Urinary System – Physiology

The normal healthy adult produces 1-2 liters of urine a day. Filtration: The movement of fluid across the filtration membrane as a result of pressure difference. The fluid that is pulled from the blood becomes The filtrate. 21% of the total blood is the Renal Faction from the total cardiac output. Due to glomerular blood pressure water and small molecules move from the glomerulus to the inside of the renal corpuscle.

Nephrons in the kidney filter 180 liters of water per Day along with a considerable amount of small molecules (such as glucose) and ions (sodium). If the composition Of urine were the same as that of the glomerular* filtrate, The body would continually lose water, salts, and Nutrients. Therefore, we can conclude that the Composition of the filtrate must be altered as this fluid Passes through the remainder of the tubule. * A glomerulus is a capillary tuft that performs the first step in filtering blood to form urine.

Reabsorption: When sodium ions (Na+) are actively reabsorbed, Chloride ions (Cl-) follow passively. The reabsorption Of salt (NaCl) increases the osmolarity of the blood Compared to the filtrate, the therefore water moves Passively from the tubule into the blood. Where is this happening? Renal tubules

Glucose and amino acids also return to the blood at the PCT (proximal convoluted tubule). Glucose is an example of a molecule that ordinarily is completely reabsorbed because there is a plentiful supply of carrier molecule for it. There is a max – the rest ends up in the urine. In diabetes mellitus – excess glucose occurs in the blood and then the urine. This increases the osmolarity compared to the blood and less water gets reabsorbed – this results in frequent urination and an increased thirst.

Secretion: Hydrogen ions, potassium ions, creatinine, and some Drugs are moved by active transport from the blood Into the distal convoluted tubule. Ammonia is synthesized in the epithelium of the Nephron and diffuses into the lumen.

Water reabsorption: ADH (Antidiuretic Hormone) is released by the Posterior lobe of the pituitary gland. When ADH Is present, more water is reabsorbed in the Collecting ducts – resulting in an increase in Blood volume and pressure and decrease Urine production.

Micturation Reflex Triggered by stretching of the urinary bladder. Triggered by stretching of the urinary bladder.