When You Gotta Go
The principal function of the urinary system is to remove toxins from the plasma of the blood and eliminate them from the body in the form of urine. In doing so, the kidneys filter a huge volume of plasma from the blood daily (around 180 liters of filtrate per day). However, all but approximately 1.5 liters (L) of fluid are returned to the body. The remainder becomes urine.
Depending upon the volume of urine produced versus fluid retained, the kidneys also function to regulate blood pressure by altering the fluid volume of the blood. Additionally, the kidneys reabsorb all of the essential components from the filtrate and return it to the blood stream. Among those elements are proteins and carbohydrates (glucose). All things considered, the kidneys actually are organs of conservation that also play a minor, yet critical, role in toxin elimination.
Kidneys
The bilateral kidneys are bean-shaped organs located in the lower lumbar region of the abdomen. Each receives a supply of blood from a single renal artery and a single renal vein returns the blood to the inferior vena cava. A continuous supply of blood is essential for kidney function, because urine production begins with filtered blood plasma entering the kidney tubules where the plasma is modified, concentrated, and excreted as urine.
Anatomy
The kidney has an indention (hilum) facing the midline of the body and a convex surface facing the latter part of the abdominal cavity. At the hilum, blood vessels enter and leave, as does the ureter, which is the tube that transfers urine from the kidney to the bladder. With two kidneys servicing the body, an individual may lose one kidney and the remaining organ will be sufficient for survival.
Blood Flow
The kidney is organized into an outer cortex and an inner medulla, much like those of organs such as the adrenal gland. From the renal artery, interlobar arteries branch and extend upward through the medulla until the boundary of the medulla and cortex is reached. At this point (the juxtamedullary zone), arcuate arteries branch and follow the curvature of this zone, which mimics the convex curvature of the kidney itself. Perpendicular branches from the arcuate artery course upward into the cortex as interlobular arteries. Here, branches called afferent glomerular arterioles provide blood to the capillary bundle called the glomerulus where the plasma is filtered from the blood and urine production begins.
While in a typical circulatory system venules follow capillaries, in the kidney, the efferent glomerular arteriole follows the capillaries of the glomerulus. It supplies a network of peritubular capillaries that carry out internal respiration for all the cells of the kidneys. This begins in the cortical region of the kidney as long straight extensions of vessels coursing first downward into the medulla as the arteria recta (straight arterioles) before returning upward into the cortex via the vena recta (straight venules). These two straight vessels compose the vasa recta component of the kidneys and extend peritubular capillaries between the two straight vessels, hence the conversion from arteriole to venule.
The venous blood is now drained into an interlobular vein with the remaining vessels removing blood from the kidneys mirroring their arterial counterparts (arcuate veins, interlobular veins, and the renal vein).
Renal Pyramids
The interior of the kidney is divided into units called renal pyramids. Essentially, the tubules and cells of the kidney are arranged as triangles with the apex of each pointing downward toward the hilum. In this way, as urine is produced, it is funneled to this single point where it leaves the kidney via the ureter. Each of the 10–12 pyramids end at their tip (renal papilla) and empty the urine into the first of 3 cone-shaped funnels called a minor calyx. Several minor calyces then empty into 3–5 larger major calyces that are drained by a single large funnel, the renal pelvis, which funnels urine to the ureter.
Nephrons
The network of tubes that transport and modify the plasma-derived filtrate into urine works its way through the cortex and medulla of the kidneys. At the beginning of the tubules is a capsule that surrounds the glomerular capillaries and captures the filtered plasma as it leaves the vessels and enters the tubules.
The glomerulus is the cluster of capillaries where plasma from the blood leaves and enters the initial space of a nephron. Bowman’s capsule (glomerular capsule) surrounds the glomerulus to trap the filtered fluid in the space between the inner layer (visceral layer) and the outer layer (parietal layer) of the capsule. You can envision this arrangement by taking your fist and placing it in the palm of your other hand with your fingers over the closed fist. The fist is similar to the glomerulus and the covering hand and fingers represents Bowman’s capsule. Together, the glomerulus and its associated capsule are termed a renal corpuscle and are only found in the cortex of the kidney.
Glomerular filtrate will flow through Bowman’s space and into the beginnings of the proximal convoluted tubule (PCT) at the urinary pole of the renal corpuscle. This section of the nephron is also restricted to the cortex of the kidney much like the renal corpuscle. The cells within the PCT are well suited to start the process of reabsorbing essential materials into the blood stream from the filtrate. All glucose and all proteins, under normal physiological conditions, are reabsorbed by the time the filtrate enters the next section of the tubular network.
Additionally, 65 percent of the sodium chloride (NaCl) and water in the filtrate is actively and consistently reabsorbed in the PCT. The salt creates a hypertonic environment (greater solute concentration) that generates an osmotic pressure and draws water out of the filtrate through the PCT cells.
What is the difference between filtration and reabsorption?
Filtration is the process in which materials leave the blood stream. Absorption (reabsorption) is the process of returning materials to the blood stream.
Intermediate Loop
After the PCT, the modified filtrate begins to descend from the cortex into the medulla in the first portion of a medullary loop, termed the intermediate loop (the loop of Henle). This is called the intermediate loop because it is between the PCT and the next cortical portion of the nephron, the distal convoluted tubule (DCT). The first portion of the loop is to the PCT, and is therefore called the thick descending limb of the loop. From this region, another 20 percent of the salt and water is reabsorbed using the same mechanisms as in the PCT. By this point, of the 180 L of filtrate that is produced per day, 27 L remain in the filtrate, which may be reabsorbed under hormonal control down to the approximate 1.5 L of urine that is eliminated.
Deeper in the medulla is a much narrower section of the loop, appropriately termed the thin descending limb and, following a curve, a thin ascending limb. These regions are actively involved in water reabsorption. Following a thick ascending portion of the loop, which resembles the next tubular section histologically and partly functionally, the filtrate enters the DCT.
Like the renal corpuscle and PCT, the DCT is found in the cortex of the kidney and transports filtrate that has been processed through the earlier regions of the nephron. This region does little for the reabsorption of water. However, it does play an essential role in the acid-base balance of the urine, and in doing so affects the pH of the blood.
Having left the DCT, the last segment of the nephron, the filtrate now enters a collecting duct with a larger diameter. Several nephrons empty their filtrate into a collecting duct and begin the process of funneling the filtrate (and urine) toward the renal pelvis and ureter. As the filtrate moves down the collecting duct, the diameter slowly increases as the duct approaches a renal papilla of one of the pyramids. The ducts are now termed papillary ducts (also called ducts of Bellini) and leave the papilla and enter the space of a minor calyx via the area cribrosa of the papilla.
Is the collecting duct part of the nephron?
Although not a portion of a nephron, the collecting duct along with a nephron is considered the functional unit of the kidney and is called a uriniferous tubule.
Ureter
This muscular tube (3–4 mm in diameter) connects the kidney with the bladder. Much like in the alimentary canal, the smooth muscle of the ureter spasmodically contracts in peristaltic movements to propel urine toward and into the bladder. This is typically the location where one becomes aware of any kidney stones that are being passed through the urinary tract. Although the lining of the ureter is transitional epithelium, which can stretch as the tract fills with urine, it cannot expand to the extent that most stones need to pass freely.
Bladder
The urinary bladder receives urine from each kidney via the ureters and can store approximately 500 ml to 1 L of urine. Lining the bladder is the same compliant transitional epithelium found in the ureter, as well as underlying layers of smooth muscle that collectively are referred to as the detrusor muscle. These layers are under autonomic nervous system control, and when stretched reflexively contract in order to urinate (micturition).
At the inferior apex of the bladder is the opening of the urethra, the tube that transfers urine from the bladder to the outside of the body. The internal urethral sphincter, which prevents urine from leaking into the urethra, is found here. As the urethra passes through the perineal muscle of the lower pelvis, the skeletal muscle creates the external urethral sphincter.
Urethra
Much shorter in females than in males, this conduit for urine has much less smooth muscle than the ureter, and shifts from transitional epithelium near the bladder to stratified squamous epithelium (similar to the skin) near the external urethral orifice (opening at the end of the urethra).