6 Renal System Physiology
Learn how kidney and nephron structure supports urine formation, fluid and electrolyte control, blood pressure regulation, and acid–base balance.
Kidney functions
The maintain the composition of the internal environment while removing metabolic wastes and excess substances. They regulate water and electrolyte levels, blood volume and pressure, and blood acidity. They also produce renin and erythropoietin and activate vitamin D, producing calcitriol.
Kidney and structure
Each kidney has an outer cortex and an inner medulla. Its functional units, the nephrons, filter blood and modify the resulting fluid.
A consists of a and a tubule. The is the glomerulus enclosed by Bowman’s capsule; the tubule includes the proximal tubule, loop of Henle, distal tubule, and collecting duct.
Blood enters the glomerulus through an afferent arteriole and leaves through an efferent arteriole. Capillaries alongside the tubule collect reabsorbed substances. Urine drains through collecting ducts into the renal pelvis and then the ureter.
How urine is formed
Urinary excretion reflects three processes:
In , blood pressure drives water and small dissolved substances from glomerular capillaries into Bowman’s capsule. Blood cells and most proteins remain in the bloodstream, and the fluid that enters Bowman’s capsule is called .
In , useful substances move from the tubule back into the blood. The proximal tubule reclaims most filtered water and sodium, along with nearly all glucose and amino acids under normal conditions. Other segments make further regulated adjustments.
In , selected substances move from blood into the tubule. This helps eliminate hydrogen ions, potassium, some drugs, and metabolic wastes.
Glucose illustrates how these processes affect what leaves the body: it may enter the , but a healthy kidney normally reabsorbs it. A substance that is not reabsorbed, or that is additionally secreted, remains in the tubular fluid and can leave in urine.
Fluid, electrolytes, and blood pressure
The adjust urine volume and composition in response to the body’s needs. The loop of Henle helps establish a concentration gradient in the medulla, allowing the collecting ducts to conserve water when needed.
When blood is concentrated or water is scarce, antidiuretic hormone (ADH, or vasopressin) increases the collecting ducts’ permeability to water. More water returns to the blood, producing a smaller volume of more concentrated urine. When ADH is low, less water is reabsorbed and urine is more dilute.
The responds to low blood pressure or reduced kidney blood flow. Aldosterone increases sodium in the distal , and water tends to follow. Aldosterone also promotes potassium . Together, these mechanisms help regulate water, sodium, potassium, blood volume, and blood pressure.
During dehydration, increased ADH helps the conserve water. If blood volume also falls, RAAS promotes sodium and water retention, helping support blood pressure.
Acid–base regulation
The work with the lungs to stabilize blood . The lungs adjust carbon dioxide removal, while the provide longer-term control by reabsorbing filtered , secreting hydrogen ions into the tubules, and excreting acid, much of it buffered as ammonium.
When the body has excess acid, the increase acid excretion and conserve or generate . When conditions shift the other way, renal handling of and hydrogen ions adjusts accordingly. These processes help keep blood within a narrow range.