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Did you know?
The world’s largest and heaviest kidney stone ever recorded weighed an astonishing 800 grams. Measuring 13.37 cm in length—roughly the length of a standard ballpoint pen—and 10.55 cm in width, this stone was actually larger than the patient’s own kidney, setting a record for its massive size and weight.
Structure of the Urinary System
The urinary system is responsible for filtering blood, removing waste, and regulating fluid and electrolyte balance. It is divided into two main components:
Upper Urinary Tract: Comprises the kidneys and ureters. The kidneys filter blood to produce urine, while the ureters transport urine from the kidneys to the bladder.
Lower Urinary Tract: Includes the bladder and urethra. The bladder stores urine until it is expelled from the body through the urethra.
Upper Urinary Tract
The Kidneys
The kidneys are two bean-shaped organs situated on either side of the spine, just below the ribcage. Each kidney is approximately the size of a fist and is protected by a layer of fat and connective tissue. They play a vital role in filtering blood, excreting waste, and maintaining the body’s balance of water, salts, and pH. Each kidney has a complex internal structure divided into the cortex, medulla, and renal pelvis, with about 1 million nephrons per kidney responsible for producing urine.
Renal Capsule: A thin, fibrous outer covering of the kidney that provides structural support and protects the internal tissues from injury and infection.
Renal Cortex: This is the outermost region of the kidney, containing the glomeruli and much of the nephron tubules. The cortex is where the initial stages of blood filtration occur. It houses the glomeruli—tiny bundles of capillaries where blood plasma is filtered to remove waste, electrolytes, and excess water.
Renal Medulla: The medulla is the inner region of the kidney, organized into several cone-shaped structures known as renal pyramids. These pyramids contain:
Loops of Henle: The part of the nephron that dips into the medulla, creating a concentration gradient that allows for water reabsorption, essential for urine concentration.
Collecting Ducts: Extend from the nephrons through the medulla and ultimately empty into the renal pelvis. These ducts are vital in the final stage of urine formation, where water reabsorption is adjusted according to the body’s hydration level.
Renal Columns: These are extensions of cortical tissue between the renal pyramids, providing structural support and housing blood vessels that supply the medulla.
Renal Pelvis: A funnel-shaped cavity at the center of the kidney that collects urine from the renal medulla. The renal pelvis branches into several major and minor calyces, which receive urine from the collecting ducts. The calyces channel urine into the renal pelvis, from which it flows into the ureter.
Nephrons: Functional Units of the Kidney
Nephrons are the microscopic filtering units within the kidneys, with each kidney containing around 1 million nephrons. Each nephron has several specialized structures that work in sequence to filter blood, reabsorb essential substances, and produce urine:
Glomerulus: A cluster of capillaries encased within Bowman’s capsule, the glomerulus is the starting point of filtration. Blood pressure forces water, electrolytes, and waste from the blood into Bowman’s capsule, forming a filtrate that will be processed into urine.
Bowman’s Capsule: A cup-shaped structure surrounding the glomerulus, Bowman’s capsule collects the initial filtrate and directs it into the nephron tubule.
Proximal Convoluted Tubule (PCT): The first section of the nephron tubule, where about 65-70% of filtered substances, including glucose, amino acids, and electrolytes, are reabsorbed back into the bloodstream. This segment helps maintain nutrient levels and begins to regulate the fluid balance.
Loop of Henle: Extending from the PCT into the medulla, the Loop of Henle has descending and ascending limbs that create a concentration gradient within the medulla, allowing for efficient water and salt reabsorption. This gradient is crucial for producing concentrated urine, especially during dehydration.
Distal Convoluted Tubule (DCT): The final portion of the nephron before the collecting duct. Here, selective reabsorption of calcium, sodium, and chloride, along with secretion of potassium and hydrogen ions, helps maintain the body’s electrolyte and acid-base balance.
Collecting Ducts: The collecting ducts receive urine from multiple nephrons and continue to adjust the water content based on the body’s needs. Under the influence of the hormone vasopressin (ADH), these ducts reabsorb additional water, concentrating the urine further. Urine then flows from the collecting ducts into the minor calyces, progressing through the major calyces to the renal pelvis.
The Ureters
The ureters are two narrow, muscular tubes approximately 25–30 cm long, responsible for transporting urine from the kidneys to the bladder. Each ureter is attached to the renal pelvis at the kidney’s hilum and extends down to enter the bladder at an oblique angle.
Structure and Composition: The ureter wall consists of three layers:
Mucosa: The innermost layer lined with transitional epithelium, which can expand and contract to accommodate varying urine volumes.
Muscularis: A middle layer of smooth muscle arranged in circular and longitudinal fibers. This muscle layer produces peristaltic waves that propel urine toward the bladder.
Adventitia: The outer layer of connective tissue that anchors the ureter to surrounding structures, providing stability and protection.
Peristalsis: The muscular layer in the ureter wall generates rhythmic contractions known as peristalsis, which moves urine from the renal pelvis toward the bladder in waves. This movement is essential for efficient urine flow, even when lying down or under different body positions.
Ureteral Orifices: The ureters enter the bladder at an angle, creating a valve-like effect that prevents backflow of urine from the bladder into the kidneys during bladder contraction. This angle is critical for protecting the kidneys from high pressure, which could damage kidney tissue.
Lower Urinary Tract
The Bladder
The bladder is a hollow, muscular organ located in the pelvis, designed to store urine temporarily. Its shape and position vary depending on the amount of urine it holds. The bladder wall can stretch to accommodate up to 400-600 ml of urine, but it sends signals to the brain to urinate at around 200-300 ml.
Detrusor Muscle: The bladder’s wall consists of smooth muscle known as the detrusor muscle. This muscle can expand to accommodate urine as the bladder fills and then contract to expel urine during urination. The contraction of the detrusor muscle is regulated by the autonomic nervous system.
Trigone: The trigone is a triangular area at the bladder’s base formed by the two ureteral openings (where urine enters the bladder) and the urethral opening (where urine exits). This area remains smooth even when the bladder is stretched, creating a stable pathway for urine flow. The trigone’s unique structure is thought to prevent urine backflow and maintain a consistent exit path.
Bladder Neck and Internal Sphincter: The bladder neck connects the bladder to the urethra. The internal urethral sphincter, located at the bladder neck, is a ring of smooth muscle that prevents involuntary urine leakage by remaining contracted at rest. This involuntary sphincter is under the control of the autonomic nervous system and relaxes in coordination with detrusor muscle contraction during urination.
Urothelium (Transitional Epithelium): The inner lining of the bladder is a specialized layer known as the urothelium. This epithelium can expand and contract with the bladder’s changing size, providing a barrier to prevent urine from re-entering the bloodstream. The urothelium also helps protect the underlying tissues from the acidic and potentially irritating nature of urine.
The Urethra
The urethra is the final passageway for urine excretion. It allows urine to exit the body and is controlled by a combination of involuntary and voluntary muscles. The urethra’s structure differs between males and females.
Male Urethra: The male urethra is approximately 20 cm long and serves a dual function, as it also carries semen during ejaculation. The male urethra is divided into three segments:
Prostatic Urethra: The portion that runs through the prostate gland. During ejaculation, prostatic fluids mix with sperm in this section.
Membranous Urethra: The shortest segment, which passes through the urogenital diaphragm, a layer of pelvic floor muscles. This part of the urethra contains the external urethral sphincter, allowing voluntary control over urination.
Spongy (Penile) Urethra: The longest portion, extending through the length of the penis and ending at the external urethral orifice. This segment is lined with mucosal glands that provide lubrication during urination and ejaculation.
Female Urethra: The female urethra is about 3–4 cm long and opens just above the vaginal opening. Due to its shorter length, the female urethra provides a direct path for urine to exit the body. However, its length also makes females more susceptible to urinary tract infections, as bacteria have a shorter distance to travel to reach the bladder.
Sphincters
Two key sphincters control the flow of urine through the urethra, regulating urination and preventing involuntary leakage:
Internal Urethral Sphincter: This involuntary sphincter is made of smooth muscle and is located at the base of the bladder. It is under the control of the autonomic nervous system and remains contracted to prevent urine leakage. During urination, it relaxes in coordination with detrusor muscle contraction to allow urine flow.
External Urethral Sphincter: Composed of skeletal muscle, the external urethral sphincter is located at the level of the pelvic floor in both males and females. This sphincter allows voluntary control over urination, giving individuals the ability to start and stop the flow of urine as needed. It is innervated by the somatic nervous system, making it under conscious control.
Blood Supply and Innervation
Blood Supply
Renal Arteries: The renal arteries originate from the abdominal aorta and provide the primary blood supply to the kidneys. Each renal artery enters the kidney at the hilum and branches into segmental arteries, which further divide into interlobar, arcuate, and interlobular arteries. These branches deliver oxygenated blood to the nephrons, where filtration of blood begins.
Segmental Arteries: After entering the renal hilum, the renal arteries split into several segmental arteries, each supplying a specific segment of the kidney. These arteries ensure that each region of the kidney receives an adequate blood supply for effective filtration and function.
Interlobar Arteries: These arteries arise from the segmental arteries and run between the renal pyramids in the medulla. They branch further into arcuate arteries at the corticomedullary junction.
Arcuate Arteries: Located at the boundary between the renal cortex and medulla, these arteries arch over the base of the renal pyramids, supplying blood to the cortical regions of the kidney.
Interlobular Arteries: These small arteries branch off the arcuate arteries and extend into the renal cortex, supplying the glomeruli with blood for filtration. They play a critical role in the delivery of blood to the nephrons.
Venous Drainage
Renal Veins: The renal veins drain deoxygenated blood from the kidneys and exit at the renal hilum. The right renal vein drains directly into the inferior vena cava, while the left renal vein is longer and crosses anteriorly to the abdominal aorta before draining into the inferior vena cava. These veins collect blood from the interlobular, arcuate, and interlobar veins that run parallel to their corresponding arteries.
Interlobar Veins: These veins collect blood from the arcuate veins and run between the renal pyramids, joining together to form the renal veins. They play a crucial role in draining the medullary regions of the kidney.
Arcuate Veins: Located at the corticomedullary junction, these veins collect blood from the interlobular veins and transport it to the interlobar veins.
Interlobular Veins: These veins drain the deoxygenated blood from the renal cortex, including the glomeruli, and deliver it to the arcuate veins.
Innervation
Sympathetic Nerves: Sympathetic fibers from the renal plexus regulate renal blood flow and the release of renin, a hormone involved in blood pressure regulation. These nerves also influence the contraction of smooth muscles in the renal arteries and affect the rate of urine production.
Parasympathetic Nerves: Parasympathetic fibers from the vagus nerve primarily innervate the renal pelvis and ureters. They help modulate peristaltic activity in the ureters, which is essential for the movement of urine from the kidneys to the bladder.
Sensory Nerves: Sensory afferent fibers relay information regarding the distension of the renal capsule and ureters, as well as the pain associated with renal colic or kidney stones. These nerves play a vital role in the perception of pain and the regulation of kidney function in response to changes in blood pressure and volume.
Common Congenital Anomalies
Congenital anomalies in the urinary system involve structural abnormalities in the kidneys, ureters, bladder, or urethra that develop during fetal life. These conditions can vary in severity, from asymptomatic to those requiring medical intervention to prevent complications such as infections, kidney damage, or obstructed urine flow. Below are some of the most commonly encountered congenital anomalies in the urinary system:
Congenital Anomaly
Description
Horseshoe Kidney
A condition where the two kidneys are fused at the lower poles, forming a horseshoe shape. This fusion can lead to urinary tract infections, kidney stones, and hydronephrosis due to impaired drainage. Although often asymptomatic, severe cases may require surgical intervention if complications arise.
Renal Agenesis
The absence of one (unilateral) or both (bilateral) kidneys at birth. Unilateral agenesis may go unnoticed and typically does not impair function due to compensatory growth in the remaining kidney, while bilateral agenesis is life-threatening.
Polycystic Kidney Disease (PKD)
A genetic disorder causing clusters of cysts to form within the kidneys, leading to enlarged kidneys and impaired function over time. PKD can result in hypertension, kidney stones, and kidney failure, with management focusing on blood pressure control and supportive care.
Vesicoureteral Reflux (VUR)
A condition where urine flows backward from the bladder into the ureters, increasing the risk of urinary tract infections and kidney damage. Treatment may involve antibiotics for infection prevention or surgery in severe cases to correct the reflux.
Duplicated Ureters
A condition where two ureters drain a single kidney instead of one. This anomaly can be asymptomatic, but it may lead to urine flow issues, recurrent infections, or obstruction if one of the ureters is poorly positioned. Surgical correction may be necessary in symptomatic cases.
Ureteropelvic Junction (UPJ) Obstruction
A blockage at the junction where the ureter meets the renal pelvis, which can impair urine flow from the kidney to the ureter, causing hydronephrosis. UPJ obstruction may require surgical intervention to restore normal urine drainage and prevent kidney damage.
Bladder Exstrophy
A rare condition where the bladder is turned inside out and exposed outside the abdomen. This anomaly requires surgical reconstruction to restore bladder function and protect surrounding organs.
Posterior Urethral Valves (PUV)
Abnormal folds of tissue in the male urethra that obstruct urine flow. PUV can cause bladder dysfunction, hydronephrosis, and kidney damage. Early surgical intervention is often necessary to remove the obstruction and prevent long-term complications.
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