The kidneys process an extraordinary volume of blood daily to remove waste and maintain fluid balance. Receiving about 20% of the heart’s total output, these organs rely on a highly specialized and structured blood supply to perform their function efficiently. The unique arrangement of the renal vasculature—a sequence of arteries, capillaries, and veins—allows for the precise filtering and reclaiming of substances. This structure enables the kidney to filter approximately 150 quarts of blood daily while only producing 1 to 2 quarts of urine.
Entry Point: High-Pressure Arterial Supply
The journey begins with the renal artery, a large vessel that branches directly off the abdominal aorta, carrying oxygenated blood under high systemic pressure toward the kidney. Inside the organ, the renal artery divides into several segmental arteries, which then further divide into the interlobar arteries that ascend between the renal pyramids.
As they reach the boundary between the outer cortex and the inner medulla, the interlobar arteries arch over the bases of the pyramids, becoming the arcuate arteries. From these arcuate arteries, smaller vessels called cortical radiate arteries (also known as interlobular arteries) project outward into the cortex. The final branch of this arterial tree is the afferent arteriole, which delivers blood to the functional unit of the kidney, the nephron.
The Glomerulus: Filtration and Resistance
The blood arrives at the nephron’s filtering component, the renal corpuscle, via the afferent arteriole. This arteriole supplies a dense ball of capillaries known as the glomerulus, which is encased within Bowman’s capsule. The glomerulus is the primary site of filtration, where fluid and small solutes are forced out of the blood plasma into the capsule space to form filtrate.
Filtration is driven by the high hydrostatic pressure within the glomerular capillaries. This pressure is maintained because the efferent arteriole, which drains the glomerulus, is noticeably narrower than the afferent arteriole supplying it. This difference creates resistance to outflow, generating the necessary force to push fluid through the filtration membrane.
The result is ultrafiltration, a passive process where about 20% of the plasma volume is filtered into Bowman’s capsule. The blood exiting the glomerulus through the efferent arteriole remains oxygenated, having lost fluid and small solutes. Non-filterable components, such as blood cells and large proteins, remain in the highly concentrated blood.
Post-Filtration: The Peritubular Exchange Network
After leaving the high-pressure environment of the glomerulus, the concentrated blood flows into a low-pressure capillary network. This network is critical for the two-way exchange of substances between the blood and the renal tubules. For the majority of nephrons in the outer cortex, the efferent arteriole branches into a web of peritubular capillaries that densely surround the proximal and distal convoluted tubules.
The low pressure and high colloid osmotic pressure of the blood entering these capillaries facilitates the reabsorption of valuable substances. Essential materials like glucose, amino acids, and the majority of water are reclaimed from the filtrate and returned to the bloodstream through this network.
For the small percentage of nephrons that extend deep into the inner medulla, the efferent arteriole forms specialized vessels called the vasa recta. These vessels run parallel to the loop of Henle and are essential for maintaining the osmotic gradient necessary for the kidney to concentrate urine.
Exit Route: Venous Collection and Return
The blood, now purified and having regained reclaimed materials, begins its final transition from the capillary beds into the venous system. The peritubular capillaries and vasa recta first converge into small venules, which then drain into the cortical radiate veins. These small veins follow a path that mirrors the arterial supply in reverse.
The cortical radiate veins merge to form the arcuate veins, which arch over the renal pyramids. From there, the blood flows into the interlobar veins, passing between the renal columns. These larger vessels ultimately converge to form the single, large renal vein. The renal vein carries the filtered, deoxygenated blood away from the kidney and empties it directly into the inferior vena cava, completing the vascular loop.