URINARY SYSTEM

There are two kidneys in the human body, each is fist-sized, and both are located in the abdomen, on either side of the spine, right behind the rib cage (Fig. 1). They span from T12 to L3. Each kidney is approximately three vertebrae in length. When interpreting radiographs, this can be operated to assess any change in size. The right kidney is observed somewhat lower due to the influence of the liver (Fig. 2). The body's circulation is cleared of water-soluble wastes by the kidneys, which act to keep blood pressure, pH levels, electrolyte concentrations, and the quantity of extracellular fluid, and consequently they maintain homeostasis in the body. Urea and uric acid are the main waste products managed by the urinary system. When these products are excessively piled up, severe diseases occur. Blood is purified through filtration, reabsorption, and secretion. These processes are accomplished by the nephrons in each kidney (Fig. 3). There are thousands of nephrons in the kidneys; each is supported by a filtration mechanism called the renal corpuscle, containing a glomerulus and a Bowman's capsule. To evaluate how well the kidneys are working, the glomerular filtration rate is examined. Kidneys are designed with external and internal structures.

Kidneys are enveloped by compound layers of fat and fascia (Fig. 4). The deepest layer is a tough fibrous capsule called the renal capsule. Then comes a collection of extraperitoneal fat named perirenal fat. Renal fascia is the next layer which encases the kidneys and the suprarenal glands. Pararenal fat is the superficial layer situated on the posterolateral aspect of the kidney (Fig. 5).  

The internal structure of a kidney includes an outer cortex and an inner medulla (Fig. 6). While the former is dark brown and granular, the latter is divided into 6-12 areas forming a pyramid (renal pyramid). A renal papilla is the peak of a renal pyramid, which is enhanced by a structure, named the minor calyx, functioning to filter the blood in the pyramid. When several minor structures or calices combine together, a major calyx is shaped. It is a pathway through which urine passes into the renal pelvis, then to the ureter where it drains. The urine is carried by the ureter to be stored in the bladder. The renal hilum is a deep fissure that runs along the medial margin of each kidney. It works as the portal through which the renal vessels and ureter can go in and leave the kidney. The base of the pyramid appears toward the cortex (Fig. 7). 

The two kidneys receive blood from the renal arteries. The nephrons inside the kidneys work to filter the coming blood. Capillaries reabsorb small molecules and water and send them back to the bloodstream (Fig. 8). This process of filtration results approximately 180 liters of filtered fluid per day. However only 2 liters are excreted as urine, with the remainder being reabsorbed. The functioning of kidneys depends upon the endocrine system which controls a variety of hormones such as aldosterone, antidiuretic hormone, and parathyroid hormone. Kidneys also play an important role in adjusting water and salt levels in the body. Water is taken into the bloodstream through the gastrointestinal system, thus diluting the blood. The kidneys filter out extra water and convert it to urine. The role of kidneys is crucial to a healthy body.

Kidneys are responsible for ejecting surplus quantities of salt in blood, filtering them out into urine, in a way that can adjust its quantities in blood. Regulating blood pressure is among the crucial functions performed by the kidneys. Also, renin is an enzyme naturally produced by these organs. It works not only to regulate blood pressure and electrolyte balance through stimulating a variety of hormones, but also to convert blood protein into angiotensin, a hormone produced due to the fall of blood pressure. The top of the kidneys includes the adrenal glands (Fig. 9) which are in turn stimulated to release another substance known as aldosterone, a hormone allowing the reabsorption of more salt and water into the blood. Blood volume is accordingly elevated, and subsequently blood pressure.

Each of our kidneys is composed of a million of tiny filtering units called nephrons which work through two main structures: the renal corpuscle and the renal tubule (Fig. 10). Two components are involved in each renal corpuscle: a glomerulus, described as a small ball-shaped bundle of capillaries, and Bowman's capsule, a cup-liked sac, enveloping the glomerulus. The remaining portion of the nephron is made up of renal tubules; each of which is roughly 3 cm in length. Thousands of collecting ducts are also present in each kidney, and each duct functions to carry fluid from multiple nephrons to the renal pelvis. These ducts are considered as the destinations of renal tubules which travel from the glomerular capsule, producing a hairpin loop that is preceded and followed by twisted movements before the tubule approaches and enters a collecting duct. The proximal convoluted tubule (PCT), the nephron loop (loop of Henle), and the distal convoluted tubule are the names of the different areas of the tubule (Fig. 11). 

Nephrons can be cortical or juxtamedullary: cortical nephrons refer to those nephrons fully situated within the cortex, and they represent the majority. Juxtamedullary nephrons refer to those which lie close to the cortical medulla junction. They are few, and the medulla is penetrated deeply by their loops (Fig. 12). After gathering urine from the nephron, collecting ducts go down through the medullary pyramids to transfer the final urinary product to the calyces and renal pelvis. These ducts stand behind the striped look of the pyramids.

Filtration of blood is done by the glomerulus which detaches blood plasma from blood protein (Fig. 13). The former is called filtrate, and it is formed in the glomerular capsule. Due to the large size of proteins and blood cells, they are stuck behind the filtration membrane. The appearance of either of them in the urine indicates a glomerular filter malfunctioning. The formation of filtrate depends upon whether the systemic blood pressure is normal or not. With the drop of arterial blood pressure, glomerular pressure is insufficient to push substances out of the blood into the tubules. Filtrate creation in turn ceases.

Water, amino acids, glucose and ions are among the beneficial materials which are included in the filtrate to be retrieved back to the blood circulation in contrary to the wastes and unwanted ions that must be eliminated from the blood. These components are carried by the tubule cells out of their posterior facet into the extracellular environment, where they are subsequently ingested into the peritubular capillary blood. It describes the process of tubular reabsorption which initiates with the arrival of the filtrate to the proximal convoluted tubule (Fig. 14). Although the distal convoluted tubule and the collecting duct are functional, the proximal convoluted tubules are where the bulk of reabsorption occurs. The actions of the reabsorption process can be passive or active. While the majority of molecules are reabsorbed using membrane carriers with the help of ATP in an active transport process, water, for instance, is re-imbibed passively through osmosis.  

Tubular secretion is the opposite of tubular reabsorption. It is a vital process necessary for removing harmful materials as a way to control and stabilize blood pH. During tubular secretion, hydrogen, creatinine and potassium ions (H+ and K+), for instance, are ejected as urine after being carried by the tubule cells from the peritubular capillaries into the filtrate (Fig. 14).

Only very little, if no, reabsorption occurs for nitrogenous waste products. To maintain the proper pH and electrolyte balance and because they are not direly needed for any vital activity in our bodies, tubule cells are designed with few membrane carriers to retain nitrogenous wastes or to release them as urine according to the requirements of the body.  Urea, uric acid and creatinine, which are produced as a result of protein breakdown, metabolism of nucleic acids and with creatine metabolism in muscle tissue, respectively, are the common nitrogenous wastes.  

When there are more solutes in urine, it is released with deep yellow color. However, urochrome, a pigment produced  when hemoglobin is dissolved in the body, causes the normal yellow color of urine. Also, a pale straw hue indicates diluted urine. The color of urine occasionally differs with eating particular foods such as beets, or with the existence of blood or bile in urine. When a bladder is emptied, the freshly voided urine is sterile and has somewhat fragrant smell. If left to stand, the urinary solutes are broken down by bacteria, and the urine gives off an ammonia odor. The basic urine smell is also affected by some vegetables, pharmaceuticals, and maladies as in DM. Urine can be acidic or alkaline. Normal urine pH is approximately 6 or acidic. However, certain diets and modifications in human metabolism can increase the acidity or make it  basic. The level of acidity rises with the excessive intake of whole wheat products and proteins. In opposite, urine becomes quite alkaline either with a vegetarian diet or as a result of urinary tract bacterial infection (Fig. 15).

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There are two ureters; each function as a gateway allowing the passage of urine from the renal pelvis to the bladder through a process called peristalsis (Fig. 16). As a hollow duct, the length of an adult ureter ranges from 25 to 30 cm, and the width is about 3 mm. Through the contraction of the smooth muscle forming the ureteric walls, peristaltic waves result pushing the urine in the direction of the urinary bladder.

The urinary bladder, a hollow muscular structure, temporarily stores the urine produced through filtering water and other wastes by kidneys (Fig. 16). The walls of the bladder are inflatable, and they are enhanced by an internal lining, giving it the ability to carry up to 600 ml of urine. The bladder muscle and the sphincter muscle work harmonically for the storage of urine inside the urinary bladder. The bladder receives urine which fills it up through the relaxation of the detrusor muscle or the bladder muscle (Fig. 17). Sphincter muscles are found at the end of the bladder, and through their simultaneous contraction, urine is preserved in the bladder (Fig. 18). During urination, the roles of these two muscles are reversed upon orders by the brain. With the contraction of the detrusor muscle, the urine is forced out, especially with loosening the muscles at the bottom of the bladder. It allows pushing the urine through the urethra. 

The capacity of the bladder differs from a person to another, and it carries less quantities with age. The brain receives an alarm from receptors in the bladder walls indicating the half fullness of the bladder of urine. This alarm goes as a signal along the pelvic nerves of the spinal cord and then to the brain. Consequently, man feels the need to use the restroom to empty the bladder. This feeling can be suppressed for a while, but at the end, and as the bladder completely fills up, urination becomes inevitable. Once the bladder is full, it resembles oval-shaped, but with emptying its contents, it becomes flat. 

The apex, body, fundus and neck are the significant exterior structures of the bladder. The apex is the top of the bladder looking at the pubic symphysis. The median umbilical ligament links the apex to the umbilicus. The body is the main and middle part of the bladder. The fundus, with a posterior location, is a triangle in shape for the tip looking downward. The fundus and the two inferolateral surfaces meet together to form the neck which connects the bladder to the urethra (Fig. 19).

It acts as a link between the bladder and the external environment, through which urine is expelled out of the body (Fig. 20). 

In males, the length of urethra ranges from 15-20 cm. It goes through the penis, functioning as an exit for both semen and urine. Four segments are involved in the anatomy of urethra: pre prostatic, prostatic, membranous, and spongy (Fig. 21). The first segment is sited at the neck of the bladder, extending from the internal urethral opening to the prostate. The Prostatic travels through the prostate gland. It is the place where the prostatic ducts and ejaculatory ducts empty their contents in the urethra. The membranous extends from the pelvic floor to the deep perineal pouch. The external urethral sphincter, which governs micturition, borders it. The spongy terminates at the distal urethral orifice after traveling through the penis' bulb and corpus spongiosum. The navicular fossa is shaped with the stretching of the urethra in the glans penis. The proximal urethra is where the bulbourethral glands drain.

In Females, the urethra originates at the bladder neck, extending inferiorly through the pelvic floor muscles and perineal membrane (Fig. 22). The urethral orifice in the vestibule lies 2-3 cm posterior to the clitoris and prior to the vaginal opening. Two mucous glands are found on either side of the urethra at the distal end of the urethra. These glands resemble the male prostate in structure. The female urethra is considerably shorter, around 4cm in length, which increases women’s vulnerability to urinary tract infection.

There are an internal and an external sphincter in human body (Fig. 23). When the urethra and the urinary bladder intersect, the internal urethral orifice exists, and it is pressured by the smooth muscle of the internal sphincter in an involuntary action. However, the external urethral sphincter performs voluntary actions controlling urination.

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It refers to the action of removing urine out of the bladder or of voiding the bladder, which is regulated by the external urethral sphincter and the internal urethral sphincter (Fig. 24). The capacity of a bladder increases, and its wall expand through stretch receptors whenever its content exceeds 200 ml. The bladder contracts reflexively as a response of impulses passing to the sacral region of the spinal cord and then rebounding to the bladder via the pelvic splanchnic nerves. As the contractions intensify, the internal involuntary urethral sphincter is forced to open, allowing stored urine to pass into the upper side of the urethra. At that time, man feels an urgent need to urinate, which can put off for a while as the lower external sphincter, a skeletal muscle, is controlled electively. After a while, urination is inevitable, and urine flows out of the body with the relaxation of the external sphincter (Fig. 25). If urination is not permitted and with reaching the content of bladder about 300 ml, micturition happens automatically whether man wills or not.  

Voiding cystourethrogram is a diagnostic procedure which aims at providing x-ray images of the bladder while it is voiding (Fig. 26). It is conducted through inserting contrast medium into the bladder so urinary reflux and urethral strictures can be recognized easily.

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Chronic kidney disease or chronic renal failure can be recognized through measuring the patient's glomerular filtration rate via a blood test for creatinine. A drop-in glomerular filtration rate due to elevated creatinine levels badly affects or restrains the kidney's capability to refine and eliminate waste materials.  Although glomerular filtration rate tends to be normal at the beginning of CKD, red blood cells or lack of protein into the urine can be detected through urinalysis tests. Depending on the severity of diminished kidney function and symptoms of kidney injury, such as the existence of protein or blood in the urine, CKD is categorized into five phases. End-stage kidney disease (ESKD), also referred to end-stage renal disease or CKD stage 5, represents the most serious phase of disease. It is recognized when functions of kidney completely fail that permanent kidney damage takes place, necessitating kidney dialysis or kidney transplantation (Fig. 27).

When kidneys fail to keep up normal filtration, balanced electrolyte levels and convenient waste excretion, renal failure occurs (Fig. 28). Acute, chronic and end-stage are the three principal phases of renal failure. In case kidneys unexpectedly lose the function to eliminate waste and concentrate urine, it is named as acute renal failure (ARF) or acute kidney injury which starts to appear with the presence of serious dehydration, infection, long-term analgesic consumption or kidney injury. This disease is curable, and it does not leave permanent damage. Once the kidneys are utterly unable to work or with the decrease of kidney function less than 10% of normal, end-stage renal disease (ESRD) is said to take place. Chronic kidney disease (CKD) stage 5 is another term of ESRF.

Whenever kidneys would not grow normally throughout embryonic development, renal agenesis happens. Single kidney is present in unilateral renal agenesis, while the non-existence of kidneys or the presence of very little kidney tissue refers to bilateral renal agenesis (dysgenesis). In URA, the existed kidney will often enlarge to compensate for the missing kidney, and it often goes undetected for a long period (Fig. 29).   

With the inflammation of the glomeruli, which function to refine blood from waste and fluids, a kidney disease called glomerulonephritis occurs, leading to the loss of blood and protein in the urine. Although the pathogens are unknown, Glomerulonephritis may be due to some problems in the immune system. In acute glomerulonephritis, an abrupt bout of inflammation is experienced while in chronic glomerulonephritis, it starts progressively (Fig. 30).   

It indicates the accumulation of hard clumps of minerals in the bladder. Vesical calculus is another name of the disease. This condition is developed when some urine remains in the bladder after urination. Without treatment, this condition can cause infections, bleeding and complicated problems in the urinary tract.  Some bladder stones usually leave the body with urine without causing any symptoms. However, larger bladder stones can stay in the bladder and cause further complication including severe pain, bleeding and uncomforting urination (Fig. 31).

It occurs when the bladder harbors cancerous cells. Bladder cancer appears in a variety of forms.  Transitional cell carcinoma, adenocarcinoma and squamous cell carcinoma are the types of bladder cancer. TCC hits the lining of bladder, urethra, kidney and ureters, affecting around 90% of the cases. Although the pathogens tend to be vague, chronic bladder infections, cigarette smoking, chemical exposure, and family history are all recognized risks factors for the disease. Also, it is more prevalent in males.  Electrocautery can be functional with superficial tumours; however, cystectomy, chemotherapy, and/or radiation therapy are more effective with more invasive tumors) (Fig. 32).   

When bacteria from the digestive tract move to the urethra and proliferate inside, the urethra is inflamed, and urethritis occurs. It is a particular category of urinary tract infection (Fig. 33).   

When some urinary minerals accumulate forming a solid stone in the urinary tract, this disease happens. As these stones obstacle the urinary flow, ache and hemoturia result. The ureteropelvic junction, the pelvic brim, and the gateway of the bladder are the areas in which a stone can more likely be trapped. Kidney stones or ureteric calculus can be easily detected via CT scans of the kidneys, ureters, and bladder (Fig. 34). 

It is the medical term of a kidney disease brought on by high blood pressure. When hypertension as a disease is not diagnosed and not properly treated, it develops to HKD.  With the hard and difficult performance of the heart as a result of high blood pressure, damaging blood vessels becomes more likely throughout the body, for example in the kidneys. The renal functioning is accordingly disrupted, allowing the accumulation of toxins and fluid in the body. Blood pressure may subsequently increase even higher as a result of the additional fluid in the blood vessels, and the cycle repeats (Fig. 35). 

Urea Nitrogen is a waste product generated by the body after eating. Blood urea nitrogen is a testing method aiming to find out how properly the kidneys eliminate waste from the body. An increased level of nitrogen in the urea signifies a renal malfunctioning (Fig. 36).

A bladder can be inspected visually through cystoscope. During this procedure, the scope is sent up to the bladder through the urethra. To get biopsies of the bladder allowing for microscopic analysis, a catheter can be attached into the scope (Fig. 37).

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  a. Renal fascia

  b. Perirenal fat

  c. Renal capsule

  d. Pararenal fat

  a. Glomerulus 

  b. Renal papilla 

  c. Collecting duct

  d. Distal convoluted tubule 

  a. Creatinine 

  b. Protein

  c. Uric acid

  d. Glucose

  a. Ureter

  b. Pelvis 

  c. Urethra 

  d. Calyx

  a. Renal failure

  b. Renal agenesis 

  c. Glomerulonephritis 

  d. Bladder calculus