Name the two major parts of a nephron. State the general function of nephrons. (p. 419)

The renal corpuscle is the initial phase of the nephron's urine formation process. It serves as the blood's first filtering point. Comprising the glomerulus and Bowman's capsule, the renal corpuscle is central to the kidneys' filtration function.

Here's how it works: Blood enters the glomerulus, which contains a cluster of tiny blood vessels with semi-permeable walls. As blood pressure forces water and solutes like salts and glucose from the blood into the Bowman's capsule, the resulting filtrate excludes larger proteins and blood cells, maintaining their presence in the blood. The glomerulus' unique structure and selective permeability are key to preventing vital components from being lost during filtration.

After filtration in the renal corpuscle, the renal tubule takes over the task of refining the filtrate. The tubule is a winding, tube-shaped structure divided into several segments, including the proximal convoluted tubule, the loop of Henle, the distal convoluted tubule, and the collecting duct.

In these segments, the renal tubule fine-tunes the filtrate through two processes: reabsorption and secretion. Reabsorption is where valuable substances like water, glucose, and certain ions are selectively moved back into the bloodstream. Conversely, secretion is the process of adding waste products or excess substances, such as hydrogen ions or drugs, into the filtrate. Thus, the renal tubule ensures that essential nutrients are conserved while waste materials are prepared for elimination.

Glomerular filtration is a pivotal part of nephron function. This process involves the movement of fluid from blood in the glomerulus into the Bowman's capsule to form filtrate.

It relies on the blood pressure gradient to push water and small solutes (like ions, glucose, and urea) across the filtration barrier, which includes the endothelial cells of glomerular capillaries, a basement membrane, and the podocytes of the Bowman's capsule. Large molecules and cells are typically too big to pass through this natural sieve, safeguarding essential proteins and cells in the bloodstream. The rate of glomerular filtration is a clinical marker of kidney function, with disturbances often reflecting kidney damage or disease. The process is influenced by factors like blood pressure and blood flow within the glomerulus.

Urine formation is the ultimate process of nephron function and a vital aspect of maintaining homeostasis in the body. It begins with glomerular filtration, continues with tubular reabsorption and secretion, and culminates in the excretion of urine.

The filtrate loses volume and changes composition as it travels through the renal tubule. Valuable substances are reclaimed, and waste materials are concentrated. By the time the filtrate reaches the collecting duct, it has been transformed into urine—now ready to be passed to the bladder through the ureters. The regulation of urine concentration is a tightly controlled process, responsive to the body's hydration levels and influenced by hormones like antidiuretic hormone (ADH) and aldosterone.