38 The Nephron
The functional units of the kidneys are called nephrons. To understand how different regions of the nephron are able to have unique, spatial functions, we will first discuss how the multicellular epithelial structures found in these structures are organized for specific functions.
Nephrons are made up of epithelial cells with an underlying non-cellular layer or basement membrane that separates the filter in the lumen fluid from the interstitial space. These epithelial cells differ in cellular anatomy along the length of the nephrons according to the filtering and processing functions of the epithelial cells.
As the chief functional organ in the urinary system, the kidneys excrete nitrogenous wastes and are involved in regulating the volume, composition, and pH of the blood. The kidneys receive one fourth of total cardiac output, a reflection of their function as blood processors. Each kidney contains about one million nephrons, the structural and functional units of the kidneys. Each nephron is made up of a high-pressure capillary bed called a glomerulus and a renal tubule, segments of which included a proximal convoluted tubule, nephron loop (loop of Henle), and distal convoluted tubule. The distal convoluted tubules from multiple nephrons join a common collecting duct. The nephrons are involved in three functions: filtration, reabsorption, and secretion.
Overview of Nephron Structure
The structure within each nephron that actually filters blood plasma is the renal corpuscle containing the glomerulus and glomerular capsule. Another nephron structure called the renal tubule receives the filtered fluid, called glomerular filtrate. Very thin and a little over an inch long, the renal tubule has three major consecutive segments that the filtrate flows through: a proximal convoluted tubule the nephron loop(loop of Henle), and a distal convoluted tubule.
The Glomerulus
The renal corpuscle is made up of a tangled capillary network called a glomerulus and a cup-shaped structure called the glomerular capsule (Bowman’s capsule) surrounding the glomerulus. The glomerular capsule has an external parietal layer made of simple squamous epithelium. Although this layer is not involved in the production of filtrate, it helps to maintain the shape of the capsule. An inner visceral layer adheres to the glomerular capillaries and is composed of a special type of simple squamous epithelial cells called podocytes. These podocytes have multiple projections called pedicels or foot processes. The pedicels of one podocyte interlock with the pedicels of adjacent podocytes. Filtrate from the glomerulus passes through filtration slits, the openings between the foot pedicels, to enter the capsular space(Bowman’s space), the area between the visceral and parietal layers of the glomerular capsule.
The Proximal Convoluted Tubule
The proximal convoluted tubule (convoluted refers to the coiled shape) tubule connects the glomerular capsule to the nephron loop. The apical surface of the simple cuboidal epithelial cells making up the proximal convoluted tubule are covered in microvilli producing a brush border. The brush border and the length of the proximal convoluted tubule dramatically increase the luminal surface area available for reabsorbing water and solutes and for secreting substances into the filtrate.