The Urinary System

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The Urinary System

The series of organs in the urinary system function to produce, store, and eliminate soluble waste products from the body. The filtrates from the kidney are transported by the ureter to the bladder for storage, and are eliminated by way of the urethra. A major function of the system is to salvage water, sugars, and ions from the filtrate and return them to the blood. In addition, specialized structures within the kidney secrete the vasopressor renin, which increases systemic blood pressure.

Kidney

For orientation, examine R-P, Figs. 20.1 & 20.6. The kidney has a thin, tough connective tissue capsule beneath which lies the glandular tissue. The glandular tissue surrounds a large central cavity called the renal sinus. Adjacent to the renal sinus on the medial side of the organ is a notch called the hilus where the renal vessels and the ureter enter and leave the organ. The glandular portion of the kidney is composed of a number of conical pyramids, the renal lobes. Each renal lobe has its base on the capsule, and its apex (the renal papilla) projects into a minor calyx. The renal lobe consists of two regions, the cortex and the medulla. The cortex has a granular appearance due to the presence of renal corpuscles and renal tubules. These are components of the nephron, the functional unit of the kidney. The medulla has a striated appearance due to the presence of collecting tubules. Structures called medullary rays appear as vertical striations in the cortical substance. Renal columns are projections of cortical tissue between the bases of medullary pyramids.

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Renal Corpuscle

A renal corpuscle is the invaginated blind end of a renal tubule called Bowman's capsule. The capsule is composed of a simple squamous epithelium referred to as the parietal layer. Internally (the visceral layer) is a capillary tuft, the glomerulus, and specialized cells (refer to R-P, Fig. 20.7 & Pls. 71 & 72) that are intimately associated with the capillary tuft. These cells are the podocytes. In H&E stained sections the podocytes are difficult to see but have pale cytoplasm, and lightly stained, slightly larger, oval nuclei. The podocytes extend finger-like processes (pedicels) that wrap around the capillaries. Between the pedicels are basal lamina-covered slits that act as a filter, allowing fluid from the blood to enter the corpuscle. In the electron microscope, podocytes are seen to consist of a central cell body and numerous foot processes applied to the basal lamina of the capillary. Note the pores between the foot processes and fenestrations in the capillary wall (R-P, Figs. 20.8 & 20.11). The other major cell type within the corpuscle is the mesangial cell. These cells are difficult to distinguish from endothelial cells. Between the visceral and parietal layers of Bowman's capsule is Bowman's space, which fills up with the filtrate.

The corpuscle is a polarized structure with a vascular pole and a urinary pole (RP, Fig. 20.7). At the vascular pole, the vessel bringing blood to the glomerulus is called the afferent arteriole, and the vessel taking blood away is called the efferent arteriole. These vessels anchor the glomerulus to the wall of Bowman's capsule.

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Proximal Convoluted Tubule (PT)

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Thin Tubules

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Review of All Urinary System Sections

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