Birds have a unique anatomy compared to other vertebrates like mammals, reptiles, amphibians and fish. One of the major differences is the location of the kidneys. In birds, the kidneys are located much higher up in the body than in mammals. The kidneys play a crucial role in regulating fluid balance, electrolytes, and removing metabolic waste from the blood. Understanding the anatomy and function of avian kidneys is important for veterinarians, ornithologists, and avian hobbyists. This article will examine the location of kidneys in birds and how their position differs from mammals.
Kidney Location in Birds
In birds, the kidneys are embedded in depressions of the synsacrum, which is formed by the fusion of the sacral vertebrae. The synsacrum is located dorsally between the two ilia bones of the pelvis. Specifically, the kidneys lie between the transverse processes of the lumbar vertebrae and the anterior portion of the synsacrum.
This high placement of the kidneys is necessary due to the unique respiratory system of birds. Unlike mammals, birds do not have a diaphragm separating the thoracic cavity from the abdominal cavity. Instead, birds have air sacs distributed throughout their bodies that function in respiration. The dorsal air sacs expand during inspiration and compress the abdominal air sacs, forcing air through the lungs. This requires space in the dorsal abdominal cavity. Consequently, the kidneys cannot be located ventral to the pelvis as they are in mammals and have moved dorsally.
Kidney Structure
The avian kidney retains the basic structure of the reptilian kidney with three divisions – the cranial, middle, and caudal groups of renal lobes. However, structural adaptations allow the avian kidney to function more efficiently.
The lobes contain many thousands of nephrons, the functional units of the kidney. In the cranial group, the nephrons are tightly packed together in a compact mass. The middle group forms a distinct oval-shaped lobe located at the depression between the synsacrum and ilium. The caudal group has variable lobulation depending on the species. Overall, the structural organization of the avian kidney allows large numbers of nephrons to fit into the limited space available dorsally.
Development of the Avian Kidney
In bird embryos, the kidneys initially develop in a similar ventral position as in mammalian embryos. However, as the embryo grows, the kidneys migrate dorsally to take their final position within the synsacrum.
Several factors drive this kidney migration during development:
- Expansion of the liver – As the embryo’s liver enlarges, it displaces the kidneys dorsally.
- Filling of the abdominal cavity by the yolk sac – The yolk sac occupies much of the ventral abdominal space as it supplies nutrition to the embryo, forcing the kidneys to shift dorsally.
- Growth of the synsacrum – The developing synsacrum provides a site for the kidney tissues to integrate into dorsally.
By the time of hatching, the kidney has completely separated from the intermediate mesoderm and taken its fixed position within the synsacrum.
Comparison to Mammals
In mammals, the kidneys are located much lower in the abdominal cavity than in birds. The kidneys sit retroperitoneally – behind the peritoneum on the dorsal body wall of the abdomen.
In humans, each kidney is located on either side of the vertebral column between the levels of T12 to L3 vertebrae. This corresponds to the area between the ribs and pelvis on the posterior abdominal wall. The right kidney sits slightly lower than the left kidney to accommodate the size of the liver.
This ventral kidney position in mammals is possible due to the presence of the diaphragm, which separates the thoracic and abdominal cavities. It allows the abdominal visceral organs like the kidneys to reside lower without compromising respiration.
Avian Kidney Advantages
The dorsal position of the kidneys in birds has several advantages:
- Allows for large kidney size – Being located over the pelvis rather than below it provides more space for the enlarged kidney needed to support high metabolic rates in birds.
- Enables higher blood pressures – Shorter renal arteries allow greater systolic blood pressure to reach the kidneys to support filtration.
- Permits lower blood pressures – Shorter renal veins mean less pressure is needed to return blood to the heart.
- Provides protection – The location within concave depressions in the rigid synsacrum protects the kidneys from trauma.
Mammalian Kidney Advantages
The lower abdominal position of kidneys in mammals also provides some advantages:
- Ease of urinary drainage – Urine can drain down via gravity into the bladder.
- Accessibility – The kidneys are easily accessed surgically due to their ventral position.
- Less constraint on respiration – Lungs have room to expand downwards without the kidneys obstructing breathing.
So while the kidney position differs between birds and mammals, both arrangements confer unique benefits suited to their different anatomies and physiologies.
Clinical Significance
Knowing the normal positioning of kidneys in avian species is clinically relevant for several reasons:
- Performing diagnostic procedures – Understanding location aids in taking samples and doing assays.
- Interpreting imaging – Radiographs must be examined taking the dorsal kidney position into account.
- Assessing function – Expected urine flow and composition depends on kidney anatomy.
- Administering treatment – Therapies rely on kidneys filtering blood from this dorsal position.
- Doing surgery – Any procedures must work around the fixed dorsal kidney placement.
Additionally, being aware that the kidneys develop ventrally and then migrate during embryogenesis provides context for understanding congenital kidney disorders in birds.
Conclusion
The kidneys in birds are located dorsally within the synsacrum, rather than ventrally as in mammals. This position between the vertebral transverse processes and pelvis allows space for the air sacs involved in avian respiration. During development, the embryonic kidneys migrate from the ventral abdomen to take their final place in the synsacrum. The dorsal kidney position provides advantages like protection, blood pressure optimization, and room for large kidneys. However, it also constrains respiration less than in mammals. Knowledge of avian kidney structure and location is clinically relevant for veterinary medicine procedures and evaluating function. Overall, the unique kidneys of birds reflect specializations for avian anatomy and flight.
