The nephron is the functional unit of the kidney. It is the site where blood is filtered and urine is formed. There are approximately one million nephrons in each kidney in humans. The nephron is divided anatomically into several segments. These are: The glomerulus which is a network of blood capillaries that form the site where blood is filtered. The glomerulus is connected and surrounded by a capsule that is called Bowman capsule. It is the site where the filtered blood enters.
The Bowman capsule is in turn attached to tubules that eventually drain into collecting ducts and from there to the urinary bladder. These tubules are divided into three main segments. These are: The proximal convoluted tubules which are connected directly with the Bowman capsule. To the proximal convoluted tubules are attached other tubules which are called the loops of henle. To the loop of henle is attached the distal convoluted tubules which are the last part of the nephron.
The type of epithelium that lines all parts of the nephron is simple epithelium that is covered at the proximal convoluted tubules with microvilli in order to increase the surface area for the reabsorption of important filtered compounds in the glomeruli. An important anatomical structure in the nephron is called the macula densa. It is a region of the loop of henle which comes in contact with arterioles of the blood.
Around cells of the macula densa, there are cells of the muscular system which contract and relax adjusting the blood pressure in the kidney. Two other types of cells in the tubules of the nephrons are located in the distal part of the convoluted tubules of the nephron. These are: The principal cells which have receptors for the hormones aldosterone and antidiuretic hormone. These hormones can adjust the amount of water and electrolytes that are reabsorbed in the tubules of the nephrons.
The other cell type of the distal convoluted tubules are the intercalated cells. This type of cells have a function that is related to blood pH monitoring. It can adjust the secretions or reabsorption of hydrogen ions based on the pH of the filtered blood in the glomerulus. The glomerulus of the nephron is composed of endothelial cells of the arterioles and a membrane which is called the basal lamina.
This membrane does not have cellular components but is composed of collagen fibers and proteoglycans. This layer has the important function of preventing large molecules from being filtered in the glomerulus such as proteins. Cells of the kidney which are called podocytes are the other part of the glomerular membrane. They encircle the blood vessels in the glomerulus. The filtration of blood in the glomerulus is regulated by several factors.
The pressure of the blood which is called the hydrostatic pressure leads to forcing the blood in the glomerular capillaries out of the glomerular capillaries into the Bowman capsule. Two other types of pressures lead to the withholding of blood from being filtered in the glomeruli into the Bowman capsule. These are: The hydrostatic pressure that is exerted by the filtered blood in the Bowman capsule itself. This typical pressure prevents further filtration of the blood in the glomeruli.
The other type of pressure which inhibits the filtration of blood in the glomeruli is the colloid osmotic pressure. In liver disease and in the nephrotic syndrome, this pressure is diminished due to the low content of proteins in the blood, thus the filtration rate in the glomeruli is increased. Mechanisms that control the filtration of blood in the glomeruli of the nephron include: Neural as well as hormonal mechanisms.
Neural mechanisms which control the filtration rate of blood in the glomeruli involve the secretion of norepinephrine by the sympathetic nervous system. The secretion of this neurotransmitter leads to decreased filtration rate of the blood in the glomeruli of the nephron. Hormonal regulation of the filtration rate of blood at the kidney glomeruli involve two hormones. These are: Angiotensin II and atrial natriuretic peptide. The first reduces the filtration rate of the glomeruli while the other increases the filtration rate.