6 The Renal System
Learn how kidney structure supports urine formation, fluid and electrolyte balance, blood-pressure regulation, and acid–base control.
Kidney roles and structure
The kidneys help maintain homeostasis by removing metabolic wastes and adjusting the amount of water, electrolytes, and acid excreted in urine. They also contribute to blood-pressure regulation, red blood cell production, and calcium balance.
Each kidney has an outer cortex and an inner medulla. Urine drains from the medulla into the renal pelvis, then passes through a ureter to the bladder. Each kidney contains roughly a million nephrons, its functional units.
A contains a and a renal tubule. The tubule consists of the proximal tubule, loop of Henle, and distal tubule, and connects to a collecting duct. Blood leaving the glomerulus travels through vessels that surround the tubule, allowing exchange between blood and tubular fluid.
Filtration and urine formation
Urine forms through three linked processes: glomerular filtration, , and .
During glomerular filtration, blood pressure drives water and small dissolved substances from glomerular capillaries into Bowman’s capsule. Blood cells and most large proteins normally remain in the bloodstream. The is the volume filtered per minute; a typical adult value is about , though it varies with factors such as age and body size.
As filtrate travels along the tubule, returns useful substances to the blood. These include most filtered water and sodium, as well as glucose, amino acids, and bicarbonate. Normally, nearly all filtered glucose is reabsorbed.
moves some substances from the blood into the tubule. These include hydrogen ions, potassium in certain conditions, and many drugs and waste products. Secretion helps regulate blood chemistry and remove substances that were not sufficiently eliminated by filtration.
The fluid remaining after filtration, reabsorption, and secretion is urine. The kidneys filter about of fluid each day in a typical adult, but most is reabsorbed; daily urine output is much smaller and varies with intake and losses. Urine flows from collecting ducts to the renal pelvis, down the ureters, and into the bladder for storage before leaving the body through the urethra.
Water, electrolytes, and blood pressure
The kidneys adjust water and solute excretion to help keep blood volume, pressure, and concentration within appropriate ranges. increases the collecting ducts’ permeability to water. When the body needs to conserve water, more water is reabsorbed into the blood and urine becomes more concentrated. When less ADH is acting, more water remains in the tubule and urine is more dilute.
The helps respond to reduced kidney perfusion or reduced sodium chloride delivery to parts of the . Renin initiates a hormone cascade that produces angiotensin II, which supports blood pressure and promotes sodium retention. Angiotensin II also stimulates aldosterone, which increases sodium reabsorption and potassium secretion in the distal . Water tends to follow retained sodium. Together, these mechanisms help regulate extracellular fluid volume and blood pressure.
The tubules also adjust the excretion of electrolytes such as sodium, potassium, chloride, calcium, and phosphate. Conserving sodium helps maintain extracellular fluid volume, while secretion of excess potassium helps control its concentration in the blood. The precise response depends on the body’s needs and hormonal signals.
Acid–base regulation
Metabolism produces acids that must be managed to keep blood within a narrow range. The lungs regulate carbon dioxide, while the kidneys provide a slower, sustained contribution.
Kidney tubules reabsorb filtered bicarbonate, a major blood buffer, and secrete hydrogen ions into tubular fluid. They also excrete acid buffered by substances such as phosphate and ammonia; this process adds newly generated bicarbonate to the blood. In this way, the kidneys help compensate for acid–base disturbances and eliminate nonvolatile acids.