THE KIDNEYS AND THE PROCESS OF URINE FORMATION

• The kidneys are part of the urinary or excretory system.
• The urinary system consists of four main components: kidneys, ureters, urinary bladder, and urethra.
• The kidneys are bean-shaped paired organs located in the posterior wall of the abdomen (retroperitoneum).
• Each kidney weighs about 150 g in the adult male and 135 g in the adult female.
• The kidneys are about 12.5 cm in length, 6 cm in width, and 2.5 cm in depth.
• The kidneys have three layers: cortex, medulla, and pelvis.
• The cortex of the kidney is about 1.4cm thick and is the exclusive site of the plasma filtration process because all of the glomeruli are located in the outer cortex.
• The medulla of the kidney is the inner layer and consists of renal tissue shaped into pyramids.
• The pelvis of the kidney is funnel-shaped, emerges from the indented region of each kidney and narrows to join with the ureter.
• Aldosterone antidiuretic hormone (ADH) or arginine vasopressin promotes sodium reabsorption in the distal convoluted tubule and potassium secretion.
• Aldosterone antidiuretic hormone (ADH) or arginine vasopressin makes the walls of the distal convoluted tubule and collecting duct permeable or impermeable to water.
• Aldosterone antidiuretic hormone (ADH) or arginine vasopressin responds to the body’s need for sodium, which is produced and released from the adrenal cortex.
• The ureters are about 25 cm long and carry the urine to the bladder.
• Secreted hydrogen ions combine with a filtered phosphate ion and are excreted.
• Aldosterone antidiuretic hormone (ADH) or arginine vasopressin responds to the body’s state of hydration, which is produced in the hypothalamus and released by the posterior pituitary gland.
• Hydrogen ions are secreted in exchange for bicarbonate ions in the posterior cortical thymus.
• The acid-base balance in the body is regulated by the secretion of hydrogen ions.
• The urinary bladder is shaped like a three-sided pyramid and stores the urine produced.
• Tubular secretion is a process that occurs in the kidney, where certain substances are secreted into the urine.
• Hormone secretion in the kidney includes the production of erythropoietin and renin.
• Collecting duct traverses the renal cortex and medulla and is the site of final urine concentration, terminating at the renal papilla, conveying urine formed into the renal calyces of the kidney.
• The kidney requires a rich blood supply to execute its primary function of regulating the internal environment of the body.
• There is a direct relationship between the kidney’s functional ability and its blood supply.
• The kidney receives approximately 25% of cardiac output, which is the blood that leaves the left ventricle of the heart.
• In a normal adult, the blood passes through the kidneys at a rate of about 1200 mL/min, or 600mL/min/kidney.
• Glomerular filtration is a process that occurs in the kidney, where blood is filtered through millions of tiny blood vessels, known as glomeruli.
• Tubular reabsorption is a process that occurs in the kidney, where excess water and solutes are reabsorbed into the bloodstream.
• Renal blood flow or renal circulation is a crucial aspect of kidney function, as it supplies the kidney with the necessary oxygen and nutrients to perform its tasks.
• The urethra is about 4 cm long in women and about 24 cm long in men and delivers the urine for excretion.
• The blood supply to the kidneys comes from the renal artery and the blood drainage is through the renal vein.
• Reabsorptive capacity varies with each solute and depends on the glomerular filtration rate.
• Reabsorption of water and sodium in the distal convoluted tubule and collecting ducts is controlled by hormones.
• Tubular secretion serves two major functions: elimination of waste products not filtered by the glomerulus and hormone secretion.
• Various ions are also secreted including hydrogen ions, ammonium ions, sodium ions, potassium ions, bicarbonate ions, uric acid, and some weak acids and bases.
• Renal concentration begins in the descending and ascending loops of Henle.
• Filtrate leaving the ascending loop is diluted (low concentration) owing to the reabsorption of salt and not water in that area of the tubule.
• Final concentration begins in the late distal convoluted tubule and continues in the collecting ducts.
• Sodium and chloride are reabsorbed in the ascending loop of Henle.
• Water is removed via osmosis in the descending loop of Henle.
• The micturition reflex is initiated when approximately 150mL of urine accumulates and results in the voiding of urine via the urethra.