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Urinary and Excretory System Likhitha Musunuru, Sal Ghodbane, and Margaret Strair Function of Excretory and Urinary Systems Physiological problem: maintaining a consistent internal environment

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urinary and excretory system

Urinary and Excretory System

Likhitha Musunuru, Sal Ghodbane, and Margaret Strair


Function of Excretory and Urinary Systems

  • Physiological problem: maintaining a consistent internal environment
  • Excretory system in all types of organisms has one main function: maintain homeostasis within a given organism
    • Homeostasis- condition in which all internal systems and chemicals of that organism are in consistent balance
    • Involves the removal and gain of equal amounts of material
mechanisms of homeostasis
Mechanisms of Homeostasis
  • Homeostatic control systems have three components: receptor, control center, and effector
  • Receptor detects a change in some variable of the animal internal environment
  • Control center processes the information it receives from the receptor and directs a response by the effector
negative feedback
Negative Feedback
  • Negative feedback is when a change in the variable triggers the control mechanism to counteract further change in the same direction
  • This prevents small changes from becoming too large
  • Most homeostatic mechanisms including human temperature is regulated this way
positive feedback
Positive Feedback
  • Positive Feedback is when a change in a variable triggers mechanisms that amplify the change
  • Childbirth occurs this way when pressure of a baby’s head pushes against the uterus triggering heightening of contractions which causes even greater pressure

Means of Maintaining Homeostasis

  • Rid organisms of waste products
  • Keep both the fluid and the salt content of the organism within normal parameters
  • Keep the concentration of other substances in body fluids at normal levels


Osmoregulation- how animals regulate solute concentrations and balance the gain and loss of water

Excretion- how animals get rid of nitrogen containing waste products of metabolism


Review of Osmosis

  • All animals face the same problem of osmoregulation: water uptake and loss must balance
  • Osmosis occurs when two solutions separated by a membrane differ in osmotic pressure or osmolarity

How Osmosis is Controlled

An animal is a regulator if it uses internal control mechanisms to moderate internal change in the face of external fluctuation

-Example: Freshwater fish are able to maintain stable internal concentration of solutes in blood and interstitial fluid even though that concentration is different from the solute concentration of the water it lives in

  • Conformer is an animal that allows its internal condition to vary with certain external changes
  • Example: Maine invertebrates such as spider crabs, live in environments with stable solute concentration. It conforms its internal solute concentration to the environment
a continuum
A Continuum
  • Regulating and conforming are two extremes of a continuum
  • No animal is a perfect regulator or conformer
  • Some animals regulate some internal conditions and allow others to conform

Function of Osmoregulation

  • Ultimate function of osmoregulation is to maintain cellular cytoplasm
    • Animals with open circulatory system (insects) manage the hemolymph, or fluid that bathes the cells
    • Animals with closed circulatory system (vertebrates), cells are bathed directly in interstitial fluid that is directly controlled by composition of the blood

Solutions to Osmolarity

  • -Marine animals can be isoosomotic to surroundings (osmoconformer)
    • -Live in stable environments
  • -Osmoregulator is an animal that controls its internal osmolarity
    • --Animals in hypoosmotic environment must discharge water and vice versa
    • --Allows animals to live in places conformers cannot like freshwater and terrestrial habitats


  • Osmoregulators maintain the osmotic gradients that cause water to move in or out by using active transport.
  • Energy cost of osmoregulation depends on how different an animal’s osmolarity is from its surrounds and how much work is required to pump solutes across the membrane.
    • Accounts for 5% of resting metabolic rate of many marine and freshwater bony fish
    • Some fish that live in extremely salty lakes like Utah’s Great Salt Lake use up to 30% of their resting metabolic rate
    • Osmoconformers expend very little energy

Stenohaline are animals that cannot tolerate substantial changes in external osmolarity

  • Euryhaline animals can survive large fluctuations in external osmolarity
    • Includes both osmoconformers and certain osmoregulators
    • Species of Salmon; Talapia can adjust to any salt concentration between freshwater and twice that of salt water

Marine Adaptations

  • Most marine animals are always losing water through osmosis
  • The sum of their total osmolarity equals that of the environment but specific solute concentrations differ
    • Even osmoconformers need to regulate their internal composition of solutes. (marine invertebrates)
  • Marine vertebrates and some invertebrates are osmoregulators


  • Marine bony fish, like cod, are hypoosmotic to seawater and constanly lose water and gain salt
    • Counteract this by drinking a lot of seawater and gills dispose of salt
  • Marine sharks and chondrichthyans have kidney’s that remove some salt and rectal gland removes the rest
    • Maintain high concentration of urea and organic solute TMAO to protect from damage from urea
    • Actually hyperosmotic to environment and urine disposes of small influx of water
freshwater animals
Freshwater Animals
  • Constantly gaining water by osmosis and lose salts by diffusion (osmolarity of internal fluids is much higher than its surroundings)
  • Body fluids are lower solute concentrations than marine relatives
  • Reduced osmotic difference between body fluids and the surroundings reduces energy needed for osmoregulation
  • Maintain water balance by execreting large amounts of very dilute urine
  • Salt is replenished by food and Cl- is actively transported across gills and Na+ follows

Marine and Freshwater Fish

  • Salmon and other euryhaline fish migrate between seawater and fresh water
  • In the ocean, osmoregulation is done like marine fish by drinking seawater and exereting excess salt from gills
  • In fresh water, salmon cease drinking and begin to produce large amounts of dilute urine and gills take up salt

Temporary Waters

  • Anhydrobiosis is an adaptation that aquatic invertebrates have that allow them to survive in a dormant state when temporary ponds and films of water dry up
    • Tardigrades, tiny invertebrates, have 85% water mass in hydrated state and 2% water in inactive state
    • Must have adaptations to keep cell’s membranes in tact--use trehalose, a disaccharide, to replace water of their membranes when dehydrated
land animals
Land Animals
  • Body coverings prevent dehydration
    • Many terrestrial animals, esp. desert dwellers are nocturnal because low temperature and high humidity
  • Animals still lose a lot of water through gas exchange, urine, feces, and across skin
  • Balance water budget by drinking liquid, eating food, and using metabolic water produced during cellular respiration

Water Gain

Ingested in food

Ingested in food

Derived from metabolism

Derived from metabolism

Ingested in liquid





Water Loss



transport epithelium
Transport Epithelium
  • Most animals have one or more kinds of transport epithelium, layer of specialized epithelial cells that regulate solute movements
    • Essential for osmotic regulation and metabolic waste disposal
    • Move specific solute in controlled amounts in specific directions
    • Some face outside directly, others line channels that connect to outside. This ensures that solutes going between animal and environment must pass through selectively permeable membrane
    • In most animals, Transport epithelium are arranged in tubular networks with extensive surface areas.

Primary Wastes

  • Primary waste products of all organisms include:
    • Nitrogen–based products such as urea created by the breakdown of proteins into amino acids
    • Water and carbon dioxide created by the breakdown of carbohydrates

Carbon dioxide and some water excretion performed by the respiratory system. These wastes are toxic to the body if not removed

Nitrogen and water are processed and released by the excretory and urinary system

nitrogenous waste
Nitrogenous Waste
  • Since water is needed to dissolve waste before it is removed, waste can have large effect on water balance
  • When proteins and nucleic acids are broken down it results in ammonia
    • Some animals convert it to other less toxic compounds which requires ATP

Forms of Nitrogenous Waste include:



Uric Acid

  • Ammonia is very soluble but only tolerable at low concentrations
    • Aquatic species excrete this because access to a lot of water. (Ammonia is toxic, must be excreted in large, dilute quantities)
    • Readily passes through membranes and lost by diffusion to the surrounding water
    • In invertebrates, it can occur across the whole body structure
    • In fishes, most ammonia is lost in form of ammonium ions across epithelium of gills, kidneys excrete minor amounts of nitrogenous wastes
    • Freshwater fish: gill epithelium takes up sodium ions from water in exchange for ammonium ions while helps maintain a higher sodium concentration in body fluids than surrounding water
  • Urea is ammonia and carbon dioxide
    • Low toxicity (100,000X less than ammonia)
    • Animals can transport and store Urea safely
    • Requires much less water, more suitable for terrestrial animals because less water is lost when a given quantity of nitrogen is excreted
    • Allows waste to be excreted in concentrated solutions (Good for land animals)
    • Must expend energy to produce it from ammonia
    • Excreted by mammals, adult amphibians, sharks and some marine bony fish, and turtles
uric acid
Uric Acid
  • Insects, land snails, and many reptiles excrete uric acid
    • Relatively nontoxic
    • Largely insoluble in water
    • Excreted as semi-solid paste with little water
    • Takes even more energy than urea but saves water
    • Excreted by insects, land snails, many reptiles, land birds


Uric Acid



  • Water seems to have most significant on evolution of wastes
    • Uric acid and urea show minimal water loss
  • Reproduction effected waste too
    • Mammals need soluble wastes so waste can diffuse out of embryo
    • Shelled eggs (produced by birds and reptiles) need uric acid because it can be stored in the egg until the animal hatches. Shelled eggs are permeable to gases, not liquids. Soluble nitrogenous wastes released by embryo would be trapped with in egg and could accumulate to dangerous levels.
Waste of vertebrates depend on habitat and evolutionary lineage
    • Terrestrial turtles excrete uric acid while aquatic excrete urea and ammonia
  • Some species that move between land and aquatic environments can change their waste products
  • Waste also depends on the energy budget
    • Endotherms eat more food and produce more waste than ectotherms
    • Predators that eat more proteins excrete more nitrogen
excretory systems are diverse but go through same basic steps
Excretory systems are diverse but go through same basic steps
  • Body fluid is collected which usually involves filtration
    • Hydrostatic pressure forces small solutes (the filtrate) into the excretory system
  • Selective Reabsorption uses active transport to put valuable solutes back into system
  • Selective Secretion uses active transport to add to the filtrate nonessential solutes that remain in the body

Phylum Porifera

  • A variety of excretory structures have evolved in the animal kingdom. Lower classes order organisms such as protozoa use a contractile vacuole. Marine animals may have evolved from a type of protozoan.
  • Sponges lack organs and instead have specialized cells for carrying out bodily functions
  • Collar cells lining the inner cavity. The beating Flagella on Collar Cells create a current which flows through pores in sponge wall into a central cavity and through an osculum.
  • 10 cm tall sponge will go through 100 Liters of water/day


The cnardians such as jellyfish are also examples of simple organisms that are able to regulate fluids and wastes without the benefit of any excretory structures.

--They have only the endoderm and ectoderm layers, making them diplobastic. They lack a mesoderm, and therefore lack organs.

--They have one opening which serves as both a mouth an anus

  • Molluscs:The mantle cavity, houses the gills; the excretory system discharge into it. Excretion is carried out by a pair of nephridia, that collect fluids from the coelom and exchange salts and other substances with body tissues as the fluid passes along the tubules for excretion. The nephridia empty into the mantle cavity.
phylum platyhelminthes protonephridia flame bulb systems
Phylum Platyhelminthes Protonephridia: Flame Bulb Systems
  • Freshwater flatworms use this system which is a network of dead end tubules lacking internal openings
    • Tubules branch throughout the body and smallest branches have a flame bulb
    • Bulb has a tuft of cilia that draws water and solutes from interstitial fluid and moves the urine outward through tubules
    • Dilute urine leaves through nephridiopores and counter balances osmotic uptake of water
  • Has internal openings that collect body fluids
    • Found in annelids like earthworms
    • Each segment of worm has pair of metanephridia
    • Internal opening are surrounded by ciliated funnels (nepthrostome)
    • Fluid enters the nephrostome and passes through a coiled collecting tubule which includes a bladder
  • Have both excretory and osmoregulatory function
    • Produce dilute urine to counter water influx
    • Transport epithelium reabsorbs most solutes and returns them to blood
malpighian tubules
Malpighian Tubules
  • Insects and terrestrial anthropods have malphihian tubules that remove nitrogenous wastes and also osmoregulate
    • Open into digestive tract and dead ends are immersed in hemolymph
    • Transport epithelium secrete solutes (wastes) into tubule
    • Waster follows and fluid passes into rectum
    • Most solutes are pumped back into hemolymph and water follows again
    • Waste is eliminated as nearly dry matter
  • Very effective in conserving water
The kidneys are important excretory and water-regulating organs that conserve or rid the body of water as appropriate in chordates.

Fishes: As with many aquatic animals, most fish release their nitrogenous wastes as ammonia. Some of the wastes diffuse through the gills into the surrounding water. Others are removed by the kidneys, excretory organs that filter wastes from the blood. Kidneys help fishes control the amount of ammonia in their bodies. Saltwater fish tend to lose water because of osmosis. In saltwater fish, the kidneys concentrate wastes and return as much water as possible back to the body. The reverse happens in freshwater fish, they tend to gain water continuously. The kidneys of freshwater fish are specially adapted to pump out large amounts of dilute urine. Some fish have specially adapted kidneys that change their function, allowing them to move from freshwater to saltwater.

Liquid wastes travel through ureters into urinary bladder.

Solid wastes pass from the large intestine into the cloaca.

Liquid and solid waste leave through cloaca and the cloacal vent.

Terrestrial amphibians excrete nitrogenous wastes in the form of urea - less toxic than ammonia and can be concentrated to conserve water.

Urea producedin liver -requires more energy to produce than ammonia.

Urogenital System

Kidneys: Filter Blood

Kidneys lobulated.

Renal arteries receive blood from the renal portal system.

Nitrogenous wastes in the form of ammonia, urea, uric acid or a combination of these.

Crocodilians, snakes and some lizards do not have a urinary bladder. In lizards with a bladder, it is connected to the cloaca by a short urethra.

Urine passes into the cloaca and then into the urinary bladder, if present, or into the distal colon where water resorption occurs.

The cloaca typically consists of 3 chambers.

1. coprodeum 2.urodeum. 3.The caudal proctodeum.

Birds eliminate uric acid with their feces.

Bird droppings is uric acid. Not very toxic and is not very soluble in water.

Uric acid conserves water since it is produced in concentrated form due to its low toxicity.

Due to insolubility and nontoxicity, can accumulate in eggs without damaging the embryos.

Synthesis of uric acid requires more energy than urea synthesis.

There is no urinary bladder in birds.

  • Two major excretory processes - formation of urine and feces.
  • Waste eliminated by urination and defecation.
  • Urine is waste product of urinary system while feces waste products of the digestive system.
  • Feces contain harmful materials.
  • Urine, contains excess water, salt, and protein waste. It seldom carries any pathogens.
Variations in nephron structure and function allow the kidneys of different vertebrates for osmoregulation in various habitats
antidiuretic harmone adh
Antidiuretic Harmone (ADH)
  • ADH is produced in the hypothalamus of the brain and is released from the posterior pituitary gland.
  • Osmoreceptor cells in the hypothalamus monitor the osmolarity of blood.
  • When osmolarity of blood is high:

- when it rises above a set point of 300mosm/L, more ADH is released into the bloodstream. This hormone increases water permeability of the distal tubules and collecting ducts, increasing water reabsorption from the urine (reduces urine volume)

-After consuming water in food or drink, negative feedback decreases the release of ADH.

  • When osmolarity of blood is low:

- very little ADH is released and this decreases the permeability of the distal tubules and the collecting ducts, so water reabsorption is reduced, resulting in increased discharge of dilute urine (diuresis).

- Alcohol inhibits ADH release and can cause dehydration.

  • When osmolarity of blood is high:

- when it rises above a set point of 300mosm/L, more ADH is released into the bloodstream. This hormone increases water permeability of the distal tubules and collecting ducts, increasing water reabsorption from the urine (reduces urine volume)

-After consuming water in food or drink, negative feedback decreases the release of ADH.

renin angiotensin aldosterone system raas
Renin-angiotensin-aldosterone system (RAAS)
  • The juxtaglomerular apparatus (JGA), located near the afferent arteriole leading to the glomerulus, responds to a drop in blood pressure or volume by releasing renin, an enzyme that converts the plasma protein angiotensinogen to angiotensin II.
  • Angiotensin II functions as a hormone and constricts arterioles, stimulates the proximal tubules to reabsorb more NaCl and water, and stimulates the adrenal glandsto release aldosterone.
  • This hormone stimulates Na+ and water reabsorption in the distal tubules.
  • The renin-angiotensin-aldosterone system (RAAS) is a homeostatic feedback circuit that maintains adequate blood pressure and volume.
  • A drop in blood pressure and volume triggers renin release from the JGA.
  • A rise in blood pressure and volume reduce the release of renin.