Kidney Structure

The Human Kidney Structure and Function

     One of the most complex, beautifully “engineered” organs of the human body, the kidneys perform several essential tasks including the excretion of waste products, the maintenance of homeostatic balance in the body and the release of important hormones. To achieve this, human kidneys have a highly developed, superbly refined anatomy and physiology.

Location and Basic Structure of the Kidneys

The kidneys are located near the vertebral column at the small of the back; the left kidney lying a little higher than the right. Each is identical in structure and function. They are bean-shaped, about 10 cm long and 6.5 cm wide. Each kidney comprises an outer cortex and an inner medulla. The kidney is supplied with oxygenated blood via the renal artery and drained of deoxygenated blood by the renal vein. In addition, urine produced by the kidney as part of its excretory function, drains out via narrow “tubules” and the ureter, in turn connected to the bladder.

The Nephron

The main functional unit of the kidney is the nephron. There are approximately one million nephrons per kidney. The role of nephrons is to make urine by:

• Filtering blood of small molecules and ions such as water, salt, glucose and other solutes including urea. Large “macromolecules” like proteins are untouched.
• Recycling the required quantities of useful solutes which then re-enter the bloodstream. (A process called reabsorption)
• Allowing surplus or waste molecules/ions to flow from the tubules/ureter as urine.

Filtration and Reabsorbtion in the Kidneys

During progress through the nephron, some solutes like sodium chloride, potassium and glucose are reabsorbed, along with water, back into the bloodstream. This maintains a correct balance of these chemicals within the blood, assisting blood pressure regulation, for example. The filtration and reabsorbtion of glucose within the kidneys also helps to maintain correct levels of vital blood sugars. When this regulation breaks down very serious health consequences can follow.

Urea and uric acid are nitrogen containing waste products from metabolic processes in the body. These substances are potentially toxic and are partially excreted by the kidneys to maintain good health. Interestingly, of the filtrate which enters each nephron from the blood, only about 1% actually leaves the body as urine because of the various reabsorbtion mechanisms driven by osmosis, diffusion, and active transport.

Tubular Secretion in the Kidneys

Another, less familiar, mechanism for urine production in the kidneys is tubular secretion. Specialised cells move solutes directly from the blood into the tubular fluid. For example, hydrogen and potassium ions are secreted directly into the tubular fluid. This process is “coupled” or balanced by the re-uptake of sodium ions back into the blood.

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Tubular secretion of hydrogen ions, augmented by control of bicarbonate levels, is important in maintaining correct blood pH. When the blood is too acidic (acidosis) more hydrogen ions are secreted. If the blood becomes too alkaline (alkalosis), hydrogen secretion is reduced. In maintaining blood pH within normal limits (about 7.35–7.45) the kidney can produce urine with pH as low as that of acid rain or as alkaline as baking soda!

The Kidney as an Endocrine Gland

In addition to its excretory and homeostatic roles, the kidneys also release two important hormones into the blood. These are:

• Erythropoietin which acts on bone marrow to increase the production of red blood cells
• Calcitriol which promotes the absorption of calcium from food in the intestine and acts directly on bones to shift calcium into the bloodstream.

Finally the kidney produces the enzyme renin, an important regulator of blood pressure.