Urinary organs and the nephron

The urinary organs and their positions

The two kidneys are bean-shaped organs lying against the back wall of the abdomen on either side of the vertebral column, between about the twelfth thoracic and third lumbar vertebrae. The right kidney sits slightly lower than the left because the liver occupies space above it. Each kidney has a concave side facing inward, called the hilum, where the renal artery, renal vein and ureter enter or leave. From each hilum, a muscular tube called the ureter carries urine downward to the urinary bladder, a hollow muscular sac in the pelvis that stores urine. When it is emptied, urine leaves the body through the urethra. Because the bladder lies in the pelvis, it expands upward as it fills, which is why a full bladder can be felt above the pubic bone.

For example: Kidney pain is usually felt in the flank, the region of the back between the ribs and the hip, because that is where the kidneys lie.

Inside the kidney: cortex, medulla and pelvis

A kidney cut in half shows three regions. The outer cortex is a rim of tissue just beneath the capsule, and it contains the renal corpuscles of the nephrons. The medulla lies deep to the cortex and is divided into triangular structures called renal pyramids, whose tips point inward; the tubules that concentrate urine run through the pyramids. Between the pyramids, extensions of cortex called renal columns pass inward. The innermost space is the renal pelvis, a funnel-shaped cavity that collects urine from the pyramids through cup-like extensions called calyces and continues as the ureter. This arrangement is functional, not just descriptive: filtration happens in the cortex, and the concentration of urine depends on the loops and collecting ducts running through the medulla towards the pelvis.

For example: A kidney stone forming in the renal pelvis can block the flow of urine and cause severe flank pain that radiates toward the groin.

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The nephron: corpuscle and tubule

The nephron is the functional unit of the kidney, and each kidney contains about a million of them. Every nephron has two main parts. The renal corpuscle is a rounded structure in the cortex consisting of a tuft of capillaries called the glomerulus enclosed in a cup-shaped chamber called the glomerular (Bowman's) capsule. Blood enters the glomerulus through an afferent arteriole and leaves through an efferent arteriole, and fluid filtered from the blood collects in the capsule. The renal tubule begins at the capsule and continues as a coiled proximal convoluted tubule in the cortex, then a descending and ascending loop of Henle that dips into the medulla, then a distal convoluted tubule in the cortex again, and finally a collecting duct that passes through the medulla to empty into a calyx. The collecting duct receives several nephrons and is the last place where urine composition is adjusted.

For example: Because the loop of Henle dips into the medulla while the convoluted tubules stay in the cortex, a single nephron spans two kidney regions.

Common misconception: The collecting duct is not part of a single nephron. Several nephrons drain into one collecting duct, which carries urine to a calyx.

Blood supply of the nephron

The nephron has an unusual blood supply because it needs both high pressure for filtration and a second capillary bed for exchange. Blood reaches the kidney through the renal artery, which divides into progressively smaller vessels until an afferent arteriole enters each glomerulus. The glomerulus is a high-pressure capillary bed, and the pressure drives fluid out of the blood into the capsule. Blood then leaves through the efferent arteriole, which is narrower than the afferent vessel and therefore helps maintain that pressure. The efferent arteriole leads to a second capillary network, the peritubular capillaries, which wrap around the tubule, and to the vasa recta, long vessels that run alongside the loop of Henle into the medulla. These capillaries reclaim the water and solutes that the tubule reabsorbs and deliver substances the tubule secretes.

For example: Because the efferent arteriole is narrower than the afferent one, pressure stays high in the glomerulus, which is what makes filtration possible.

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Cortical and juxtamedullary nephrons

Not all nephrons are identical. Cortical nephrons, which make up the majority, have their corpuscles in the outer part of the cortex and relatively short loops of Henle that barely enter the medulla. Juxtamedullary nephrons have their corpuscles close to the medulla and long loops that descend deep into it. This difference matters because the ability to produce concentrated urine depends on the long loops and their associated vasa recta, which build up a gradient of solutes in the medulla. Animals that must conserve water, such as desert species, have a high proportion of juxtamedullary nephrons with very long loops. In humans, the same principle applies: the longer the loops, the more water can be reclaimed and the more concentrated the urine can become.

For example: A desert rodent can produce urine far more concentrated than human urine because its loops of Henle are much longer.