Kidney Function and Urine Composition
Definitions:
- Bulk Filtration: A non-specific process during which most constituents in the blood plasma, except red blood cells and proteins, pass from the glomerulus into the lumen of Bowman’s capsule.
- Ultrafiltrate: Similar in composition to blood plasma, it’s a product of membrane filtration where forces like pressure or concentration gradients cause separation through a semipermeable membrane.
- Antidiuretic Hormone (ADH): A small peptide molecule released by the pituitary gland. Synthesized in the hypothalamus, ADH has an antidiuretic action, preventing the production of dilute urine.
- Aldosterone: A mineralocorticoid hormone produced by the adrenal cortex. Aldosterone regulates water and electrolyte balance, encouraging the kidney to excrete potassium and retain sodium, thus retaining water.
- Proteinuria: The presence of abnormal quantities of protein in the urine, potentially indicating kidney damage. It can also result from overproduction of proteins by the body.
- Hemoglobinuria: The presence of free hemoglobin in the urine, which may make the urine look dark. Normally absent in urine, hemoglobinuria can signify various abnormal conditions, including bleeding.
Volume Chart (Time vs. Volume):
Antidiuretic hormone (ADH) controls kidney function. Synthesized in the hypothalamus and released by the posterior pituitary in response to decreased plasma volume or increased plasma osmolarity, ADH acts on epithelial cells lining the distal tubule and collecting duct. Excessive sodium consumption leads to increased blood plasma osmolarity, triggering ADH release. This enhances water reabsorption, reducing urine output and concentrating urine with Na⁺, resulting in a darker urine color.
Glucose and Urine:
Glucose is normally filtered out of the plasma as part of the ultrafiltrate. However, it is usually absent in urine because all filtered glucose is reabsorbed in the proximal convoluted tubule. This reabsorption is mediated by carrier proteins that co-transport sodium ions. When plasma and ultrafiltrate glucose concentrations are within normal limits, sufficient carrier molecules transport all glucose back into the blood. However, if blood glucose levels exceed the renal plasma threshold for glucose (180 mg/100ml plasma), the glucose per sodium ion co-transport molecules become saturated, unable to transport all the glucose from the filtrate. The excess glucose is excreted in the urine, a common occurrence in patients with Diabetes Mellitus.
Ketone Bodies:
Ketone bodies appear in individuals with Diabetes Mellitus due to insufficient insulin production, which is necessary for glucose transport into cells. When insulin is deficient, cells lack sufficient glucose and rely on the metabolism of stored fatty acids. Ketone bodies, byproducts of fat metabolism, increase in the blood. When the body’s ability to metabolize ketone bodies is exceeded, they accumulate in the blood and are eventually excreted in the urine. The presence of ketone bodies in urine is termed ketonuria.