Urinary System

  • Functions of the Urinary System:

    • Major excretory system of the body, essential for maintaining homeostasis.

    • Comprises two kidneys, which filter blood and produce excretory products.

    • Ureters transport urine from the kidneys to the urinary bladder, where it is stored until expelled via the urethra.

    • Major components of urine include:

    • Excess water, which helps regulate hydration levels.

    • Excess ions such as sodium (Na⁺) and potassium (K⁺) that are regulated by renal function.

    • Metabolic wastes like urea, a by-product of protein metabolism, harmful if allowed to accumulate in the blood.

    • Toxic substances that need to be eliminated for overall health.

    • In addition to excretion, kidneys play roles in various metabolic processes, including regulation of blood pressure, electrolyte balance, and acid-base homeostasis.

  • Functions of the Kidneys:

    • Excretion of waste products from the blood: Approximately 21% of cardiac output is filtered per minute, facilitating the removal of toxins and excess substances.

  • Regulatory Functions:

    • Blood volume and pressure regulation through extracellular fluid volume control, influencing overall cardiovascular health.

    • Regulation of blood solute concentrations—including ions such as Na⁺, K⁺, and Cl⁻—and urea, crucial for maintaining osmotic balance.

    • pH regulation of extracellular fluid by secreting H⁺ ions, ensuring the blood remains within a healthy pH range (7.35-7.45).

    • Erythropoietin secretion stimulates red blood cell production in response to low oxygen levels.

    • Regulation of vitamin D synthesis is vital for calcium homeostasis, impacting bone health and function.

  • Anatomy of the Kidneys:

    • Location:

    • Positioned as retroperitoneal organs on the posterior abdominal wall, providing some protection from the lumbar vertebrae and rib cage.

    • The right kidney is usually positioned slightly lower than the left due to the presence of the liver.

    • External Anatomy:

    • Renal capsule: A fibrous connective tissue that envelops each kidney, providing a protective layer.

    • Adipose tissue: Surrounds and cushions the kidneys, serving as both protection and an energy reserve.

    • Renal fascia: Connective tissue anchoring the kidneys to the abdominal wall, helping maintain their position.

    • Hilum:

    • The entry point for renal artery and nerves, and the exit point for renal vein and ureter, facilitating the vascular supply and drainage.

    • Internal Anatomy:

    • Cortex: Outer area of the kidney that contains renal columns extending into the medulla, involved in filtration and blood processing.

    • Medulla: Inner area containing cone-shaped renal pyramids; these structures lead to the renal papillae, which point towards the renal sinus.

    • Renal pelvis: A funnel-shaped space that collects urine from the major calyces, leading it to the ureter for transport.

  • Functional Unit - Nephron:

    • Each nephron comprises:

    • Renal corpuscle: The site of blood filtration, where plasma is filtered to form filtrate.

    • Proximal convoluted tubule: Reabsorbs most of the water and nutrients from the filtrate.

    • Nephron loop (Loop of Henle): Creates a concentration gradient essential for the reabsorption of water.

    • Distal convoluted tubule: This area further regulates ion balance and fluid volume.

    • Path of urine: distal convoluted tubule → collecting duct → renal papilla → minor calyx, illustrating the progression from formation to release.

  • Urine Production Process:

    1. Filtration:

    • Blood pressure in glomerular capillaries forces fluid (filtrate) out through the filtration membrane, allowing for the non-selective separation of substances based on size; large proteins and blood cells do not pass.

    • Filtration pressure consists of:

      • Glomerular capillary pressure (50 mm Hg)

      • Capsule hydrostatic pressure (10 mm Hg)

      • Blood colloid osmotic pressure (30 mm Hg)

      • Net filtration pressure: (50 - 10 - 30) = 10 mm Hg, critical for effective urine formation.

    1. Tubular Reabsorption:

    • Water and essential nutrients are salvaged from the filtrate back into the blood through peritubular capillaries, primarily in the proximal convoluted tubule.

    • Approximately 99% of water and 100% of glucose and amino acids are effectively reabsorbed, ensuring nutrient conservation.

    • Regulated reabsorption occurs in the distal convoluted tubule and collecting ducts, adjusting to the body’s needs.

    1. Tubular Secretion:

    • Involves the movement of non-filtered substances from the blood into the filtrate, facilitating the elimination of waste products and toxins.

  • Hormonal Regulation:

    • Renin-Angiotensin-Aldosterone System:

    • Activated by low blood pressure, this system regulates blood volume and pressure effectively.

    • Renin release leads to the production of angiotensin II, promoting aldosterone secretion, which increases sodium reabsorption from urine back into the blood.

    • Antidiuretic Hormone (ADH):

    • This hormone promotes water reabsorption in the kidneys by inducing the integration of aquaporins in collecting ducts, which regulates urine concentration and volume.

    • Atrial Natriuretic Hormone (ANH):

    • Functions to reduce blood volume and pressure by inhibiting sodium reabsorption and promoting increased urine output, countering aldosterone.

  • Urine Movement and Micturition:

    • Ureters transport urine via hydrostatic pressure and peristalsis, moving urine from the kidneys to the bladder.

    • The micturition reflex is triggered by bladder stretch, leading to detrusor muscle contraction, allowing for the relaxation of the external sphincter and resulting in urination.

  • Effects of Aging on Kidneys:

    • Gradual reduction in kidney size and blood flow with age, resulting in decreased nephron numbers, less effective waste filtration, reduced vitamin D synthesis, and impaired hormonal responsiveness, which can impact overall kidney function and health.