Concepts to remember on Human Physiology Excretory Products and Their Elimination

• Structure of the kidney

• Renal cortex: The outermost layer of the kidney involved in the initial processing of blood.

• Renal medulla: The inner layer of the kidney where further processing of the filtrate takes place.

• Renal pelvis: The funnel-shaped structure that collects urine from the renal calyces and directs it to the ureter.

• Ureter: The tube that carries urine from the renal pelvis to the urinary bladder.

• Urinary bladder: The muscular organ that stores urine until it is released during urination.

• Urethra: The tube that allows urine to pass from the urinary bladder to the outside of the body.

• Functions of the kidney:

• Filtration of blood: In the glomerulus, the kidney filters excess water, ions, small molecules, and waste products from the bloodstream, forming a filtrate that enters the renal tubules.

• Reabsorption of water, glucose, amino acids, and other essential substances: As the filtrate passes through the renal tubules, essential substances such as glucose, amino acids, and most of the filtered water are reabsorbed back into the bloodstream.

• Secretion of waste products: The renal tubules also perform tubular secretion, where certain waste products (e.g., hydrogen ions, creatinine, uric acid, and drugs) are actively transported into the tubules from the bloodstream and added to the filtrate.

• Formation of urine: Urine formation involves three processes:

• Glomerular filtration: The initial step where blood is filtered to form the filtrate.

• Tubular reabsorption: The process where essential substances are selectively reabsorbed back into the bloodstream.

• Tubular secretion: The process where waste products are actively transported into the tubules from the bloodstream.

• Regulation of urine output:

• Antidiuretic hormone (ADH): ADH, also known as vasopressin, regulates the reabsorption of water in the collecting tubules, influencing the concentration and volume of urine.

• Aldosterone: Aldosterone is a hormone produced by the adrenal cortex that controls the reabsorption of sodium and the secretion of potassium in the collecting tubules, influencing the electrolyte balance of the body and blood volume.

• Micturition:

• Voluntary control of urination: Once the urinary bladder reaches a certain level of fullness, the desire to urinate arises. Voluntary control allows us to consciously postpone or initiate urination to an appropriate time and place.

• Involuntary control of urination: Involuntary micturition is regulated by the micturition reflex, a complex neural pathway that involves the spinal cord and brain. **

• Reflexes involved in micturition:

-Stretch reflexes: When the bladder fills up, it stretches, triggering stretch reflexes that initiate bladder contraction and relaxation of the urethral sphincter, leading to urination.

• Voluntary reflexes: higher centers in the brain, primarily the cerebral cortex, can voluntarily control micturition by overriding or facilitating the micturition reflex.

• Disorders of the urinary system:

• Kidney stones: Hard deposits of minerals and salts that form in the kidneys and can cause severe pain when passing through the urinary tract.

• Urinary tract infections (UTIs): Infections caused by bacteria or other microorganisms anywhere along the urinary system, including the kidneys, bladder, and urethra.

• Renal failure: A condition where the kidneys lose their ability to function effectively, leading to the accumulation of waste products, electrolyte imbalances, and fluid overload in the body.

• Diabetes insipidus: A disorder characterized by the inability of the kidneys to concentrate urine due to deficient production or action of antidiuretic hormone (ADH), resulting in excessive urine output and dehydration.

• Diabetes mellitus: A chronic metabolic disorder characterized by elevated blood glucose levels due to impaired insulin production or resistance, which can also affect kidney function and contribute to kidney damage over time.