Kidneys Health Article

Definition

The kidneys maintain body fluid volumes and blood pressure, filter blood, and contribute to waste removal by producing urine.

Description

The kidneys are two bean-shaped organs that sit just below the rib cage on either side of the spinal cord. Each is about the size of a bar of soap. At any one time 20–25% of the body's blood flows through them, even though they comprise only 0.5% of the body's total weight. At this rate, the kidneys filter the entire blood supply 60 times per day.

Blood flows into the kidney through the renal artery and exits through the renal vein. Within the kidney are many small capillaries that perfuse it with blood, giving the organ its reddish-brown color.

The gross anatomy of the kidney can be divided into four parts:

• Capsule: A thin but tough outer membrane that protects the kidney against infection and trauma.
• Cortex: The outer layer of the kidney's interior, about 1 cm (0.4 inch) thick.
• Medulla: The inner layer of the kidney's interior, which contains triangular structures called renal pyramids. Between pyramids are sections of cortex called renal columns.
• Renal pelvis: A large funnel for collecting urine from all parts of the kidney, connected to the bladder by the ureter.

Each cortex and medulla together contain about a million nephrons, microscopic filtering systems that are the basic unit of each kidney. Each nephron has two main components. The first is a vascular system that includes1) the glomerulus, 2) afferent and efferent arterioles, and3) peritubular capillaries. The second, tubular component contains five main parts: Bowman's capsule, proximal tubule, loop of Henle, distal tubule, and the collecting duct.

Bowman's capsule forms one end of each nephron. It contains a bundle of tiny capillaries called the glomerulus, which receives its bloodflow from the afferent arteriole. The glomerulus filters minerals, nutrients, wastes, and water from the blood that flows through it, and passes them down into the proximal tubule. The glomerulus also returns large plasma proteins and red blood cells to the blood supply through the efferent arteriole. The efferent arteriole is connected to a second capillary bed called the peritubular capillaries. These two successive capillary beds create a pressure difference that forces fluid through the nephron.

Once filtrate enters the proximal tubule, specialized cells reabsorb sodium and other ions, water, glucose, and amino acids back into the blood. The fluid then goes into the loop of Henle, which helps concentrate the waste products to be excreted in urine. After the loop of Henle, fluid then flows into the highly coiled distal tubule, where potassium is secreted and more water and sodium are reabsorbed back into the blood. The fluid then flows into the last part of the nephron, the collecting duct, where final adjustments are made to the urine concentration. The collecting ducts respond to the antidiuretic hormone (ADH), which regulates the amount of water reabsorbed by the blood. The urine then flows through the renal pelvis to the ureter, which delivers urine to the bladder for excretion.