Anatomy of the Urinary System_rev 1 PDF

Anatomy of Excretory System The excretory system also known as the urinary or renal system, is responsible for filtering waste products from the bloodstream, maintaining electrolyte balance, regulating blood pressure, and excreting these waste products in the form of urine. The major organs of the e...

Anatomy of Excretory System The excretory system also known as the urinary or renal system, is responsible for filtering waste products from the bloodstream, maintaining electrolyte balance, regulating blood pressure, and excreting these waste products in the form of urine. The major organs of the excretory system the kidneys, ureters, urinary bladder and urethra. These organs work together to form, transport, store, and excrete urine from the body. Organs of the Excretory System Kidneys The kidneys are two bean-shaped organs located in the posterior abdominal wall, on either side of the vertebral column, behind the peritoneum and below the rib cage (they extend from T12-L3 vertebrae). The right kidney is typically slightly lower than the left due to the position of the liver. Each kidney is about 10-12 cm long, 5-7 cm wide, 3 cm thick and weighs about 150 grams. Although the two kidneys constitute only about 0.4% of the total body weight, they receive a combined blood flow of about 1100 ml/min of blood flow, or about 22% of the cardiac output. The kidneys play a vital role in filtering blood to Figure 1: Anterior View of the Kidneys Showing the Areas remove metabolic waste products (urea, creatinine, of Contact with Associated Structures and excess salts), maintain electrolyte balance, regulate blood pressure and manage the body’s fluid level. The filtered fluid (urine) is then transported to the urinary bladder for storage and eventual excretion. Organs Associated with the Kidneys: Right Kidney Superiorly – the right adrenal gland Anteriorly – the right lobe of the liver, the duodenum and the hepatic (right colic) flexure Posteriorly – the diaphragm, 12th rib and muscles of the posterior abdominal wall Figure 2: Anterior View of the Kidneys Showing Associated Organs Left Kidney Superiorly – the left adrenal gland Anteriorly – the spleen, stomach, pancreas, jejunum and splenic flexure (left colic) Posteriorly – the diaphragm, 11th & 12th ribs, and muscles of the posterior abdominal wall Figure 3: Posterior View of the Kidneys Showing Associated Organs 1  Anatomy of Excretory System Macroscopic Structure of the Kidney: The kidney is a complex organ with distinct regions: Renal Capsule: This is an outer fibrous membrane that covers the kidney. Renal Cortex: The reddish-brown layer of tissues immediately below the capsule, which contains the renal corpuscles (glomeruli and Bowman’s capsules), which are the initial filtering units. Renal Medulla: The innermost layer of the kidney. It is divided into 8 to 10 cone- shaped masses of tissue called “renal pyramids”. The apex of each pyramid forms a renal papilla, which drains urine into the minor calyces. Renal Pelvis: A funnel-shaped structure of the kidney. The outer border of the pelvis is divided into open- ended pouches called major calyces that extend downward and divide into minor calyces, which collect urine from the tubules of each papilla and channels it into the ureter (the Figure 4: Macroscopic Structure of the Kidney renal pelvis is continuous with the ureter). The walls of the calyces, pelvis, and ureter are lined with transitional epithelium and contain smooth muscles. These smooth muscles enables/permits peristalsis1 in these walls which aid to propel urine through the renal pelvis and ureters toward the bladder, where urine is stored until it is emptied by micturition. The renal pelvis. Hilum: The concave medial border of the kidney where the renal artery, vein, lymph vessels, nerves and ureter enter and exit. Microscopic Structure of the kidney: Nephron: is the basic structural and functional unit of the kidney, responsible for filtering blood, removing waste products, balancing electrolytes, and forming urine. Each kidney is composed of about 1-2 million nephrons2. The nephron consists of two main parts: the Renal Corpuscle and the Renal Tubule. These parts play a distinct role in the processes of filtration, reabsorption, secretion, and excretion that ultimately result in urine formation. Parts of the Nephron are described below: 1 Peristalsis refers to series of muscle contraction hat move foods and fluids 2 The kidney cannot regenerate new nephrons. Therefore, with renal injury, disease, or normal aging, the number of nephrons gradually decreases. After age 40 years, the number of functioning nephrons usually decreases about 10% every 10 years; thus, at age 80 years, many people have 40% fewer functioning nephrons than they did at age 40 years. This loss is not life- threatening because adaptive changes in the remaining nephrons allow them to excrete the proper amounts of water, electrolytes, and waste products. 2  Anatomy of Excretory System Renal Corpuscle: The Renal Corpuscle is located within the renal cortex. It is the initial filtering unit of the nephron and consist of two main parts: Glomerulus: A ball-like network of capillaries, surrounded by the Bowman’s capsule, where blood plasma is filtered under pressure. Bowman’s Capsule: A double- layered, cup-shaped sac surrounding the glomerulus. It collects the filtered fluid (called filtrate) that passes through the glomerular capillaries and directs it into the renal tubule for further processing. It is made up of Figure 5: Diagram of a Nephron Showing the Renal Corpuscles two layers: and the Renal Tubule ✓Parietal layer: The outer layer made of simple squamous epithelium. ✓Visceral layer: Inner layer composed of specialized cells called podocytes, which have foot- like extensions called pedicels. These pedicels interlock to form a filtration barrier with tiny silts, called silt pores, through which blood plasma passes, playing a key role in the filtration of blood. Renal Tubule: A series of tubules that extends from the Bowman's capsule, where the filtrate is processed and converted into urine. It consists of four main segments: Proximal Convoluted Tubule (PCT): Located in the renal cortex, originating from the bowman’s capsule. It is lined with cuboidal epithelial cells. it is responsible for about 65-70% reabsorption of water, ions, and nutrients from the filtrate. Loop of Henle (Nephron/Medullary Loop): It begins in the cortex, dips into the medulla, and returns to the cortex and is made up of two limbs: ✓ Descending Limb: Thin segment that descends into the medulla, lined with simple squamous epithelium. It is permeable to water. ✓ Ascending Limb. Thick segment that ascends back toward the cortex, Lined with cuboidal to columnar epithelium. It is permeable to salts. Both limbs create a concentration gradient that helps to concentrate the urine. Distal Convoluted Tubule (DCT): Located in the renal cortex, lined with cuboidal epithelial cells. Has smoother appearance in histological sections as a result of the lack of microvilli. It is shorter and less convoluted than the PCT it and continues the reabsorption of ions and plays a role in acid-base balance. Collecting Duct: Extends from the cortex, through the medulla, to the renal papillae. In each kidney, there are about 250 large collecting ducts, each of which collects urine from about 4000 nephrons. It is lined with cuboidal and columnar epithelial cells, depending on the region. Several nephrons drain into a single collecting duct and transports it to the renal pelvis. The collecting ducts transport urine through the pyramids to the calyces (at the renal papilla) and renal pelvis, giving the pyramids their striped appearance. The collecting ducts are supported by a small amount of connective tissue, containing blood vessels, nerves and lymph vessels. The permeability of the collecting duct to water is regulated by hormones like Antidiuretic Hormone (ADH). 3  Anatomy of Excretory System Blood Flow Through the Kidneys The kidneys receive a significant portion of the body's blood supply, about 22% of the cardiac output (approximately 1100 mL per minute). Renal Artery: Blood enters each kidney through the renal artery, which branches off from the descending aorta (the major artery from the heart). This artery enters the kidney at the hilum (the central part of the kidney where blood vessels, nerves, and the ureter enter and exit). and divides into smaller (segmental) arteries. Segmental Artery: Once inside the kidney, renal artery branches into smaller (segmental) arteries deliver to blood to nephrons. Interlobar arteries: The segmental arteries further branches into interlobar arteries which run between the renal pyramids toward the cortex. Arcuate arteries: As the interlobar arteries reach the boundary between the medulla and cortex of the kidney, they branch into arcuate arteries. These Efferent arteriole arteries arch over the base of the renal pyramids, running parallel to the kidney’s outer surface. Interlobular arteries (Cortical Radiate Arteries): Figure 6: Schematic of Blood Flow through the arcuate arteries branches into interlobular the Kidney arteries and travel into the renal cortex. Afferent Arterioles: The interlobular arteries supply blood to the glomeruli by giving rise to afferent arterioles which lead directly to the glomerular capillaries. This is where filtration of blood occurs, removing large amounts of fluid and solutes (except for plasma proteins) to begin the process of urine formation. Efferent Arterioles: After blood is filtered in the glomerulus, the remaining blood exits through the efferent arteriole, which connects to a second capillary network called the peritubular capillaries. Peritubular Capillaries and Vasa Recta: Surrounding the renal tubules, the peritubular capillaries are responsible Figure 7: Diagram of Blood Flow through the Kidney for reabsorbing most of the filtered fluid and solutes from the tubules back into the bloodstream. In nephrons located deeper in the kidney, especially in the juxtamedullary nephrons, the efferent arterioles form a network called the vasa recta, which extends into the medulla and helps concentrate the urine by maintaining a gradient of solutes. Venous Drainage: After passing through the peritubular capillaries or vasa recta, blood is collected by venous vessels that runs parallel to the arteries: Interlobular veins: Drain blood from the peritubular capillaries and vasa recta. Arcuate veins: Collect blood from the interlobular veins. 4  Anatomy of Excretory System Interlobar veins: Drain the arcuate veins and converge toward the renal hilum. Finally, all the venous blood is collected into the renal vein, which exits the kidney and returns filtered blood to the inferior vena cava, where it will return to the heart. The blood vessels of the kidney are supplied by both sympathetic and parasympathetic nerves. Ureters These are two narrow muscular tubes (one from each kidney) that transport urine from the kidneys to the urinary bladder. They are about 25-35 cm long with a diameter of about 3 mm. The ureter is continuous with the funnel-shaped renal pelvis. It passes downwards through the abdominal cavity, behind the peritoneum in front of the psoas muscle into the pelvic cavity, and passes obliquely through the posterior wall of the bladder. Microscopically, the ureter wall consists of three layers: Adventitia: The outer layer of fibrous and connective tissue, which is continuous with the fibrous capsule of the kidney. The adventitia anchors the ureters in place. Muscular Layer (Muscularis): The middle layer consists of smooth muscle, which contracts in waves (peristalsis) to propel urine toward the bladder. Mucosa: The inner layer, composed of transitional epithelium. This epithelium allows the ureter to stretch as urine passes through. Figure 8: The Ureters and its Relationship to the Kidneys and Urinary Bladder Urinary Bladder This is a hollow, pear shaped (but becomes more oval as it fills with urine) muscular sac located in the pelvic cavity, posterior to the pubic bone. In males, the bladder is situated superior to the prostate gland, while in females, it lies anterior to the uterus and vagina. It serves as a temporary storage/reservoir organ for urine before it is excreted from the body. Its size and position vary, depending on the volume of urine it contains. It can hold approximately 300-500 mL of urine comfortably, though it can expand to hold more. When distended, the bladder rises into the abdominal cavity. The detrusor muscle in the bladder wall contracts during urination to expel urine. The urinary bladder has the following regions: Apex: The pointed anterior end. Base (Fundus): The posterior surface, where the ureters enter. Neck: The funnel-shaped narrow region, passing inferiorly and anteriorly into the urogenital triangle that connects to the urethra. Trigone: A triangular area located on the posterior wall of the urinary bladder, lying immediately above the bladder neck that helps funnel urine from the bladder into the urethra during urination. Figure 9: Section of the Bladder Showing the Trigone 5  Anatomy of Excretory System It is bounded by three points: a) the upper two orifices on the posterior wall (points where the ureters enter the bladder) and one lower orifice (the point where the urethra exits the bladder at the bladder neck). Organs Associated with the Bladder In Females Anteriorly: Symphysis pubis Posteriorly: uterus and upper part of the vagina Superiorly: small intestine Inferiorly: urethra and muscles forming the pelvic floor Figure 10: Organs Associated with the Female Bladder In Males Anteriorly: Symphysis pubis Posteriorly: rectum and seminal vesicles Superiorly: small intestine Inferiorly: urethra and prostate gland Figure 11: Organs Associated with the Male Bladder Microscopically, the bladder wall is made up of three layers: Adventitia: The outer layer of loose connective tissue, it provides support and contains blood, lymphatic vessels and nerves. Detrusor Muscle: A thick middle layer of smooth muscle that contracts during urination to expel urine Mucosa: The innermost layer, composed of transitional epithelium that readily permits distension of the bladder as it fills with urine. Figure 12: Diagram of Bladder Showing the Three Layers Urethra The urethra is a tube/canal extending from the neck of the bladder to the external environment (outside of the body) at the external urethral orifice. It is longer in the male than in the female. The urethra serves as the exit passage for urine from the bladder. In males, it also serves as a conduit for semen during ejaculation. Male Urethra: is about 18-20 cm long, passing through the prostate and penis and consists of four parts: 6  Anatomy of Excretory System Prostatic Urethra: is the first portion of the male urethra and is about 3-4cm long. It originates at the urethral orifice of the bladder and passes through the prostate gland. Membranous Urethra: The shortest and narrowest part, about 1-2cm long and extends from the prostate gland to the bulb of the penis. Bulbar Urethra: Is the beginning of the spongy urethra and lies in the bub of the penis. Spongy (Penile) Urethra: is the longest portion of the male urethra about 15-18cm (inclusive of the bulbar urethra) running Figure 13: Diagram of Male Urethra also through the length of the penis and is surrounded by the corpus Showing the Detrusor Muscle spongiosum. it extends from the bulb of the penis to the external urethral orifice at the tip of the penis (glans penis) Female Urethra: It is about 3-4 cm long, 6 mm in diameter. It runs anterior to the vaginal canal and opens at the between the clitoris and vaginal orifice. The external urethral orifice is guarded by the external urethral sphincter. Microscopically, the wall of the female urethra has a muscle layer and an inner mucosa lining: Muscle Layer: this is an outer muscle layer composed of two parts: Inner Smooth Muscle: This layer is under autonomic nerve control, Figure 14: Diagram of Female Urethra meaning it functions involuntarily. Outer Striated Muscle: Surrounds the smooth muscle and forms the external urethral sphincter, which is under voluntary control, allowing conscious control over urination. Mucosa (Inner Lining): is continuous with the bladder and is supported by loose fibroelastic connective tissue containing blood vessels and nerves. Proximally (near the bladder), it consists of transitional epithelium while distally (near the external urethral orifice), it is composed of stratified epithelium. Note: There are two urethral sphincters: a) the internal sphincter consists of smooth muscle fibres at the neck of the bladder above the prostate gland and b) the external sphincter consists of skeletal muscle fibres surrounding the membranous part. Process of Urine Formation and Micturition The process of urine formation and its excretion from the body is essential for maintaining the body’s internal balance of water, electrolytes, and waste. The kidneys perform the complex task of filtering blood, producing urine, and regulating essential bodily functions like blood pressure and pH balance. The urinary bladder and urethra then help in storing and releasing urine through a process called micturition (urination). Urine Formation Urine formation is a multi-step process that occurs in the nephrons (the functional units of the kidney). Each kidney contains about 1 million nephrons, and these are responsible for filtering the blood and transforming the filtrate into urine. The process involves three main stages: 7  Anatomy of Excretory System Glomerular Filtration: This is the first step in urine formation. It occurs in the glomerulus. Blood enters the glomerulus through the afferent arteriole and due to high pressure, water, electrolytes (like sodium, potassium, chloride), and small molecules like glucose, amino acids, and waste products (urea) are pushed through the capillary walls (glomerular membrane) into the Bowman’s capsule (This fluid is called filtrate). Larger molecules such as blood cells and proteins remain in the bloodstream (because they are too large to pass through the glomerular membrane) and exit the glomerulus through the efferent arteriole. The filtrate, which consists of water and small solutes, moves into Figure 15: Diagram of the Process of Urine Formation the next part of the nephron: the renal tubule. Tubular Reabsorption: Occurs mainly in the Proximal Convoluted Tubule (PCT), loop of Henle, Distal Convoluted Tubule (DCT) and Collecting Duct. As the filtrate passes through the renal tubules, essential substances like water, glucose, amino acids, and ions (e.g., sodium, chloride) are reabsorbed into the bloodstream. PCT: About 65% of the filtrate is reabsorbed in the PCT. Water, glucose, amino acids, and important ions like sodium, potassium, and bicarbonate are returned to the bloodstream through surrounding capillaries. This prevents dehydration and helps conserve Figure 16: Schematic of the Process of Urine Formation essential nutrients. Loop of Henle: This is crucial for concentrating urine. In the descending limb, water is reabsorbed into the blood of the surrounding tissue, making the filtrate more concentrated. In the ascending limb, sodium and chloride are reabsorbed, but water remains, so the filtrate becomes diluted. I.e.: The descending limb of the loop of Henle is permeable to water, allowing it to be reabsorbed, while the ascending limb is impermeable to water but actively reabsorbs ions. DCT: Further reabsorption of ions occurs in the DCT. This process is regulated by hormones such as aldosterone (which increases sodium reabsorption) and parathyroid hormone (which increases calcium reabsorption). This in turn influences fluid balance and blood pressure. Collecting Duct: The final adjustment of urine concentration occurs in the collecting duct. The amount of water reabsorbed is controlled by Antidiuretic Hormone (ADH). If ADH is present, more water is reabsorbed, making the urine more concentrated. If ADH is absent, more water is lost, and urine becomes diluted. Tubular Secretion: Occurs in the DCT and collecting duct. Waste products and excess ions (such as hydrogen, potassium, and creatinine), are actively secreted from the blood and transported into the renal tubules. This process helps maintain acid-base balance (pH levels) by secreting hydrogen ions, regulate electrolyte balance by removing excess potassium and other ions, and eliminate drugs and toxins from the body. The remaining fluid in the renal tubule is now recognized as urine and contains 5% waste products (urea, ammonia, and creatinine, excess ions {sodium, potassium and calcium}), and 95% water. 8  Anatomy of Excretory System Excretion of Urine: After passing through the collecting ducts, urine (containing waste products and excess ions) flows into the renal pelvis (the funnel-like structure at the center of the kidney), and from there, into the ureters. The ureters transport urine to the urinary bladder, where it is stored until micturition (urination). Out of the large quantity of fluid that is filtered by the glomerular (180 L/day), only about 1 liter (1%) of fluid is excreted each day, the remaining fluid (99%) is reabsorbed. However, the renal excretion rate is highly variable depending on fluid intake. Micturition (Urination) Micturition is the process by which the bladder empties and urine is expelled from the body through the urethra. This process is controlled by a combination of involuntary and voluntary actions. It involves two phases: Bladder Filling and Storage Phase: As urine fills the bladder, the bladder wall stretches activating the stretches receptors in the bladder’s wall. These receptors then send signals to the spinal cord (specifically the sacral region) and the brain (specifically the cerebral cortex) to alert the body about the level of bladder fullness. Initially, the bladder can store urine without a strong urge to urinate. During this time, sympathetic nervous system activity is dominant, causing the detrusor muscle (the bladder's smooth muscle) to remain relaxed, and allow the bladder to expand and store increasing amounts of urine without generating high pressure, and the internal urethral sphincter (involuntary smooth muscle) stays contracted to prevent urine leakage into the urethra. When the bladder reaches a certain volume (around 300- 500 mL in adults), these stretch receptors send more frequent and stronger signals, leading to an urge to urinate. At this point, the cerebral cortex becomes involved in voluntary control, allowing a person to consciously decide whether or not to void (urinate). Figure 17: Schematic Showing the Micturition Process Voiding Phase: When the bladder reaches its maximum capacity (about 300-500 mL) and is full, the micturition reflex is triggered. The cerebral cortex sends signals to the body to prepare for urination, initiating a decrease in sympathetic activity (which had maintained bladder relaxation) and increase parasympathetic activity. The parasympathetic activity stimulates contraction of the detrusor muscle, simultaneously, the internal urethral sphincter relaxes, allowing urine to move towards the urethra. The external urethral sphincter (a skeletal muscle under voluntary control) provides the final barrier to urination. This muscle allows a person to consciously decide when to urinate. If it's not an appropriate time to urinate, the cerebral cortex can counteract the micturition reflex by keeping the external urethral sphincter contracted, delaying urination. When it’s socially appropriate or convenient, parasympathetic activity increases which leads to the complete contraction of the detrusor muscle, and the individual consciously relaxes the external urethral sphincter, allowing urine to be expelled through the urethra. After urination, the detrusor muscle relaxes, sympathetic activity increases to contract the internal urethral sphincter, and the external urethral sphincter returns to its contracted state to prevent further urine release. and the bladder begins to fill again, restarting the cycle. 9