Urinary System Review

• Primary organs: The urinary system is comprised of the kidneys, ureters, urinary bladder, and urethra. Each organ plays a vital role not only in waste excretion but also in maintaining various physiological processes that ensure homeostasis within the body.

• Functions:

  • Waste removal: The kidneys function as the body's natural filtration system, filtering the blood to remove waste products such as urea, creatinine, and excess ions. The filtration process converts the filtrate into urine, which is then passed through the urinary tract for excretion.

  • Urine storage: The urinary bladder is a highly elastic muscular sac that serves as a reservoir for urine. It can expand significantly to accommodate volumes up to 1 liter, allowing for the storage of urine until it is convenient to be emptied. This capability prevents continuous urination and provides the body with the flexibility to excrete waste at suitable intervals.

  • Urine expulsion: The urethra is a tubular structure that transports urine from the urinary bladder to the outside of the body. It is crucial for the expulsion of waste and aids in the disposal of toxins and excess substances gathered in the urine.

  • Blood volume regulation: The kidneys are integral in maintaining homeostasis through the regulation of blood volume. They achieve this by controlling the levels of interstitial fluid and blood plasma through hormonal influences, which help sustain adequate blood pressure and nutrient distribution to tissues.

  • Regulation of erythrocyte production: The kidneys secrete a hormone called erythropoietin in response to low blood oxygen levels. This hormone stimulates the bone marrow to produce and release more red blood cells, ensuring that the body's oxygen-carrying capacity is sufficient to meet metabolic demands.

  • Ion balance regulation: The kidneys help maintain the balance of essential ions in the body. They carefully regulate the levels of sodium, potassium, calcium, and phosphate, ensuring that these ions are present in appropriate concentrations necessary for various bodily functions, including nerve transmission and muscle contraction.

  • Acid-base balance regulation: The kidneys play a critical role in managing the body's acid-base balance. They adjust the levels of hydrogen ions and bicarbonate in the blood, helping maintain a stable pH level. This function is vital for the proper functioning of enzymes and overall metabolic processes within the body.

Kidneys

• Anatomy: The kidneys are bean-shaped, reddish-brown organs, approximately 12 cm in length, and are located retroperitoneally, meaning they are situated behind the peritoneum of the abdominal cavity.

  • Each kidney is surrounded by a protective fibrous capsule, a layer of perinephric fat that cushions them, and renal fascia which provides support. The kidneys are divided into two main regions: the outer cortex, which contains the nephrons where filtration occurs, and the inner medulla, featuring renal columns and renal pyramids which play a role in urine formation. The hilum serves as the point of entry and exit for blood vessels and the ureter.

• Blood Supply: The kidneys receive 20-25% of the heart's total blood flow via the renal arteries. Blood flow follows a specific pathway that includes segmental arteries branching off the renal artery, interlobar arteries traveling up through the renal columns, arcuate arteries at the boundary of the cortex and medulla, and interlobular arteries that supply the renal cortex. Blood travels through afferent arterioles into the glomerulus for filtration, then exits via efferent arterioles, proceeding to the peritubular capillaries or vasa recta before returning through interlobular veins, arcuate veins, interlobar veins, and finally into the renal vein.

• Nephrons: Nephrons are the fundamental functional units of the kidneys, with approximately 2.5 million located in each kidney. Each nephron consists of a renal corpuscle, which includes the glomerulus and Bowman's capsule, and a renal tubule composed of the proximal convoluted tubule (PCT), nephron loop (loop of Henle), and distal convoluted tubule (DCT). Nephrons facilitate urine formation through three key processes:

  1. Glomerular Filtration: This initial stage involves the movement of water, ions, and small molecules from the blood in the glomerulus into the capsular space of Bowman's capsule, driven by hydrostatic pressure differences.

  2. Tubular Reabsorption: In this process, essential substances and most water are reabsorbed from the tubular fluid back into the bloodstream, ensuring that vital nutrients are not lost in urine.

  3. Tubular Secretion: This phase involves the active transport of additional waste products and excess ions from the blood into the tubular fluid, further refining the composition of urine before it is excreted.

Urinary Tract

• Ureters: The ureters are fibromuscular tubes that conduct urine from the kidneys to the urinary bladder. Positioned inferiorly to the kidneys, they transport urine via peristaltic movements. The ureters are composed of three layers: an inner mucosa with transitional epithelium that allows for stretching, a muscularis layer containing two smooth muscle layers that facilitate peristalsis, and an outer adventitia for structural support.

• Urinary Bladder: This expandable muscular sac is located posterior to the pubic symphysis, functioning as a reservoir for urine. Its structure includes the mucosa, a submucosa that provides supportive connective tissue, the muscularis layer made of the detrusor muscle responsible for bladder contractions during urination, and the adventitia for additional protection. The bladder features a trigone area—defined by ureteric openings and the urethra entrance—that funnels urine into the urethra when the bladder contracts.

• Urethra: The urethra is a tubular structure that conveys urine from the bladder to the exterior of the body. It contains both internal (involuntary) and external (voluntary) sphincters that help control the release of urine. The male urethra is longer and serves a dual purpose, facilitating both urine and semen expulsion, while the female urethra is shorter and exclusively dedicated to urine.

• Micturition: Micturition is the process of urination, initiated reflexively when the bladder fills to about 200-300 ml. This reflex involves the contraction of the bladder wall and the relaxation of the sphincters, allowing for the release of urine from the body. Factors such as emotional state and the presence of external stimuli can also influence this reflex.

Aging and Development of Urinary System

• Aging Effects: With aging, structural changes in the urinary system can lead to decreased kidney function, reduced bladder capacity, and altered micturition patterns, which may result in increased urinary incontinence or retention issues.

• Kidney and Ureter Development: The kidneys and ureters develop from the intermediate mesoderm in the embryo. Three sets of excretory organs emerge during development: pronephros, which is transient; mesonephros, which also serves a temporary function; and metanephros, which forms the permanent kidneys.

• Urinary Bladder and Urethra Development: The urinary bladder and urethra originate from the cloaca, an area of the hindgut during early embryonic development. The cloacal membrane eventually gives rise to the urinary bladder and the urethra, establishing their anatomical foundations for later life.