Osmoregulation and Excretion

Osmoregulation and Excretion

• Animal physiological systems operate in a fluid environment. Water and solute concentrations must be balanced.

• Osmoregulation controls solute concentrations and balances water gain and loss. Excretion rids the body of nitrogenous and other waste products.

• Homeostasis is the maintenance of internal environment regardless of external environment fluctuations above or below a set point are detected by a sensor, triggering a response to return the body to the set point.

• Water enters and leaves cells by osmosis.

• Osmolarity (solute concentration) determines the movement of water across a selectively permeable membrane.

  • Isoosmotic - equal osmolarity, water molecules cross the membrane equally in both directions.
  • Hypoosmotic - low osmolarity, water moves out of the cell.
  • Hyperosmotic - high osmolarity, water moves into the cell.

Animal Osmoregulation

• Osmoconformers are isoosmotic with their surroundings and do not regulate osmolarity.
• Osmoregulators expend energy to control water uptake and loss in a hyperosmotic or hypoosmotic environment.

Osmoregulation in Marine Animals

• Most marine invertebrates are osmoconformers. Marine bony fishes are hypoosmotic to seawater.
• They balance water loss by drinking large amounts of seawater and eliminating the ingested salts through their gills and kidneys.

Osmoregulation in Freshwater

• Freshwater animals constantly take in water by osmosis from their hypoosmotic environment.
• Water balance is achieved by drinking almost no water and excreting large amounts of dilute urine.
• Salts lost by diffusion are replaced in foods and by uptake across the gills.

Osmoregulation on Land

• Adaptations to reduce water loss are key to survival on land.
• Body coverings of terrestrial animals help prevent dehydration.
• Desert animals get major water savings through anatomical features and behaviors such as nocturnal lifestyles.
• Land animals maintain water balance by eating moist food and producing water metabolically through cellular respiration.
• Other adaptations such as kangaroo rats concentrate their urine (12-15x more concentrated than human blood plasma; ~4x in humans).
• Camels tolerate a 7°C increase in body temperature, reducing water loss from sweat. They can lose 25% of their water and still survive.

Energetics of Osmoregulation

• Osmoregulators must expend energy to maintain osmotic gradients.
• The amount of energy differs based on how different the animal's osmolarity is from its surroundings, how easily water and solutes move across an animal's surface, and the work required to pump solutes.
• Osmoregulators maintain osmotic gradients using active transport to manipulate solute concentrations.

Transport Epithelia

• Transport epithelia are specialized cells for moving solutes in specific directions. They are typically arranged into complex tubular networks that lead to the external environment (e.g., kidneys, salt glands in birds, sea turtles).
• Some animals forcefully eject excess salts.

Excretion

• Excretion is ridding the body of toxic metabolites (ammonia) produced from breaking down nitrogenous (nitrogen-containing) molecules.

• Nitrogenous wastes include ammonia, urea, and uric acid converted from toxic ammonia through different processes

  • Ammonia is extremely toxic, and its excretion requires lots of water.
  • Urea is less toxic than ammonia, requiring less water for excretion; however it is energy costly.
  • Uric acid is non-toxic, requires little water & most energy

• Hydrostatic pressure (blood pressure) in animals drives the process of filtration.

• Excretion occurs in a 4 step process:

• Filtration (blood filtered into excretory tubule)

• Reabsorption (valuable substances are reabsorbed from the filtrate into the blood)

• Secretion (extra substances, such as toxins and excess ions, are added to contents of tubule) Excretion (altered filtrate)

Protonephridia

• found in flatworms, some annelids, mollusc larvae, and lancelets)
• A pair of these nephridia are found in each segment of an annelid)

Malpighian Tubules

• Insects and other terrestrial arthropods have Malpighian tubules that remove nitrogenous wastes as well as function in osmoregulation

Kidneys

• Kidneys are used in osmoregulation and excretion. They filter blood and generate urine and are found in vertebrates.

• Tubules in the kidneys are closely associated with a network of capillaries. The ducts carry the urine from tubules out of the kidneys

• Kidneys in mammals are important for homeostasis (endocrine and urinary systems). They filter blood for nitrogenous waste and contain approximately 1 million nephrons (excretory tubules) consisting of a long tube and capillary ball (glomerulus).

• Nephrons (functional unit of the kidneys) have 3 processes:

  • Filtration (forces blood fluid to Bowman's capsule)
  • Reabsorption (water and minerals actively and passively transported to blood)
  • Secretion (H and K ions transported to tubules in proximal and distal tubules)

• Filtrate becomes urine as it moves through the nephron.

• Countercurrent multiplier system actively transports NaCl to regulate osmolarity

• Kidneys are categorized as cortical (most nephrons, filter wastes) and juxtamedullary (long, conserve water, regulate ion concentrations)