Excretion Physiology in Birds

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Dr. Amar Chaudhary, ^{DVM, MS}

Assistant Professor

Department of Physiology & Biochemistry

Agriculture and Forestry University,

Rampur, Chitwan, Nepal

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Excretion Physiology in Bird

The avian excretory system consists of

• Kidneys
• Ureters
• Cloaca
• Ureters and Urodeum (part of the cloaca)
• Kidneys
• Birds have paired kidneys divided into cranial, middle and caudal divisions
• located near the backbone, close to the synsacrum (fused part of the vertebral column)
• Each contains numerous nephrons, cortical and medullary regions, with a relatively smaller medulla compared to mammals
• Unlike mammals, birds have both reptilian-type nephrons (lacking loops of Henle) and mammalian-type nephrons (with loops of Henle)
• The primary function is filtration of blood to remove nitrogenous waste, and osmoregulation

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Excretion Physiology in Bird

• Ureters
• The ureters are long tubes that transport urine from the kidneys to the cloaca
• The ureters are unique in that they are capable of reabsorbing water from urine before it reaches the cloaca, conserving water in the process
• Cloaca
• The cloaca is a common chamber where the excretory, digestive, and reproductive systems empty
• It is divided into three parts:
• Proctodeum: Stores faecal matter
• Urodeum: Receives urine and reproductive fluids
• Coprodeum: Receives the digestive waste
• Play a role in the reabsorption of water and electrolytes from urine
• The vent is the external opening through which excretory products are expelled

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Excretion Physiology in Bird

• Filtration
• Blood enters the kidneys via the renal arteries, and the renal corpuscles (similar to glomeruli in mammals) filter out waste products
• Due to the reduced size of the glomerulus and the absence of renal pyramids in the kidneys minimal filtration in the renal corpuscles compared to mammals
• Reabsorption
• Proximal Convoluted Tubule (PCT):
• Active reabsorption of essential molecules like glucose, amino acids, and electrolytes (e.g., sodium and potassium).
• Loop of Henle:
• Plays a crucial role in water reabsorption, particularly in desert-adapted birds
• The loop's relative length varies depending on the species and its habitat
• Distal Convoluted Tubule (DCT):
• Regulates the final composition of urine, adjusting for electrolytes and acid-base balance

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Excretion Physiology in Bird

• Excretion of Uric Acid
• Birds primarily excrete nitrogenous waste in the form of uric acid
• Uric acid is excreted as a paste or solid, which minimizes water loss
• This is particularly beneficial in environments with limited water resources
• Uric acid is formed in the liver, and its excess is transported to the kidneys for filtration
• Role of Ureters and Cloaca in Water Conservation
• Water is reabsorbed by the ureters, and cloaca and this is a critical function for water conservation, especially in avian species in hot or arid environments
• Semiliquid with white precipitated uric acid play role in water conservation

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Water and Electrolyte Balance

• Birds are highly adapted for maintaining water and electrolyte balance

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• Birds have a high metabolic rate and produce significant amounts of waste, but they must conserve water

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• Specialized glands, such as the salt glands (located near the eyes), help to excrete excess salt, especially in seabirds

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• Salt Glands
• Found in marine birds (e.g., seagulls, penguins) that consume salty seawater
• These glands concentrate the salt and excrete it through a duct near the beak, allowing the bird to drink seawater without becoming dehydrated

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Physiological adaptations in different bird species

• Desert Birds:
• Birds like the sandgrouse or doves are adapted to conserve water.
• Their kidneys are highly efficient in reabsorbing water, and their urine is highly concentrated
• Waterfowl:
• Birds like ducks and swans have specialized excretory systems that allow them to excrete excess water consumed from their aquatic environment
• Seabirds:
• Species like pelicans, seagulls, and penguins possess well-developed salt glands to excrete excess salt from seawater

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Disorder of the avian system

• Renal Failure:
• In birds, kidney failure can occur due to infections, toxins, or dehydration
• Early symptoms may include changes in excrement consistency, lethargy, or dehydration
• Urolithiasis:
• Formation of uric acid crystals or stones in the kidneys or ureters can block the flow of urine
• It is more common in captive birds due to imbalanced diets
• Gout:
• A condition associated with excess uric acid, which can accumulate in the joints, kidneys, and soft tissues
• Kidney dysfunction can lead to the accumulation of uric acid, causing gout (visceral or articular)
• Gout is a common condition in birds and is often associated with high-protein diets or renal failure
• Dehydration:
• Birds are highly susceptible to dehydration due to their efficient water conservation mechanisms
• Clinical signs include lethargy, sunken eyes, and dry mucous membranes.
• Salt Toxicity:
• In birds without functional salt glands, excessive salt intake can lead to electrolyte imbalances and dehydration

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Difference in Urine Formation Between Mammals and Birds

Nitrogenous Waste Excretion

• Mammals:
• Urea is the primary nitrogenous waste in mammals
• Urea is highly soluble in water, which means it can be easily excreted through urine.
• Urea is formed in the liver via the urea cycle (also known as the ornithine cycle)
• The kidneys play a major role in excreting urea efficiently
• Birds:
• Uric acid is the primary nitrogenous waste in birds
• Uric acid is relatively insoluble in water, which allows birds to conserve water by excreting it as a paste or solid
• Uric acid is formed in the liver from ammonia and carbon dioxide and then transported to the kidneys for excretion

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Difference in Urine Formation Between Mammals and Birds

Mammals:

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• Urine Formation Process:
• Filtration:
• Blood is filtered in the glomerulus, where water, salts, glucose, amino acids, and nitrogenous waste (urea) are filtered out
• Reabsorption:
• Most water and essential molecules (e.g., glucose, amino acids, electrolytes) are reabsorbed in the proximal convoluted tubule (PCT)
• Loop of Henle:
• Plays a major role in the concentration of urine
• The descending loop is permeable to water, while the ascending loop actively transports sodium and chloride out
• This creates a concentration gradient that allows for water reabsorption
• Excretion:
• The remaining filtrate, which is now concentrated, is excreted as liquid urine via the distal convoluted tubule (DCT) and the collecting ducts
• Water Conservation:
• Mammals with a longer loop of Henle (e.g., desert species) can concentrate urine more effectively to conserve water.

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Birds

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• Urine Formation Process:
• Filtration:
• Blood is filtered through the renal corpuscle, but less filtrate is produced compared to mammals due to the smaller glomerulus
• Reabsorption:
• Similar to mammals, the proximal convoluted tubule (PCT) reabsorbs most essential nutrients, and water is reabsorbed along the ureter
• Ureters and Urodeum:
• The ureters are capable of absorbing water back into the system before the waste reaches the cloaca, conserving water
• Excretion:
• Instead of producing liquid urine, birds excrete uric acid as a paste or solid, reducing water loss.
• The uric acid is largely retained in the ureter before being passed into the cloaca.
• This conserves water, which is vital for survival, especially in arid regions.
• Water Conservation:
• Birds have highly efficient mechanisms for conserving water, with the ability to concentrate uric acid in their kidneys and reabsorb water along the ureters

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Difference in Urine Formation Between Mammals and Birds

• Urine Composition
• Mammals:
• Urine is primarily composed of urea, water, electrolytes (e.g., sodium, potassium), and waste products such as creatinine, uric acid, and other metabolic by products
• Urea is highly soluble in water, so it is excreted in liquid form
• Birds:
• Urine is excreted as a paste or semi-solid mass containing uric acid and only a small amount of water
• The solid excretion helps birds conserve water while still removing nitrogenous waste

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Difference in Urine Formation Between Mammals and Birds

Osmoregulation and Concentration Mechanism

• Mammals:
• Loop of Henle plays a significant role in creating a concentration gradient in the kidney medulla, allowing for the production of concentrated urine
• Desert mammals have a longer Loop of Henle to maximize water reabsorption, producing highly concentrated urine with a low volume
• Birds:
• Birds have a shorter loop of Henle compared to mammals, which means their kidneys are less capable of concentrating urine
• To compensate, birds have evolved the ability to reabsorb water through the ureters as urine moves toward the cloaca, significantly reducing the water content of excretory products.

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�Unique Features of the Avian Excretory System�

• Kidney Structure
• Compact Kidneys:
• The avian kidney is relatively small and lacks the extensive renal pyramids seen in mammals
• Instead, it has more compact nephrons with relatively short loops of Henle, making the ability to concentrate urine less efficient than in some mammals
• Renal Papillae:
• The avian kidney is composed of a continuous system of ducts - whereas in a mammals the collecting ducts terminate at the renal papilla, urine is collected in the pelvis, and this drains into the ureter. 
• There is no renal pelvis in birds or reptiles; therefore, they cannot develop pyelonephritis
• Nitrogenous Waste:
• Unlike mammals, which primarily excrete urea, birds excrete uric acid
• Uric acid is insoluble in water, which allows it to be excreted as a paste or solid rather than a liquid
• This form of nitrogenous waste excretion is highly advantageous in conserving water, especially in species that live in arid or water-scarce environments

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Unique Features of the Avian Excretory System

Ureters

• The ureters in birds are long tubes that transport urine from the kidneys to the cloaca
• Unlike mammals, the ureters in birds play a more significant role in water reabsorption before excretion
• Water Reabsorption:
• The ureters are able to reabsorb water from the excretory fluid before it enters the cloaca, conserving valuable water.
• Cloaca (Urodeum) allows for the reabsorption of water from the waste products before they are excreted through the vent
• This is particularly important for birds that need to conserve water
• Salt Glands (in Some Species)
• Marine and some desert-adapted birds have salt glands, located near the eyes or around the beak.
• These glands help birds to excrete excess salt that they consume from seawater or in their diet, allowing them to drink seawater without becoming dehydrated

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Post-Renal Modifications in the Avian Excretory System

• In birds, post-renal modification refers to the physiological processes essential for water conservation, electrolyte regulation, and efficient waste removal
• Unlike mammals, birds have a cloaca, a common chamber where the digestive, urinary, and reproductive tracts converge
• The cloaca plays a critical role in modifying the composition of urine before excretion, particularly in terms of water and electrolyte reabsorption
• This adaptation is essential for water conservation, especially in birds that live in arid environments or have limited access to freshwater
• The primary post-renal modification in birds is the formation of uric acid paste
• This paste-like substance minimizes water loss, as uric acid is excreted in an insoluble form
• The relatively small amount of water present in the paste allows birds to excrete their nitrogenous waste with minimal fluid loss
• The cloaca also serves to further reabsorb water from the excretory products
• This is a final step in minimizing water loss from the waste before it is eliminated from the body

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Mechanism of Post renal Modification

• Water Reabsorption:
• Birds have a high capacity for water reabsorption in the cloaca, which is crucial for conserving water
• The cloacal epithelium contains transport proteins that actively reabsorb water and electrolytes.
• Electrolyte Reabsorption:
• Sodium, chloride, and potassium ions are reabsorbed in the cloaca to maintain electrolyte balance
• This process is regulated by hormones such as aldosterone and antidiuretic hormone (ADH).
• Uric Acid Precipitation:
• Uric acid, which is relatively insoluble in water, precipitates out of the urine in the cloaca
• This results in the formation of a semi-solid paste, which is excreted along with faecal matter

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Post-Renal Modifications in the Avian Excretory System

Salt Glands (in Marine Birds)

• Marine Birds:
• In species like seagulls and pelicans, excess salts from seawater are excreted through specialized salt glands
• These glands are located near the eyes or beak and help to excrete hypertonic solutions of salt, enabling the bird to drink seawater without becoming dehydrated
• The salt glands are a crucial adaptation for birds living in coastal or marine environments where drinking seawater is necessary for survival

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