What anatomy is labeled as letter D in the image below? 
Definitions:
ADH Production
The process by which the posterior pituitary gland secretes antidiuretic hormone (ADH), which helps control the balance of water in the body by affecting the kidneys.
Na+
Sodium ions, essential electrolytes in bodily fluids that play key roles in nerve impulse transmission and muscle contraction.
Extracellular Fluid
The fluid that exists outside the cells, providing a medium for the transport of substances and facilitating cellular interactions.
Intracellular Fluid
The fluid contained within the cells, making up about two-thirds of the total body water content and providing a medium for cellular processes.
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