What are They?

The kidneys are bean-shaped, reddish brown paired organs, concave on one long side and convex on the opposite. They are normally located high in the abdominal cavity and against its back wall, lying on either side of the vertebral column between the levels of the 12th thoracic and third lumbar vertebrae, and outside the peritoneum, the membrane that lines the abdomen.

The long axes of the kidneys are aligned with that of the body, but the upper end of each kidney (pole) is tilted slightly inward toward the backbone (vertebral column). Situated in the middle of the medial concave border is a deep vertical cleft, the hilus, which leads to a cavity within the kidney known as the renal (kidney) sinus. The hilus is the point of entry and exit of the renal arteries and veins, lymphatic vessels, nerves, and the enlarged upper extension of the ureters.

Renal vessels and nerves

The renal arteries arise, one on each side, from the abdominal aorta at a point opposite the upper border of the second lumbar vertebra (i.e., a little above the small of the back). Close to the renal hilus each artery gives off small branches to the adrenal gland and ureter and then branches into anterior and posterior divisions. The large veins carrying blood from the kidneys usually lie in front of the corresponding arteries and join the inferior vena cava almost at right angles. The left vein is longer than the right vein because the inferior vena cava lies closer to the right kidney.

The kidneys are supplied with sympathetic and parasympathetic nerves of the autonomic nervous system, and the renal nerves contain both afferent and efferent fibres (afferent fibres carry nerve impulses to the central nervous system; efferent fibres, from it).

Internal configuration

A cross section of a kidney reveals the renal sinus and two layers of kidney tissue distinguishable by their texture and colour. The innermost tissue, called the renal medulla, forms comparatively dark cones, called renal pyramids, with bases outward and apexes projecting, either singly or in groups, into the renal sinus. Each projection of one or more pyramid apexes into the sinus is known as a renal papilla. The bases of these pyramids are irregular, with slender striations extending toward the external kidney surface. The paler, more granular tissue external to the medulla is the cortex. It arches over the bases of the pyramids and fills gaps between the pyramids. Each group of pyramids that projects into a papilla, together with the portion of cortex that arches over the group, is called a renal lobe.

The renal sinus includes the renal pelvis, a funnel-shaped expansion of the upper end of the ureter, and, reaching into the kidney substances from the wide end of the funnel, two or three extensions of the cavity called the major calyxes. The major calyxes are divided in turn into four to 12 smaller cuplike cavities, the minor calyxes, into which the renal papillae project. The renal pelvis serves as the initial reservoir for urine, which flows into the sinus through the urinary collecting tubules, small tubes that open into the sinus at the papillae.

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