The kidneys are a pair of bean-shaped organs responsible for filtering blood. Found in the retroperitoneal space, meaning they lie behind the lining of the abdominal cavity, they are protected by the lower ribs. The primary function of these organs is to filter metabolic waste products from the bloodstream and manage the body’s fluid and electrolyte levels. This process produces urine, which is then expelled from the body.
Macroscopic Kidney Anatomy
Externally, each kidney is protected by three layers of tissue. The innermost layer, the renal capsule, is a tough fibrous layer that helps to support the kidney’s soft internal tissue. The second layer is a cushion of fat called the perirenal fat capsule. The outermost layer, the renal fascia, is dense connective tissue that anchors the kidney in place against the posterior abdominal wall. Adult kidneys are typically about 11 to 14 cm in length and weigh between 115 and 175 grams, with the right kidney often sitting slightly lower than the left to accommodate the liver.
When a kidney is viewed in a frontal section, its internal structure reveals three distinct regions. The outer layer is the renal cortex, which has a granular appearance. Beneath the cortex lies the renal medulla, a darker, reddish-brown region. The medulla is composed of several cone-shaped tissue masses known as renal pyramids. These pyramids appear striated because they contain bundles of tiny tubes and blood vessels.
The tips of these pyramids, called renal papillae, point toward the center of the kidney. Each papilla projects into a cup-like structure called a minor calyx, which collects urine produced by that pyramid. Several minor calyces merge to form a larger collecting structure called a major calyx. The major calyces then join to form the renal pelvis, a central, funnel-shaped area that collects urine before it passes into the ureter.
The Nephron as the Functional Unit
The filtering of blood occurs at a microscopic level within structures called nephrons. Each kidney contains over a million of these tiny functional units, located in the parenchyma, the functional tissue of the cortex and medulla. The granular look of the renal cortex is a direct result of the dense packing of these nephrons.
Nephrons are responsible for three main functions: filtration, reabsorption, and secretion, which modify a blood filtrate into urine. There are two types of nephrons, distinguished by their location and the length of their components. Cortical nephrons, making up about 85% of the total, are located primarily in the cortex, while juxtamedullary nephrons have long loops that extend deep into the medulla.
Key Components of the Nephron
Each nephron is composed of two main parts: the renal corpuscle and the renal tubule. The renal corpuscle is the initial filtering component and is located in the renal cortex. It consists of a tuft of tiny capillaries known as the glomerulus, which is enclosed by a cup-shaped structure called the Bowman’s capsule (or glomerular capsule).
Following the renal corpuscle is the renal tubule, a long, coiled tube that processes the filtrate. The first segment is the proximal convoluted tubule (PCT), directly attached to the Bowman’s capsule. The middle section is the loop of Henle, a hairpin-shaped structure that dips into the medulla. The final segment is the distal convoluted tubule (DCT), which empties the refined fluid into a collecting duct.
Blood Supply and Filtration
The kidneys receive a substantial amount of blood, approximately 20-25% of the heart’s output at rest. Blood enters each kidney through the renal artery, which branches directly from the aorta. The renal artery divides into progressively smaller arteries, eventually forming afferent arterioles. Each afferent arteriole supplies blood to an individual nephron, leading into the glomerulus.
The glomerulus acts as a high-pressure filter. As blood flows through this capillary network, water and small solutes are forced out of the blood and into the Bowman’s capsule, forming the initial filtrate. Blood cells and large molecules like proteins are retained in the blood. The filtered blood then exits the glomerulus through the efferent arteriole.
After leaving the glomerulus, the efferent arteriole does not immediately lead to a vein but instead branches into a second capillary network. These are the peritubular capillaries, which surround the proximal and distal convoluted tubules, and the vasa recta, which are associated with the loop of Henle. This network allows for the reabsorption of water and useful substances from the filtrate back into the blood and the secretion of additional waste products into the tubule. Finally, the purified blood is collected into progressively larger veins, exiting the kidney through the renal vein.