Excretory Systems in Animals

Module BL1004: Animal Physiology

• This module focuses on animal physiology, specifically osmoregulation and excretion.
• The professor is Rob McAllen, in the School of Biological, Earth & Environmental Sciences, University College Cork, Ireland.
• Contact information is provided.

Osmoregulation and Excretion

• Animal excretory systems and Chapter 44 of Campbell's Biology (pg 1029) are the focus.
• Physiological systems in animals operate within a fluid environment.
• Maintaining the relative concentration of water and solutes is crucial for normal function..
• Osmoregulation is vital for balancing solute concentrations and managing water gain and loss.
• Freshwater animals have adaptations that limit water uptake and conserve solutes.
• Desert and marine animals face challenging environments that can quickly lead to dehydration.
• Excretion involves removing nitrogenous metabolic wastes and other waste products.

Overview: A Balancing Act

• Animal physiological systems operate in fluid environments which have narrow limits and balances of water and solutes.
• Osmoregulation is a process regulating solute concentrations and balancing water gain and loss.
• Freshwater animals reduce water uptake and preserve solutes.
• Desert and marine animals counteract water loss in dry and salty environments respectively.
• Excretion eliminates nitrogenous and other metabolic waste products.

Osmotic Challenges

• Osmoconformers are marine animals that have osmolarity similar to their surroundings and don't control it.
• Most marine invertebrates are osmoconformers.
• Stenohaline and euryhaline tolerances apply to osmoconformers and regulators.
• Osmoregulators control water uptake in hypotonic and loss in hypertonic environments.
• Marine bony fishes are hypoosmotic to seawater, losing water and gaining salt.
• Fishes balance water loss by drinking seawater and then excreting salts.
• Freshwater animals continuously absorb water and lose salts via osmosis.
• Salts lost are re-acquired from food and gill uptake.

Animals That Live in Temporary Waters

• Some aquatic invertebrates in temporary ponds lose nearly all their body water and survive a dormant state.
• This adaptation is called anhydrobiosis.
• Anhydrobiosis is a survival strategy where the animal becomes dehydrated, and can survive harsh environments.

Osmotic Challenges (Land Animals)

• Land animals manage water budgets by consuming moist food and drinking water.
• Desert animals conserve water through anatomical adaptations and behaviours such as nocturnal or underground lifestyles.

Water balance in two terrestrial mammals

• The comparison shows different water intake, metabolism, and water loss.

Conservation of Water

• Kidneys are adapted to minimize water loss.
• Concentrated urine, like in kangaroo rats, conserves water.
• Droppings from kangaroo rats are significantly drier than other mammals.
• Uric acid requires less water to excrete compared to urea.

Storage of Water

• Animals store water in fatty deposits and tissues (e.g., Gila monster's tail).
• Water is also obtained from the food consumed.
• Example mentioned is the Kangaroo rat's primarily seed-eating diet.

Energetics of Osmoregulation

• Osmoregulators need energy to maintain osmotic gradients.
• Precise control of body fluid composition is vital for cellular function.
• Transport epithelia are critical for solute movement and waste disposal.
• These specialized cells are arranged in complex networks to facilitate these functions.

How do seabirds eliminate excess salt?

• Seabirds, for example, have salt glands that remove excess sodium chloride from the blood.
• Salt glands use a countercurrent exchange mechanism for maximum efficiency in this function.

An animal's nitrogenous wastes

• Nitrogenous wastes reflect evolutionary history (phylogeny) and the environment (habitat) of the animal.
• The type and amount of waste products can greatly affect water balance.
• Examples include ammonia, urea, and uric acid.

Nitrogenous wastes (Ammonia, Urea, Uric Acid)

• Ammonia is toxic and requires lots of water for excretion.
• Urea is less toxic, requiring less water for excretion.
• Most mammals and adult amphibians use urea.
• Uric acid is largely insoluble in water, is excreted as a paste, and conserves water. This is typical in insects and reptiles (and birds).*

Diverse excretory systems

• Excretory systems regulate solute movement between internal fluids and the external environment.
• Most excretory systems create urine by processing filtrate.
• Some functions include filtration, reabsorption, secretion and excretion.

Excretory Systems

• Systems for waste removal vary across animal groups.
• The systems most often involve complex networks of tubules; examples include protonephridia, metanephridia, Malpighian tubules and nephrons.

Protonephridia

• Protonephridia form a network of dead-end tubules connected to external openings.
• Smallest branches are capped by cells (flame bulbs).
• The tubules excrete dilute fluid and are involved in osmoregulation.

Metanephridia

• Earthworms have a pair of open-ended tubules (metanephridia) per segment.
• These tubules collect coelomic fluid to produce dilute urine.
• Metanephridia function in both excretion and osmoregulation as well.

Malpighian Tubules

• In insects and other terrestrial arthropods, Malpighian tubules remove nitrogenous wastes from hemolymph.
• Malpighian tubules open into the digestive tract and create uric acid-based dry waste.
• This adaptation is highly efficient for water conservation in these animals.

Kidneys

• Kidneys are the primary excretory organs in vertebrates.
• Kidneys function in both excretion and osmoregulation.
• Mammalian kidneys are principally responsible for regulating water balance and salt.
• Each kidney has renal arteries for blood supply and renal veins for drainage.
• Urine exits the kidneys via the ureter, passing to a urinary bladder, and out through the urethra.

Kidneys: Nephrons (Functional Units)

• The nephron is a functional unit in vertebrate kidneys, consisting of a long tubule and a capillary network (glomerulus).
• Bowman's capsule surrounds glomerulus and collects filtrate.

Filtration: Glomerulus → Bowman's Capsule

• Filtration occurs due to blood pressure.
• Small molecules like water, salts, glucose, etc. are filtered from blood into Bowman's capsule; the process is not selective.

Pathway of the Filtrate

• Filtrate travels through the proximal tubule, loop of Henle, and distal tubule.
• Multiple nephrons drain into a collecting duct, entering the renal pelvis, and eventually the ureter.

Pathway of the Filtrate (Vasa Recta)

• Vasa recta are capillaries that surround the loop of Henle.
• Vasa recta and loop of Henle work as countercurrent systems for water conservation in the kidney.
• Urine becomes more concentrated than body fluids.

Proximal Tubule

• Reabsorption of ions, water and nutrients occurs in the proximal tubule.
• Active and passive transport processes move molecules into the interstitial fluid and then capillaries.
• Some toxins are also actively secreted into the filtrate.
• Filtrate volume decreases in the proximal tubule.

Descending Limb of the Loop of Henle

• Water reabsorption continues in this section via aquaporin channels.
• High interstitial fluid osmolarity drives water movement out of filtrate.
• Creates increasingly concentrated filtrate as it descends.

Ascending Limb of the Loop of Henle

• Salt reabsorption occurs here with little to no water movement.
• Filtrate gradually becomes more dilute.

Distal Tubule

• This segment regulates the precise K+ and NaCl concentration in body fluids.

Collecting Duct

• The collecting duct carries filtrate through the medulla to the renal pelvis.
• Water, some salts, and urea are reabsorbed.
• Urine becomes hyperosmotic to body fluids (increasingly concentrated).

The Nephron (Summary)

• Human kidneys filter about 180 litres of fluid daily.
• Reabsorption is highly efficient (approximately 99% of water, and nearly all sugars, amino acids, vitamins.)

Adaptations of the Vertebrate Kidney to Diverse Environments

• Nephron form and function are adapted to osmoregulatory needs based on the environment.
• Juxtamedullary nephron contributes to water conservation in terrestrial mammals by having longer loops of Henle (compared to those in freshwater environments).
• Birds and other reptiles conserve water because they excrete uric acid (instead of urea) with shorter loops of Henle.

Acknowledgements

• The majority of the text and power point slides came from Campbell's Biology.
• Dr Ramiro Crego contributed to the content of the slides.