Location and External Anatomy
The kidneys are bean-shaped organs that lie retroperitoneal in the superior lumbar region.
The medial surface is concave and has a renal hilus that leads into a renal sinus, where the blood vessels, nerves, and lymphatics lie.
The kidneys are surrounded by a fibrous, transparent renal capsule; a fatty adipose capsule that cushions the organ; and an outer fibrous renal fascia that anchors the kidney to surrounding structures.
There are three distinct regions of the kidney: the cortex, the medulla, and the renal pelvis.
Major and minor calyces collect urine and empty it into the renal pelvis.
Blood and Nerve Supply
Blood supply into and out of the kidneys progresses to the cortex through renal arteries to segmental, lobar, interlobar, arcuate, and cortical radiate arteries, and back to renal veins from cortical radiate, arcuate, and interlobar veins.
The renal plexus regulates renal blood flow by adjusting the diameter of renal arterioles and influencing the urine-forming role of the nephrons.
Nephrons are the structural and functional units of the kidneys that carry out processes that form urine .
Each nephron consists of a renal corpuscle composed of a tuft of capillaries (the glomerulus), surrounded by a glomerular capsule (Bowman’s capsule) and a renal tubule.
The renal tubule begins at the glomerular capsule as the proximal convoluted tubule, continues through a hairpin loop, the loop of Henle, and turns into a distal convoluted tubule before emptying into a collecting duct.
The collecting ducts collect filtrate from many nephrons, and extend through the renal pyramid to the renal papilla, where they empty into a minor calyx.
There are two types of nephrons: 85% are cortical nephrons, which are located almost entirely within the cortex; 15% are juxtamedullary nephrons, located near the cortex-medulla junction.
The peritubular capillaries arise from efferent arterioles draining the glomerulus, and absorb solutes and water from the tubules.
Blood flow in the renal circulation is subject to high resistance in the afferent and efferent arterioles.
The juxtaglomerular apparatus (complex) is a structural arrangement between the afferent arteriole and the ascending limb of the loop of Henle just at the junction with the distal convoluted tubule. The JG apparatus includes granular cells (specialized smooth muscle cells of the afferent arteriole), macula densa cells (specialized cells in the wall of the nephron loop) and extraglomerular mesangial cells.
The filtration membrane lies between the blood and the interior of the glomerular capsule, and allows free passage of water and solutes.
Kidney Physiology: Mechanisms of Urine Formation
Step 1: Glomerular Filtration
Glomerular filtration is a passive, nonselective process in which hydrostatic pressure forces fluids through the glomerular membrane.
The net filtration pressure responsible for filtrate formation is given by the balance of glomerular hydrostatic pressure against the combined forces of colloid osmotic pressure of glomerular blood and capsular hydrostatic pressure exerted by the fluids in the glomerular capsule.
The glomerular filtration rate is the volume of filtrate formed each minute by all the glomeruli of the kidneys combined. The normal adult GFR is 120 - 125 ml/min.
Maintenance of a relatively constant glomerular filtration rate is important because reabsorption of water and solutes depends on how quickly filtrate flows through the tubules.
The glomerular filtration rate is held relatively constant through intrinsic autoregulatory mechanisms, and extrinsic hormonal and neural mechanisms.
Renal autoregulation uses a myogenic control related to the degree of stretch of the afferent arteriole, and a tubuloglomerular feedback mechanism that responds to the rate of filtrate flow in the tubules.
Extrinsic neural mechanisms are stress-induced sympathetic responses that inhibit filtrate formation by constricting the afferent arterioles.
The renin-angiotensin mechanism causes an increase in systemic blood pressure and an increase in blood volume by increasing Na+ reabsorption.
Step 2: Tubular Reabsorption
Tubular reabsorption begins as soon as the filtrate enters the proximal convoluted tubule, and involves near total reabsorption of organic nutrients, and the hormonally regulated reabsorption of water and ions.
The most abundant cation of the filtrate is Na+, and reabsorption is always active.
Passive tubular reabsorption is the passive reabsorption of negatively charged ions that travel along an electrical gradient created by the active reabsorption of Na+.
Obligatory water reabsorption occurs in water-permeable regions of the tubules in response to the osmotic gradients created by active transport of Na+.
Secondary active transport is responsible for absorption of glucose, amino acids, vitamins, and most cations, and occurs when solutes are cotransported with Na+ when it moves along its concentration gradient.
Substances that are not reabsorbed or incompletely reabsorbed remain in the filtrate due to a lack of carrier molecules, lipid insolubility, or large size (urea, creatinine, and uric acid).
Different areas of the tubules have different absorptive capabilities.
The proximal convoluted tubule is most active in reabsorption, with most selective reabsorption occurring there.
The descending limb of the loop of Henle is permeable to water, while the ascending limb is impermeable to water but permeable to electrolytes.
The distal convoluted tubule and collecting duct have Na+ and water permeability regulated by the hormones aldosterone, antidiuretic hormone, and atrial natriuretic peptide.
Step 3: Tubular Secretion
Tubular secretion disposes of unwanted solutes, eliminates solutes that were reabsorbed, rids the body of excess K+, and controls blood pH.
Tubular secretion is most active in the proximal convoluted tubule, but occurs in the collecting ducts and distal convoluted tubules, as well.
Regulation of Urine Concentration and Volume
One of the critical functions of the kidney is to keep the solute load of body fluids constant by regulating urine concentration and volume.
The countercurrent mechanism involves interaction between filtrate flow through the loops of Henle (the countercurrent multiplier) of juxtamedullary nephrons and the flow of blood through the vasa recta (the countercurrent exchanger).
Since water is freely absorbed from the descending limb of the loop of Henle, filtrate concentration increases and water is reabsorbed.
The ascending limb is permeable to solutes, but not to water.
The vasa recta aids in maintaining the steep concentration gradient of the medulla by cycling salt into the blood as it descends into the medulla, and then out again as it ascends toward the cortex.
In the collecting duct, urea diffuses into the deep medullary tissue, contributing to the increasing osmotic gradient encountered by filtrate as it moves through the loop.
Since tubular filtrate is diluted as it travels through the ascending limb of the loop of Henle, production of a dilute urine is accomplished by simply allowing filtrate to pass on to the renal pelvis.
Formation of a concentrated urine occurs in response to the release of antidiuretic hormone, which makes the collecting ducts permeable to water and increases water uptake from the urine.
Diuretics act to increase urine output by either acting as an osmotic diuretic or by inhibiting Na+ and resulting obligatory water reabsorption.
Osmotic: substances like mannitol, which are filtered but not reabsorbed, remain in the filtrate and hold water in the filtrate by increasing the osmolality of the filtrate.
Loop diuretics - act on the ascending limb of the loop of Henle, prevent Na+ from entering the interstitial fluid, disrupting the osmotic gradient and preventing water reabsorption.
Thiazides - act on the distal convoluted tubule and inhibit the sodium-chloride symporter, which inhibits Na+ (and water) reabsorption . Thiazides may also be potassium sparing and are considered to be calcium sparing.
Renal clearance refers to the volume of plasma that is cleared of a specific substance in a given time.
Inulin is used as a clearance standard to determine glomerular filtration rate since it is not reabsorbed, stored, or secreted.
If the clearance value for a substance is less than that for inulin, then some of the substance is being reabsorbed; if the clearance value is greater than the inulin clearance rate, then some of the substance is being secreted. A clearance value of zero indicates the substance is completely reabsorbed.
Freshly voided urine is clear and pale to deep yellow due to urochrome, a pigment resulting from the destruction of hemoglobin.
Fresh urine is slightly aromatic, but develops an ammonia odor if allowed to stand, due to bacterial metabolism of urea.
Urine is usually slightly acidic (around pH 6) but can vary from about 4.5–8.0 in response to changes in metabolism or diet.
Urine has a higher specific gravity than water, due to the presence of solutes.
Urine volume is about 95% water and 5% solutes, the largest solute fraction devoted to the nitrogenous wastes urea, creatinine, and uric acid.
Ureters are tubes that actively convey urine from the kidneys to the bladder.
The walls of the ureters consist of an inner mucosa continuous with the kidney pelvis and the bladder, a double-layered muscularis, and a connective tissue adventitia covering the external surface.
The urinary bladder is a muscular sac that expands as urine is produced by the kidneys to allow storage of urine until voiding is convenient.
The wall of the bladder has three layers: an outer adventitia, a middle layer of detrusor muscle, and an inner mucosa that is highly folded to allow distention of the bladder without a large increase in internal pressure.
The urethra is a muscular tube that drains urine from the body; it is 3–4 cm long in females, but closer to 20 cm in males.
There are two sphincter muscles associated with the urethra: the internal urethral sphincter, which is involuntary and formed from detrusor muscle; and the external urethral sphincter, which is voluntary and formed by the skeletal muscle at the urogenital diaphragm.
The external urethral orifice lies between the clitoris and vaginal opening in females, or occurs at the tip of the penis in males.
Micturition, or urination, is the act of emptying the bladder.
As urine accumulates, distention of the bladder activates stretch receptors, which trigger spinal reflexes, resulting in storage of urine.
Voluntary initiation of voiding reflexes results in activation of the micturition center of the pons, which signals parasympathetic motor neurons that stimulate contraction of the detrusor muscle and relaxation of the urinary sphincters.
Developmental Aspects of the Urinary System
In the developing fetus, the mesoderm-derived urogenital ridges give rise to three sets of kidneys: the pronephros, mesonephros, and metanephros.
The pronephros forms and degenerates during the fourth through sixth weeks, but the pronephric duct persists, and connects later-developing kidneys to the cloaca.
The mesonephros develops from the pronephric duct, which then is named the mesonephric duct, and persist until development of the metanephros.
The metanephros develops at about five weeks, and forms ureteric buds that give rise to the ureters, renal pelvises, calyces, and collecting ducts.
The cloaca subdivides to form the future rectum, anal canal, and the urogenital sinus, which gives rise to the bladder and urethra.
Newborns void most frequently, because the bladder is small and the kidneys cannot concentrate urine until two months of age.
From two months of age until adolescence, urine output increases until the adult output volume is achieved.
Voluntary control of the urinary sphincters depends on nervous system development, and complete control of the bladder even during the night does not usually occur before 4 years of age.
In old age, kidney function declines due to shrinking of the kidney as nephrons decrease in size and number; the bladder also shrinks and loses tone, resulting in frequent urination.