renal system

anatomy

In humans, the renal system includes the kidneys, which produce urine, and the ureters, bladder, and urethra for passage, storage, and voiding of urine.

The human excretory system, or urinary tract, is similar to other mammalian species in many ways, but it has its own unique structural and functional characteristics. The terms excretory and urinary refer to the system's elimination function. In addition to secreting and actively retaining certain substances, the kidneys also eliminate others that are equally essential for survival.

In the human body, there are two kidneys that control the electrolyte composition of the blood and eliminate dissolved waste products as well as excess amounts of other substances from the blood; these are excreted in urine, which passes from the kidneys to the bladder via two thin muscular tubes known as ureters. The bladder stores urine until it is expelled through the urethra.

Human excretory organs

The kidneys

General description and location

Reddish brown, bean-shaped kidneys, they are concave on one long side and convex on the other. Normal locations are on either side of the vertebral column, between the 12th thoracic and third lumbar vertebrae, and outside the peritoneum, the membrane that lines the abdominal cavity.

Although the kidneys are aligned along their long axes with that of the body, their upper end (poles) is tilted slightly inward toward the spine (vertebral column). The hilus lies in the middle of the medial concave border and leads to a cavity within the kidney known as the renal (kidney) sinus. In the hilus, the renal arteries and veins, lymphatic vessels, nerves, and the enlarged upper extension of the ureters emerge and exit.

Renal vessels and nerves

Two renal arteries arise from the abdominal aorta opposite the upper border of the second lumbar vertebra (i.e., a few centimeters above the small of the back). Each renal artery gives off a small branch to the adrenal gland and the ureter, then divides into anterior and posterior branches. Most large veins carrying blood from the kidneys arise from the corresponding arteries and join the inferior vena cava almost at right angles. The left vein is longer than the right vein because the inferior vena cava is close to the right kidney.

The kidneys are supplied by sympathetic and parasympathetic nerves of the autonomic nervous system, and the renal nerves contain both afferent and efferent fibres (afferent fibres carry nerve impulses to the central nervous system; efferent fibres, from it).

Internal configuration

In a cross section of a kidney, the renal sinus and two layers of kidney tissue can be distinguished based on their texture and color. As the innermost tissue, the renal medulla is composed of relatively dark cones called renal pyramids, whose apexes project either singly or in groups into the renal sinus. Each projection of a pyramid apex into the sinus is called a renal papilla. Slender striations extend toward the external surface of these pyramids from their irregular bases. External to the medulla, the cortex is paler and more granular. It covers the bases of the pyramids and fills the gaps between them. A renal lobe is made up of a group of pyramids that projects into a papilla, plus the portion of cortex that overlies it.

It consists of the renal pelvis, a funnel-shaped expansion of the upper end of the ureter, and the major calyxes, which branch into the kidney substances from the wide end of the funnel. The major calyx is divided into four to twelve smaller cup-like cavities called minor calyxes, into which the renal papillae are inserted. The renal pelvis serves as the initial reservoir for urine, which drains into the sinus through the urinary collecting tubules, tiny tubes that open into the sinus at the papillae.

Minute structure

The kidneys' structural units are nephrons, which produce urine. The kidneys contain approximately one million nephrons each. Each nephron is comprised of a long tube (or extremely fine tube) that is closed, expanded, and folded into a double-walled cup-like structure at one end. In the kidney, a structure called the renal corpuscular capsule, or Bowman's capsule, encloses a cluster of capillaries (microscopic blood vessels) called the glomerulus. Together, the capsule and glomerulus make up a renal corpuscle, also known as a malpighian body. A small artery (arteriole) enters and exits the glomerulus through the open end of the capsule where blood flows into and out of the glomerulus. It is known as the vascular pole.

30 to 55 millimeters (1.2 to 2.2 inches) is the length of the nephron tubules. In the renal cortex, you will find the corpuscle and the proximal convoluted tubules. The tubule descends into a renal pyramid, makes a U-shaped turn, and returns to the cortex near its entry point into the medulla. The loop of Henle, or the nephronic loop, is the section of the tubule made up of parallel lengths and a bend. The tubule returns to the vascular pole (the opening in the cuplike structure of the capsule) of its own nephron after reentering the cortex. A short junctional tubule links the distal convoluted tubule and the collecting tubule of the corpuscle. In some cases, the collecting tubules unite to form a larger tube that transports urine to the renal pelvis and a renal papilla.

While all nephrons in the kidney have a general disposition, there are regional differences in the length of the loops of Henle. Juxtamedullary glomeruli are located deep within the renal cortex, near the medulla, and have long loops of Henle. These are more superficial than cortical glomeruli. Animal species differ in the length of their loops, which can affect their ability to concentrate urine above the osmotic concentration of plasma.

Nephron tubule sections differ in shape and calibre, and these differences, as well as the differences in the cells lining the sections, are associated with specific functions during the production of urine.

A network of blood vessels within the kidney.

The kidneys have a network of blood vessels within them that function in addition to processing blood.

Arteries and arterioles

Each renal artery has anterior and posterior segments, each of which enters the kidney substance through or near a renal papilla. The interlobar arteries that connect adjacent renal pyramids are derived from each lobar artery. Between the cortex and medulla, they divide almost at right angles into arcuate arteries that run parallel to the surface of the kidney. From the arcuate arteries, the interlobular arteries branch out into the cortex to create capillary networks in the capsule. In each glomerulus, they form four to eight loops of capillaries by splitting into afferent arterioles en route to the glomeruli.

Before the afferent arteriole enters the glomerulus, its lining layer becomes enlarged and contains secretory granules. The juxtaglomerular apparatus (JGA) is a composite structure that releases renin (see below The role of hormones in renal function). The efferent arterioles carry blood away from the glomeruli after they are reconstituted near the entry point of the afferent arterioles. Despite their thicker muscular coats, afferent arterioles are nearly twice as thick as efferent arterioles, but their channels are nearly the same size.

In the proximal and distal renal tubules, the efferent arterioles divide into capillaries.

The base of the renal pyramids are supplied by the efferent glomerular arterioles of juxtaglomerular glomeruli. Located in close proximity to the loops of Henle is the vasa silencio, or silence vessels. Arcuate arteries follow the arcuate veins, making hairpin bends, retracing their routes, and empties into arcuate veins.

The blood circulating in the cortex exceeds that in the medulla by over 90 percent in normal conditions, but in some circumstances, such as those associated with severe trauma or blood loss, the cortical vessels may become constricted, while the juxtamedullary circulation remains intact. When blood is deprived from the cortical glomeruli and tubules, urine flow decreases, and in extreme cases it may cease completely.

Veins and venules

In addition to arteries and arterioles, there are venules (small veins) in the kidney. The venules below the renal capsule, which drain into interlobular venules, are called stellate venules because of their radial arrangement. Eventually, these tributaries combine to form the interlobar, lobar, and arcuate veins. From the renal pyramids, the blood travels through the rectae veins, which join the arcuate veins. Normally, the lobar veins in the renal sinus form veins that correspond to the major branches of the renal arteries, and they unite to form a single renal vein in or near the renal hilus.

Lymphatic network

Lymphatic capillaries are located just inside the renal capsule, while deeper capillaries are located between and around the renal blood vessels. It contains very few lymphatic capillaries, and those that are present are associated with the connective tissue framework, whereas glomeruli lack lymphatics. Within the capsule and around the renal blood vessels, lymphatic channels accompany interlobular and arcuate blood vessels. The lymph channels that follow the main renal arteries and veins terminate in lymph nodes near the aorta and at the sites where the renal arteries originate.

The ureters

General characteristics

A urinary ureter is a narrow, thick-walled duct that transports urine from the kidneys to the bladder, measuring approximately 25–30 centimetres (9.8–11.8 inches) in length and 4 to 5 millimetres (0.16 to 0.2 inches) in diameter. Connective tissue attaches the peritoneum, the lining of the abdomen and the pelvis, to them.

When the bladder is distended with urine, the distance between the ureters that enter the bladder wall increases. The ureters run obliquely through the muscular wall of the bladder for nearly two centimeters before entering the bladder cavity. Owing to the oblique course of the ureters, the bladder acts as a valve; when it becomes distended, it presses against the part of each ureter that is in the muscular wall of the bladder, preventing the flow of urine back into the ureters.

Structure of the ureteric wall

The ureter wall has three layers: the adventitia, or outer layer; the intermediate, muscular layer; and the lining, composed of mucous membrane. A fibroelastic connective tissue connects the adventitia with the connective tissue behind the peritoneum. The muscular coat is composed of smooth (involuntary) muscle fibers and is divided into two layers, one inner layer arranged longitudinally and another outer layer arranged circularly in the upper two-thirds of the ureter. An additional longitudinal layer lies outside the ureter in the lower third. As the ureter extends into the bladder wall, the circular fibers disappear, but longitudinal fibers extend almost as far as the mucous membrane lining the bladder.

The mucous membrane lining becomes thicker downward from the renal pelvis. Two to three cells thick line the kidney pelvis and calyxes, four to five cells thick line the ureter, and six to eight cells thick line the bladder. As ureters have longitudinal folds, their mucous membrane can dilate considerably. Neither the ureter nor the renal pelvis contain glands. Urea passes from the kidneys to the bladder through peristaltic movements in the ureter muscles.

The urinary bladder

General description

Urinary bladders are hollow, muscular organs that serve as the main reservoir of urine in the body. Under the peritoneum, it rests on the anterior part of the pelvic floor. In the midsection of the body, the symphysis pubis joint connects the hip bones. The size and shape of the bladder are determined by how much urine it contains. Normally, the organ is tetrahedral and lies within the pelvis; when enlarged, it expands into the lower abdomen and becomes ovoid. It consists of a body, which has a fundus, or base; a neck; an apex; and superior (upper) and inferolateral (below and to the side) surfaces; though these features are not visible except when the bladder is empty or only slightly enlarged.

This is the area immediately surrounding the urethral opening; it is the lowest and most fixed part of the bladder. It is firmly attached to the base of the prostate, a gland that surrounds the urethra in males.

Triangular-shaped bladders have peritoneum covering their superior surfaces. The muscles of the levator ani help keep the bladder in place in the pelvic cavity. As a result of the visceral layer of the pelvic fascia, the bladder is covered and supported. In the pelvic cavity, fascia surrounds the organs, blood vessels, and nerves. Ligaments called pubovesical ligaments lie in front and to the side of the fascia and act like a hammock under the inferolateral surfaces and neck of the bladder.

Blood and nerve supplies

Blood is supplied to the bladder by the superior, middle, and inferior vesical arteries. Among the branches of the superior vesical artery, one passes through the ductus deferens, which is a passageway for sperm. The ductus deferens, which is part of the passageway for sperm, is one of the branches of the superior vesical artery in males. Superior vesical artery passes through ductus deferens, which is the passageway for sperm. In males, the middle vesical artery supplies the base of the bladder. Inferolateral surfaces of the bladder are supplied by the inferior vesical artery, as well as the base of the bladder, the lower end of the ureter, and other structures adjacent to the bladder.

The nerves that control the urinary bladder are part of the sympathetic and parasympathetic divisions of the autonomic nervous system. The sympathetic nerve fibers are located in the hypogastric plexus, which lies just behind the fifth lumbar vertebra. In addition to conveying to the central nervous system the sensations of distention of the bladder, sympathetic nerves are also believed to assist in relaxing the muscular layer of the sacral wall and regulating the sphincter mechanism that shuts the opening into the urethra. Second through fifth sacral spinal segments carry parasympathetic nerves to the bladder. As a result of parasympathetic nerves, the walls of the bladder contract and the bladder's sphincter relaxes. Thus, they are referred to as the emptying, or detrusor, nerves, as they are actively involved in urination.

Structure of the bladder wall

Over its upper surface is a serous coat. The covering of the abdominal cavity is part of the peritoneum; it is called serous because it exudes lubricating fluid called serum. Other layers of the bladder wall are the fascial, muscular, submucous, and mucous coats.

Fascial coats are layers of connective tissue that cover muscles. It is composed of coarse fascicles, or bundles, of smooth (involuntary) muscle fibres arranged in three layers, the outer and inner layers running lengthwise, the middle layer running circularly. The fibres in the layers are intermingled heavily. The smooth muscle coat forms the powerful detrusor muscle, which is responsible for emptying the bladder.

Vesical wall layers are thicker in the circular or intermediate layer. Although its fibres are generally circular, they interlace. The internal muscular stratum is an indefinite layer of fibres that mostly run longitudinally. The submucous coat is made up of loose connective tissue containing elastic fibres. In the trigone, a triangular area whose angles are at the two urethral openings and the single internal urethral opening, it is absent. Between each ureteric opening and the internal urethral orifice run bands of muscle that maintain the oblique direction of the ureters during contraction. Between the two ureteric openings, another bundle of muscle fibers creates a slightly downwardly curved fold of mucous membrane.

As the bladder's innermost lining, the mucous coat is impervious to urine. Regardless of whether the bladder is contracted or distended, it adheres firmly to the muscular coat and always remains smooth and pink. Additionally, the mucous coat has multiple folds and a red, velvety appearance when the bladder is contracted. In distended bladders, the folds are obliterated, but the paler trigonal area still stands out from the other parts of the mucous membrane. There is a continuous mucous membrane lining the bladder, ureters, and urethra.

The urethra

General description

Urination exits the bladder via the urethra. In the male, it is about 20 centimetres long and carries not only urine but also semen, as well as secretions from the prostate, bulbourethral, and urethral glands. It opens during urination and ejaculation, and its diameter then varies from 0.5 to 0.8 centimetres along its length. At other times, however, its walls touch and its lining is raised into longitudinal folds. There are three distinct parts of the male urethra, the prostatic, the membraneous, and the spongy, all named after the structures through which they pass rather than from any inherent characteristics.

In males, the prostatic section of the urethra begins at the internal urethral orifice and descends almost vertically through the prostate to the apex, describing a slight curve with its concavity forward. It is about 2.5 to 3 centimeters long and spindle-shaped; the middle portion is the widest and most dilatable part of the urethra. Between two layers of a membrane called the urogenital diaphragm lies the membranous part of the male urethra. The urethra is narrower here than anywhere else, except at its external opening, and is encircled by a muscle, the sphincter urethrae. Two small bulbourethral glands are located on either side of it. The membranous urethra is not strongly attached to the layers of the urogenital diaphragm. It is the part of the urethra that crosses the penis that is spongy. In the penis, it passes through the corpus spongiosum. About 2.5 centimeters below the lower layer of the urogenital membrane, the ducts of the bulbourethral glands enter the spongy urethra; except near its outer end, many mucous glands also open into it.

The female urethra is much shorter (between 3 and 4.5 centimeters) and more distensible than the male urethra, carrying only urine and mucous secretions. The urethra begins at the opening into the bladder and curves gently downward and forward through the urogenital diaphragm, where it is surrounded by the sphincter urethrae, as in the male. Under the symphysis pubis, it lies behind. The urethra is embedded in the anterior wall of the vagina, with the exception of its uppermost portion. The external urethral orifice is located immediately in front of the vaginal opening, about 2.5 centimeters behind the clitoris, and between the labia minora, the inner folds at the outer opening of the vagina.

Structure of urethral wall

Males have a mucous membrane tube with an incomplete muscular coat and a submucous layer. When the tube is empty, the membrane forms longitudinal folds; these folds are more prominent in the membranous and spongy parts. Mucous membranes contain many glands, and most of them are found in the posterior wall of the spongy part. Among the many small blood vessels in the submucous layer are venules, which are more numerous than arterioles. It is composed of smooth (involuntary) and striated (voluntary) muscle fibers. The smooth muscular layer, longitudinally disposed, is continuous above with the detrusor muscle of the bladder and extends distally as far as the membraneous urethra, where it is partly replaced by striated muscle of the external sphincter. In the spinal cord, the second, third, and fourth sacral segments form the somatic nerves to the external sphincter.

Muscular, mucous, and sub-mucous coats cover the female urethra. Similar to the male, the lining of the empty channel is folded longitudinally. In addition to the mucous glands, it also depicts the male urethra, which was mentioned in the preceding paragraphs. Unlike the male, the female's submucous coat has prominent venules. This layer appears to be a type of erectile tissue in both sexes, though it is most apparent in females. The urethra of the female is covered with a muscular coat that extends along its length and is continuous with the musculature of the bladder above. There are inner longitudinal and outer circular layers, and fibres from the latter mix with those in the anterior wall of the vagina, which houses the urethra.

Human excretion

General function of the kidney

Our kidneys have evolved to allow humans to live on land where water and salt must be conserved, wastes excreted in concentrated form, and the blood and tissue fluids strictly regulated in volume, chemical composition, and osmotic pressure. Throughout the glomerulus, water and salts are filtered out of the blood through capillaries, into the lumen of the nephron, and then they are reabsorbed back into the blood. Remaining filtrate is excreted as urine. Thus, the kidneys help to maintain a constant internal environment despite a variety of external changes.

Regulatory functions

In order to maintain electrolyte and water equilibrium, the kidneys regulate three essential and interrelated properties of tissues: water content, acid-base balance, and osmotic pressure; in other words, the kidneys can balance the amounts of water with such chemicals as calcium, potassium, sodium, phosphorus, and sulfate in solution. Body malfunction occurs when the concentrations of mineral ions such as sodium, crystalloids such as glucose, and wastes such as urea do not remain within narrow normal limits.

When both kidneys are removed, urine constituents accumulate in the blood and result in death in 14–21 days if untreated. (Ureemia does not mean that urea is a toxic compound that causes illness or death.) The kidneys expel abnormal components of the blood until normal composition is restored. Only the kidneys are capable of removing wastes generated by protein metabolism. The waste products they excrete are not modified, but are transferred to the urine in the form in which they are produced elsewhere in the body. Ammonia is the one exception to this rule. Drugs and toxic substances are also eliminated by the kidneys. Therefore, the kidneys eliminate waste products from metabolism, such as urea, while limiting the loss of valuable substances, such as glucose. In order to maintain acid-base equilibrium, the kidneys remove excess hydrogen ions from the normally acid-forming diet and convert them into ammonia in the urine.

A relatively large amount of blood flows into the kidney in order for it to perform its functions. Blood is processed in the kidneys at a rate of about 1,200 milliliters a minute, or 1,800 litres (about 475 gallons) a day, which is 400 times the volume of total blood, and roughly one-fourth that of the volume pumped by the heart each day. Approximately 170 litres (45 gallons) of water are filtered from the bloodstream into the renal tubules every 24 hours, but the vast majority of this, 168.5 litres of water along with salt dissolved in it, is reabsorbed by the cells lining the tubules and returned to the bloodstream. In a 24-hour period, glomerular filtrate represents at least 50-60 times the volume of blood plasma (the blood minus its cells) in the entire body. In a 24-hour period, a man eliminates approximately 1.5 litres of water containing waste products from his metabolism, although the actual volume depends on fluid intake, occupational factors, and the environment. When sweating vigorously, it may fall to 500 millilitres (about a pint) a day; with a large amount of water consumed, it may increase to three litres. In response to changes in plasma volume caused by dehydration or overhydration, the kidney can change its reabsorption of water.

Nonexcretory functions

In addition to excretory functions, the kidneys also perform some nonexcretory functions. They secrete substances into the bloodstream. There are three types: renin, which is involved in the regulation of electrolyte balance and blood pressure, erythropoietin, which regulates hemoglobin and red blood cells in response to anemia or low oxygen levels, and 1,25-dihydroxycholecalciferol, the metabolically active form of vitamin D. rol, the metabolically active form of vitamin D. Furthermore, although the kidneys are subject to both nervous and humoral (hormonal) control, they do possess a considerable degree of autonomy; that is, function is maintained in a separate organ but kept alive by circulating fluid. In fact, kidney transplantation would not be possible if this were not the case.