Urine Formation

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Valerie Lovelace Copyright September 2, 2005

Kidneys, Nephrons, and Urine Production

Part of the urinary system, our kidneys are vital organs that serve to remove waste from the bloodstream through ultrafiltration and the formation of urine, and to aide the body in maintaining proper hydration through a process called osmoregulation. Situated to the back of the abdominal wall, the kidneys are snugged up underneath the diaphragm, behind the liver on one side and the stomach on the other, partially shielded in the back by the ‘floating’ ribs. Figure 1 illustrates the basic structure of a human kidney:

Figure 1: Basic Kidney Structure

Structures called nephrons residing in the cortex and medulla produce urine from filtrate removed from the bloodstream, passing it to the bladder via a series of collecting tubules that

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Valerie Lovelace Copyright September 2, 2005

continuously merge, ultimately reaching the renal pelvis and finally the ureter. The question is, “Exactly how is urine produced?” It can hardly pass into the bladder until it actually exists. In order to understand that process, we have to take a deeper look into the structure of the kidney – specifically looking to the nephron and the renal vessels, since this is where waste products leave the blood and enter kidney tissue. There are about one million nephrons in a kidney, each feeding into a nest of collecting ducts. A nephron is a rather intricate structure and it serves two basic purposes: to filter and remove waste products and maintain the body’s water supply. At one end of the nephron, residing in the cortex, is an approximately 0.2mm diameter structure known as the Malpighian Corpuscle, 1 and about 3.0cm away at the other end, a collecting duct. A complicated array of blood vessels intertwine this structure. Figure 2 shows the basic organization of a nephron:

Figure 2: Kidney Nephron

1

http://www.thefreedictionary.com/, Malpighian Corpuscle: 1. A mass of arterial capillaries enveloped in a capsule and attached to a tubule in the kidney. Also called Malpighian body, renal corpuscle; after Marcello Malpaghi. This is considered the ‘closed’ or ‘blind’ end of a nephron.

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Valerie Lovelace Copyright September 2, 2005

Osmoregulation and the production of urine literally keeps the entire organism properly ‘afloat.’ Without correct kidney function, the human body would collapse quite quickly into a highly toxic and unstable state. 2 A simple flow diagram outlining the process can help provide a basic overview and understanding of the process before diving into the details: Bowman's Capsule consists of a bundle of arterial capillaries (glomerulus) surrounded by semi-permeable single-layer walls of flattened epithelial cells. The glomerulus membranes are more permeable than those found in other capillaries. Blood Pressure = 50mmHg Osmotic Pressure = 25mmHg Capsule Tissue Pressure = 10mmHg Unique pressure system within capsule enables ultrafiltration of blood. Net Pressure for Ultrafiltration: 50mmHg - (25mmHg + 10mmHg) = 15mmHg Filtrate consists of water, mineral salts, amino acids, keto-acids, glucose, hormones, creatinine, urea, uric acid, toxins, some types of drugs. Similar in composition to blood plasma. Larger molecules, such as leukocytes, erythrocytes, platelets, plasma proteins, and some larger-molecule drugs remain in the bloodsream. Filtrate is formed at a rate of approximately 125ml per minute(or about 180 litres a day), resulting in the production of approximately 1.0L of urine daily. 99% of the fluid in filtrate is returned to the body. Substances needed by the body are 'recycled' for use to maintain pH, electrolyte levels, and body fluids. Filtrate is selectively reaborbed during passage through convoluted tubules. Foreign substances and other substances not required are secreted as urine.

Arterial blood enters glomerulus of Bowman's Capsule via renal artery and afferent arteriole....
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