the inside and outside of the body

Vertebrate Osmoregulatory Organs

• Vertebrate kidneys create a tubular fluid by filtering the blood under pressure through the glomerulus.

• Filtrate contains many small molecules, in addition to water and waste products.

• Most of these molecules and water are reabsorbed into the blood.

• Selective reabsorption provides great flexibility.

• Waste products are eliminated from the body in the form of urine.

49.4 Evolution of the Vertebrate Kidney

• Made up of thousands of repeating units – nephrons

• Although the same basic design has been retained in all vertebrate kidneys, a few modifications have occurred.

• All vertebrates can produce a urine that is isotonic or hypotonic to blood.

• Only birds and mammals can make a hypertonic urine.

• Amphibian kidney is identical to that of freshwater fish

• Kidneys of reptiles are very diverse

  • Marine reptiles drink seawater and excrete an isotonic urine, eliminating excess salt via salt glands.

  • Terrestrial reptiles reabsorb much of the salt and water in their nephron tubules

  • Don’t excrete urine, but empty it into cloaca

49.5 The Mammalian Kidney

• Each kidney receives blood from a renal artery and produces urine from this blood.

• Urine drains from each kidney through a ureter into a urinary bladder.

• Urine is passed out of the body through the urethra.

• Within the kidney, the mouth of the ureter flares open to form the renal pelvis which receives urine from the renal tissue.

• Divided into an outer renal cortex and inner renal medulla

Functions of the Mammalian Kidney

The kidney has three basic functions:

• Filtration: Fluid in the blood is filtered out of the glomerulus into the tubule system.

• Reabsorption: Selective movement of solutes out of the filtrate back into the blood via peritubular capillaries.

• Secretion: Movement of substances from the blood into the extracellular fluid, then into the filtrate in the tubular system.

• In humans, approximately 2,000 L of blood passes through the kidneys each day and 180 L of water leaves the blood and enters the glomerular filtrate.

• Most of the water and dissolved solutes that enter the glomerular filtrate must be returned to the blood by reabsorption.

• Water is reabsorbed by the proximal convoluted tubule, descending loop of Henle, and collecting duct.

Reabsorption of glucose and amino acids is driven by active transport and secondary active transport. Maximum rate of transport. Glucose surpassing saturation remains in the urine of untreated diabetes mellitus patients.

Secretion of waste products involves transport across capillary membranes and kidney tubules into the filtrate. Penicillin must be administered several times a day.

Loop of Henle

• Creates a gradient of increasing osmolarity from the cortex to the medulla

• Actively transports $Na^+$ and $Cl^−$, and $H_2O$ follows from the ascending loop, creating an osmotic gradient

• Allows reabsorption of water from descending loop and collecting duct

• Two limbs of the loop form a countercurrent multiplier system, creating a hypertonic renal medulla

49.6 Hormonal Control of Osmoregulatory Functions

• Kidneys maintain relatively constant levels of blood volume, pressure, and osmolarity.

• Also regulate the plasma $(Na^+)$ and $(K^+)$

• Concentrations and blood pH within narrow limits

• These homeostatic functions of kidneys are coordinated primarily by hormones

Antidiuretic Hormone (ADH)

• Produced by the hypothalamus and secreted by the posterior pituitary gland.

• Stimulated by an increase in the osmolarity of blood.

• Causes walls of distal tubule and collecting ducts to become more permeable to water (aquaporins).

• More ADH increases reabsorption of water, making a more concentrated urine.

Aldosterone

• Secreted by the adrenal cortex

• Stimulated by low levels of $(Na^+)$ in the blood.

• Causes distal convoluted tubule and collecting ducts to reabsorb $(Na^+)$.

• Reabsorption of $(Na^+)$ and $(Cl^-)$, and water follows.

• Low levels of $(Na^+)$ in the blood are accompanied by a decrease in blood volume, activating the renin-angiotensin-aldosterone system.

The Renin-Angiotensin-Aldosterone System

• Activated by low blood pressure or low blood flow

• Involves the juxtaglomerular apparatus, kidney, angiotensinogen, renin, angiotensin I and II, adrenal cortex, and aldosterone to increase blood volume and constrict blood vessels. It is a response to maintain blood pressure and volume.