Renal System 1, Kidneys

Components

The urinary system consists of the organs and structures which produce, transport and store urine. Urine is produced in the kidneys and transported to the urinary bladder via the right and left ureter. From time to time the bladder is emptied and urine is voided from the body via the urethra. The urinary system describes the whole of the kidney, ureter, bladder and urethra system but the term renal is specific to the kidneys.

The kidneys

Normally more water and ions are ingested than are required to meet the requirements of homeostasis. Ions include potassium from fruit and sodium and chloride from salt. In order to prevent the body becoming overloaded with water and ions the excess must be excreted. The way water and ions are eliminated from the body is to incorporate them into urine which is then voided. This means the kidneys are responsible for regulation of the volume as well as the composition of body fluids.

In addition the kidneys excrete waste products from metabolic activity. Metabolic biochemical processes are carried out by all living cells. As a result of this biochemical activity, waste products are produced. If these are allowed to accumulate in body fluids they will soon reach toxic levels. The kidneys are able to isolate and concentrate waste products in urine, which is then excreted from the body.

There are two kidneys located high in the abdominal cavity, on the posterior abdominal wall. Kidneys are dark red bean shaped organs. The right kidney is a little lower than the left due to the presence of the liver above. Because of the domed nature of the diaphragm the upper part of the kidneys receive some protection from the lower ribs. In addition the kidneys are embedded in and completely surrounded by a protective layer of perirenal fat. Surrounding the perirenal fat is a further protective layer of connective tissue called the renal fascia.

Macroscopic renal structure

Surrounding the outer surface of each kidney is a layer of collagen rich fibrous tissue which comprises the renal capsule. This is a thin but tough layer of supportive connective tissue which contains many pain receptors (nociceptors). If swelling occurs within a kidney this capsule will be stretched causing pain. It also explains why it is so painful if we are poked in the kidneys.

The outer layer of the kidney itself is called the cortex. Under the cortex is a layer referred to as the medulla. The renal medulla is composed of structures called renal pyramids. Numbers of renal pyramids vary between individuals; there are normally 5 to 11 per kidney. Between the renal pyramids of the medulla are projections from the cortex referred to as renal columns. The inner area of the kidney is called the renal pelvis and is continuous with the ureter.

An arrangement of minor and major calyces connects the renal medulla with the renal pelvis. Calyces are essentially branches of the renal pelvis. Minor calyces form around the apex of the pyramids of the medulla and merge into larger calyces which then expand to form the pelvis.

At the centre of the concaved side of each kidney is the region called the renal hilum. A hilum is a relatively small area where blood vessels, nerves and lymphatic vessels enter and leave an organ. The inner surface of the hilum is called the renal sinus; this is a hollow area within the kidney which contains the renal pelvis. It is via the hilum that the arteries, veins, lymphatics, nerves and ureters enter and leave the kidneys. There are two adrenal glands, located one on top of each kidney; this is why these structures used to be called the suprarenal glands. Despite their position these endocrine glands do not have any direct anatomical connections with the kidneys. There is usually a thin layer of connective tissue separating the kidneys from the adrenal glands.

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