Urinary System and Homeostasis
Urinary System Functions
Excretion
Removes indigestible chemicals and waste from the blood.
pH Regulation
Removes hydrogen and adds bicarbonate ions to adjust blood pH.
Blood Pressure Regulation
Secretes renin, increasing water and salt in the blood, causing blood vessel narrowing.
Osmoregulation
Removes excess salt and water to balance body fluids.
Blood Volume and Composition
Maintains adequate blood volume and composition.
Other Homeostatic Functions
Releases erythropoietin for red blood cell production and synthesizes active vitamin D.
Kidneys
Approximately 10cm long and 3cm thick, surrounded by the renal capsule, with three distinct areas:
- Cortex: The outer layer where urine production begins. Filters fluid from the blood.
- Medulla: Reabsorbs substances not absorbed in the cortex and extracts unnecessary substances.
- Pelvis: Collects urine and transports it to the ureter.
Each kidney contains about a million nephrons. Each nephron consists of Bowman’s capsule (double-walled cells) surrounding the glomerulus (cluster of capillaries) and the renal tubule (3cm long) containing the U-shaped Henle’s loop.
Ureter
A 30cm hollow tube that carries urine from the kidneys to the bladder. Muscle contractions facilitate urine transport.
Bladder
A muscular bag that stores urine. Located in the pelvic girdle, its muscles relax when empty and stretch receptors signal the central nervous system to initiate emptying through the urethra as it fills.
Urine Composition
Composed of 95% water, uric acid (nucleic acid waste), urea (toxic waste product), creatinine (muscle waste), and ions (from food).
Urine Formation Stages
- Filtration: Body fluids pass through excretory tubes, creating initial urine containing waste and necessary molecules.
- Reabsorption: Excretory tubes extract water and useful substances from the initial urine.
- Secretion: Materials, such as K+ ions, are transferred from the blood to the excretory tubes.
- Concentration: Water and ions are absorbed as the filtrate moves through the collecting duct. ADH presence increases water permeability, concentrating urine. ADH absence makes the duct impermeable, resulting in hypotonic urine.
Urine flows through collecting tubules, into the pelvis, through the ureter, stored in the bladder, and expelled through the urethra.
Water in the Body
Water enters through drinking, food, and metabolic processes. It exits through urine, sweat, respiration, and feces. Water provides a medium for chemical reactions, regulates temperature, and facilitates transport and waste disposal. It constitutes 60% of body weight, with intake and output balanced.
Water Intoxication
Excessive sweating without proper fluid replacement (with electrolytes) can cause water to enter cells by osmosis, leading to central nervous system imbalances, and in severe cases, convulsions, coma, and death.