Kidney

• The urinary system consists of two kidneys, two ureters, urinary bladder and urethra.
• The functions of kidneys include excretion of waste products, homeostatic function, and endocrine function.
• The nephron is the structural and functional unit of kidney, each kidney contains one million nephron.
• The glomerulus consists of glomerular capillaries and Bowman’s capsule, they arise from afferent arteriole and drain into the efferent arteriole.
• The renal tubules consist of proximal convoluted tubule (PCT), loop of Henle (LH), distal convoluted tubule (DCT), and collecting duct.
• Urine passes from the collecting duct to the ureter and finally to the urinary bladder.
• Glomerular filtration is the process where plasma is filtered through pores of capillaries to Bowman’s capsule, the filtered fluid is called glomerular filtrate, it contains all constituents of the blood except blood cells and plasma proteins because they are larger than the pores.
• Glomerular filtration rate (GFR) is the volume of fluid filtered by glomerular capillaries/minute, it equals 125 ml/minute (180 L/day) and renal tubules reabsorb 99% of the filtrate and the remaining 1% is excreted as urine.
• Renal tubular reabsorption is the process by which essential substances pass from tubular filtrate to peritubular capillaries (H2O, glucose, Na+ & Ca++).
• Renal tubular secretion is the process by which harmful substances pass from peritubular capillaries to tubular filtrate (Urea, H+, NH3, drugs).
• Micturition is the natural evacuation of bladder through urethra.
• Renal failure is the inability to remove waste products from blood, due to reduction in number of working nephrons leading to accumulation of waste products.
• Filling of bladder with urine stimulates bladder receptors, which in turn activates micturition centre, causing contraction of bladder wall and relaxation of internal sphincter, leading to evacuation of the bladder.
• The wall of urinary bladder contains receptors which sense bladder distension, then send signals to micturition centre in the spinal cord (S 2,3,4).
• Glucosuria is the presence of increased amounts of glucose in urine, which can occur in diabetes mellitus due to increased blood glucose level or renal glucosuria due to decreased number of carrier for glucose in the kidney.
• pH ˃ 7 is considered alkalosis, while pH ˂ 7 is considered acidosis.
• In adult, cerebral cortex can inhibit the reflex if the conditions are unsuitable and can facilitate it if the conditions are suitable.
• Normal pH of blood is 7.4, which depends on Hydrogen ions.
• Micturition reflex is involuntary in infants.
• There are two sphincters around the inlet of urethra: the Internal urethral sphincter, made of smooth muscle fibers and its contraction is involuntary, and the External urethral sphincter, made of striated muscle fibers and its contraction is voluntary.
• Kidneys play most important role in acid-base regulation, by secreting H+, reabsorbing filtered HCO3-, and synthesizing new HCO3-.
• Normal metabolism of body releases waste products which cause blood pH to decrease.
• With growth, cerebral cortex acquires control on micturition reflex.
• Blood pH is kept constant by chemical buffers and respiratory system.
• These toxic substances can be removed by dialysing the blood in a machine called artificial kidney (hemodialyser) in a process called hemodialysis.