Nephron

Nephron
It is basic functional unit of the kidney and capable of forming urine. There are about million nephrons in each kidney in human body. Kidneys cannot regenerate new nephrons and their number decrease with aging.
Each Nephron Consist of:
1- Glomeruli forming from:
a. Bowman’s capsule
b. Glomerulus
2- Tubular system consist of:
a. Proximal convoluted tubule
b. Loops of Henle
c. Distal convoluted tubule
d. Collecting tubules and ducts

Figure (1): Nephron
Glomeruli forming from
1-Bowman’s capsule: It is the invaginated end of the tubule that covered the
2- glomerulus (branching capillaries). The pressure in the glomerular capillaries is higher than that in other capillary beds.

Figure(2): glomerulus
The glomerular capillaries membrane is different from other capillary membrane by having three layer instead of two. These three layers are: Endothelial layer, Basement membrane and Epithelial cells.
Despite the number of layers, the permeability of the glomerular membrane is from 100-500 times as great as that the usual capillary due to presence of thousands of small holes which are called fenestrate in the endothelial cells, presence of large spaces in the basement membrane and incontinuity of cells that form the epithelial layer.
Tubular system consist of: Proximal convoluted tubule, Loops of Henle, Distal convoluted tubule and Collecting tubules and ducts

Functions of Nephrons (Filtration, Reabsorption, and Secretion)
Each Nephron begins with a filtration component that filters the blood entering the kidney. This filtrate then flows through a tubular structure lined by a single layer of specialized cells and surrounded by capillaries. The major functions of these lining cells are reabsorption of water and small molecules from the filtrate into the blood and secretion of wastes from the blood into the urine.


Filtration: It is the process that your kidneys use to filter excess fluid and waste products out of the blood into the urine collecting tubules of the kidney. This occurs in glomeruli
Reabsorption: Tubular reabsorption is the process by which solutes and water are removed from the tubular fluid and transported into the blood.
Reabsorption with an active or passive extraction of substances from the tubule fluid into the renal interstitium (the connective tissue that surrounds the nephrons), and then the transport of these substances from the interstitium into the bloodstream.
Secretion: Tubular secretion is the transfer of materials from peritubular capillaries to renal tubular lumen. Tubular secretion is caused mainly by active transport. In nature only a few substances are secreted. These substances are present in great excess, or are natural poisons. many drugs are cleared by tubular secretion
Tubules
1- Proximal convoluted tubule (PCT):
They lie in the renal cortex along with the glomerulus. The epithelial cells of them are highly metabolic cells, with large number of mitochondria to support rapid active transport process. It contains a brush border due to the presence of microvilli to increase the surface area of reabsorption.
Basic mechanism of absorption and secretion of PCT:
Normally reabsorb here:
• about 65% of filtered Na and water.
• much of Cl, K, Ca, phosphate and urea.
• almost all the filtered glucose, amino acid and organic acids.
• small amount of protein.
Na ions are reabsorbed by: passive diffusion, facilitated diffusion, co-transport with glucose, amino acids and counter-transport in an exchange with H ions.
Cl ions are reabsorbed by: Passive and co-transport mechanisms.
Water is reabsorbed by osmosis.
Proteins are reabsorbed through the brush border of the proximal tubule by process of pinocytosis.
The concentration of Na and the total osmolarity remains relatively constant (isotonic) because water permeability of the proximal tubules is so great that water reabsorption proportional to Na reabsorption.
***The proximal tubules is also the site for secretion of organic acids and bases (bile salts, oxalate, urate, catecholamines), drugs and toxins.