The juxtaglomerular apparatus (JGA) is a specialized structure in the nephron, located where the distal convoluted tubule comes into contact with the glomerulus. It plays a key role in regulating blood pressure and the rate of glomerular filtration. The JGA consists of juxtaglomerular cells, which are modified smooth muscle cells of the afferent arteriole, and the macula densa, a group of specialized cells in the distal tubule. The juxtaglomerular cells produce and secrete renin, an enzyme that is pivotal in the renin-angiotensin-aldosterone system (RAAS), which regulates blood pressure and fluid balance. The macula densa detects changes in sodium chloride concentration in the filtrate and signals the juxtaglomerular cells to release renin accordingly. This response mechanism is essential for maintaining homeostasis, particularly in response to changes in blood volume, blood pressure, or electrolyte balance.

The counter-current multiplier system in the Loop of Henle plays a critical role in concentrating urine and conserving water. This system involves the interaction between the descending and ascending limbs of the Loop of Henle. The descending limb is permeable to water but impermeable to solutes, allowing water to be reabsorbed into the surrounding medullary tissue, thereby increasing the osmolarity of the filtrate as it descends. In contrast, the ascending limb is impermeable to water but actively transports salts (sodium and chloride ions) out of the filtrate into the medulla. This active transport of solutes creates a high salt concentration in the medulla, which in turn draws water out of the descending limb. The 'counter-current' arrangement, where the flow of filtrate in the descending limb is opposite to that in the ascending limb, amplifies this process. The concentrated medullary environment enables the collecting duct to reabsorb more water, under the influence of antidiuretic hormone (ADH), thus producing concentrated urine. This mechanism is crucial for water conservation and the maintenance of blood osmolarity.

Mesangial cells, located between the capillaries of the glomerulus, play several important roles in the functioning of the glomerulus. Firstly, they provide structural support to the glomerular capillaries, helping to maintain the integrity and shape of the glomerulus. Secondly, they are involved in the regulation of glomerular filtration rate (GFR) through their contractile properties; by contracting and relaxing, they can alter the surface area available for filtration, thereby modulating the rate of blood filtration. Additionally, mesangial cells have a phagocytic function, helping to clear away trapped residues and macromolecules from the glomerular basement membrane, which is essential for maintaining the filtration efficiency of the glomerulus. They also produce extracellular matrix components and cytokines, which can influence glomerular repair and response to injury. Thus, mesangial cells are integral to the normal function and maintenance of the glomerulus.

The endothelium of the glomerulus is uniquely structured to maximize its filtration efficiency. It consists of fenestrated endothelial cells, which are perforated with tiny holes or fenestrations. These fenestrations are large enough to allow the passage of water, ions, small molecules, and metabolic waste products, but small enough to prevent the loss of larger molecules like proteins and blood cells. This selective permeability is crucial for the filtration process in the kidney, as it ensures that essential components such as proteins and blood cells remain in the bloodstream. Additionally, the negatively charged glycoproteins present on the endothelial surface repel negatively charged blood proteins like albumin, further aiding in their retention within the blood vessels. This selective barrier forms the first step in the multi-stage process of urine formation, allowing the nephron to effectively filter the blood plasma while maintaining the composition of the blood.

The proximal convoluted tubule (PCT) plays a vital role in maintaining the body's pH balance by regulating the bicarbonate (HCO3-) concentration in the blood. It does this through a process called bicarbonate reabsorption and acid secretion. The cells of the PCT reabsorb bicarbonate from the filtrate back into the bloodstream, while simultaneously secreting hydrogen ions (H+) into the filtrate. This process is facilitated by the enzyme carbonic anhydrase, which catalyzes the conversion of carbon dioxide and water into carbonic acid, which then dissociates into hydrogen and bicarbonate ions. The PCT cells use different transport mechanisms to exchange these ions, effectively reclaiming bicarbonate, which acts as a buffer, and excreting excess hydrogen ions in the urine. This bicarbonate buffering system is crucial in maintaining the acid-base balance in the body, ensuring that the blood and other bodily fluids remain within the narrow pH range necessary for normal physiological functions.