Question

Outline how the orientation of nephrons relative to each other imparts gross structure to the kidneys of mammals and birds.

Short answer

The orientation of nephrons in mammals, divided into cortical and juxtamedullary types, creates a layered structure in the kidney evident in a cross-section, featuring an outer cortex and an inner medulla. In contrast, birds have a singleton nephron type with shorter loops of Henle that barely penetrate the medulla resulting in their kidneys lacking a distinct layered structure.

Step-by-step solution

Interpretation of Nephrons

Nephrons are the functional units of the kidney, responsible for filtering blood and removing waste products. They have a distinct structure composed of the glomerulus, proximal convoluted tubule, loop of Henle, distal convoluted tubule and collecting duct.

Orientation of Nephrons in mammals

In mammals, nephrons are categorized into two types: cortical and juxtamedullary nephrons. Cortical nephrons, which constitute 85% of nephrons, are mostly located within the cortex of the kidney. Their loop of Henle is short and extends slightly into the medulla. Juxtamedullary nephrons, however, have much longer loops of Henle that reach deep into the medulla. This arrangement imparts a distinctly layered structure to the kidney and allows mammals to produce concentrated urine.

Orientation of Nephrons in birds

Birds have only one type of nephron, located almost entirely within the cortex, with relatively short loops of Henle that only slightly penetrate the medulla. Due to this orientation, bird kidneys do not exhibit a distinct layered structure. Moreover, unlike mammals, birds can't produce highly concentrated urine.

Overview of Gross Structure

On the basis of these orientations, the gross structure of mammalian kidneys is split into two distinct regions – the outer cortex and the inner medulla which is the reason for layered structure visible in a longitudinal section. In contrast, bird kidneys do not exhibit this layered structure due to their different nephron orientation.

Key concepts

Kidney Structure

The kidney is a vital organ responsible for filtering blood, removing waste, and maintaining water balance in the body. Structurally, a kidney is divided into two main parts: the outer cortex and the inner medulla.
The cortex is the outer layer, where most filtering takes place, while the medulla contains structures that concentrate urine.
These structures include the renal pyramids and columns, which provide support and transport pathways for filtered substances.
This layered appearance is particularly evident in mammalian kidneys due to the specific organization of nephrons within the cortex and medulla.
This structural complexity enables efficient filtration and reabsorption processes, crucial for maintaining homeostasis.

Cortical Nephrons

Cortical nephrons are one of the two basic types of nephrons in mammalian kidneys. They make up about 85% of all nephrons.
These nephrons are primarily located in the kidney's cortex.
Their structure includes a short loop of Henle that just dips into the outer medulla.

• Primary location: Kidney cortex
• Short loop of Henle
• Not deeply involved in producing highly concentrated urine

Despite this, cortical nephrons play a crucial role in the bulk reabsorption of filtered substances.
Because their loop of Henle is shorter, they do not contribute significantly to the kidney's ability to produce hyper-concentrated urine like their juxtamedullary counterpart.

Juxtamedullary Nephrons

Juxtamedullary nephrons are distinct due to their location and functional role in the kidneys. In contrast to cortical nephrons, they have a long loop of Henle that extends deep into the medulla.
This long extension is key to creating a concentration gradient, which is essential for water reabsorption.

• Positioned near the medulla
• Long loop of Henle
• Crucial for urine concentration

Due to their design and placement, juxtamedullary nephrons enable mammals to conserve water efficiently.
This is especially important in arid environments or situations where water conservation is necessary. The presence of these nephrons in significant numbers is one primary reason why the mammalian kidney can produce highly concentrated urine.

Mammalian Kidney

Mammalian kidneys have a characteristic layered structure due to the specific arrangement of nephrons. Unlike bird kidneys, mammalian kidneys contain both cortical and juxtamedullary nephrons.
This dual-nephron system allows for efficient processing and concentration of urine.
The outer cortex is where most of the nephron structures reside, including the glomeruli and proximal tubules.

• Distinct cortical and medullary regions
• Cortical nephrons concentrated in the cortex
• Juxtamedullary nephrons extend into the medulla

This organization ensures that substances like sodium and water can be reabsorbed properly, enabling mammals to maintain fluid balance and excrete waste efficiently.
The ability to produce concentrated urine allows mammals to manage hydration effectively, which is vital for survival in various environments.

Bird Kidney

Bird kidneys differ structurally and functionally from their mammalian counterparts. They have only one type of nephron, located mainly in the cortex, with very short loops of Henle.
This structural feature limits their ability to produce highly concentrated urine.
The absence of a distinct medullary layer imparts a unique simplicity to bird kidneys.

• One nephron type
• Cortical location
• Limited concentration of urine

Birds compensate for this limitation by excreting nitrogenous wastes primarily as uric acid, which requires less water for excretion.
This adaptation is suitable for flight, as it minimizes water loss and reduces overall body weight. Although birds cannot concentrate urine in the way mammals can, they have developed efficient mechanisms to maintain water and electrolyte balance.