BIOLOGY Volume 3 - A Guide to General Biology - 2004

20. EXCRETION AND OSMOREGULATION

20.5. The Human Kidney

In mammals, the Kidneys serve as the primary Organs of excretion and osmoregulation. Their Functions include: 1) the removal of Metabolic waste products from the body; 2) the Regulation of Water content in Body Fluids; 3) The regulation of body fluid pH; 4) the regulation of The chemical composition of body fluids by eliminating substances present in excess of current requirements.

The kidneys receive a rich Blood supply and maintain the COMPOSITION OF THE blood in a steady state. This preserves the optimal composition of tissue fluid and, consequently, of the intracellular Contents of the Cells bathed by it, thereby ensuring their efficient functioning.

20.5.1. Location and Structure of the Kidneys

Humans have a pair of kidneys located against the posterior abdominal wall on either side of the lumbar spine. The left Kidney is positioned slightly higher than the right.

Blood enters the kidneys via the renal Arteries, which branch off the aorta, and leaves via the renal Veins, which drain into the INFERIOR VENA CAVA. Urine produced in the kidneys flows down through two Ureters into the Urinary Bladder, where it accumulates until it is expelled through the Urethra (Fig. 20.11). At the neck of the bladder, the urethra is encircled by two sphincter Muscles, one of which allows for the voluntary control of urination.

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Fig. 20.11. The human excretory system. For clarity, the organs are shown disproportionately large relative to the body outline.

A cross-section of the kidney reveals two distinct zones: the outer cortex and the inner medulla (Fig. 20.12). The renal cortex is covered by a fibrous capsule and contains glomeruli—barely visible to the naked eye—along with their surrounding capsules and other PARTS OF THE nephrons (see below). The medulla consists of tubules, collecting ducts, and Blood Vessels grouped together into renal pyramids. The apices of the pyramids, known as papillae, open into the renal pelvis, which forms the expanded upper end of the Ureter (Fig. 20.12). A dense network of blood vessels permeates the kidneys.

Fig. 20.12. Diagram of a Cytology/practical/54.html">Longitudinal section of the kidney, showing the arrangement of cortical and juxtamedullary nephrons.