Summary
Highlights
Reabsorption and secretion in the distal convoluted tubule are hormonally controlled, allowing kidneys to adjust urine composition based on body needs. The collecting duct receives tubular fluid from several nephrons and primarily concentrates urine to conserve water, thanks to the osmolarity gradient generated by the loop of Henle. As the filtrate flows deeper into the salty medulla, it loses water. The collecting duct is also under hormonal control to regulate water reabsorption based on the body's hydration state; for example, more water is reabsorbed during dehydration, resulting in concentrated urine.
The kidneys filter blood plasma, removing metabolic wastes and toxins to excrete them in urine, while also maintaining blood volume and composition (homeostasis). Blood enters the kidney via the renal artery, branching into smaller arteries and then arterioles, which contact nephrons. Filtered blood exits through the renal vein, and urine is collected and leaves via the ureters.
Each kidney contains over a million nephrons, the functional units where blood filtration and urine formation occur. A nephron consists of a glomerular capsule (Bowman’s capsule) and a long renal tubule. Urine formation involves three steps: glomerular filtration, tubular reabsorption and secretion, and water conservation.
Blood enters Bowman's capsule through the afferent arteriole, passes through the glomerulus (a ball of capillaries), and exits via the efferent arteriole. The larger afferent arteriole creates high hydrostatic pressure, driving water and solutes from blood plasma into the nephron's capsular space through a filtration membrane. This membrane allows small molecules like water, ions, glucose, amino acids, and metabolic wastes (glomerular filtrate) to pass. Glomerular filtration rate (GFR) is maintained by renal autoregulation, sympathetic, and hormonal control, primarily through afferent arteriole constriction or dilation.
Only about 1% of the daily filtrate is excreted as urine; the remaining 99% is reabsorbed back into the blood by peritubular capillaries surrounding the renal tubule. The proximal convoluted tubule reabsorbs about two-thirds of the filtrate. Sodium reabsorption is crucial, creating osmotic pressure for water movement and an electrical gradient for negatively charged ions. Sodium-potassium pumps maintain low intracellular sodium, facilitating its diffusion into cells, often with glucose and other solutes. Nearly all glucose and amino acids are reabsorbed, along with about half of nitrogenous wastes, reducing but not eliminating them from the blood.
Tubular secretion involves additional wastes and solutes moving from the bloodstream into the tubular fluid. Reabsorption and secretion continue in the nephron loop (loop of Henle) and the distal convoluted tubule. The loop of Henle's main function is to create and maintain an osmolarity gradient in the medulla. The ascending limb actively pumps out sodium, making the medulla salty. The descending limb is permeable to water, causing filtrate to become concentrated. The ascending limb is permeable to ions but not water, making the filtrate more diluted as it moves up.