Nephron

               Structure of Nephron





Nephrons (Greek nephros=kidney) are renal or kidney tubules. Each kidney has over one million nephrons that are responsible for removing waste products from blood and maintaining water, salt and pH balance in the body. This vital job results in the formation of urine. 

1.Afferent Arteriole 
The afferent arteriole receives blood rich in oxygen from the renal artery. This blood is transported to the glomerulus of the nephron where it is pressure filtered. 

2.Glomerulus 
The glomerulus (Latin glomus=ball) is a knotted up capillary that contains small pores. The plasma of the blood and the small molecules contained in plasma are pressure filtered into the Bowman's capsule. Large blood proteins and formed elements like blood cells and platlets are too big to be pressure filtered and remain in the arteriole. 

3.Efferent Arteriole 
The efferent arteriole is smaller in diameter than the afferent arteriole and increases the pressure in the glomerulus aiding pressure filtration. The blood entering the efferent arteriole is thicker, as plasma has entered the tubule, but still contains oxygen that is moved on to the peritubular network to provide for the metabolic needs of the tubule. 

4.Bowman's Capsule 
The filtrate produced in the glomerulus is collected in a bulblike start to the nephron called the glomerular capsule or Bowman's capsule (named for British surgeon Sir William Bowman). The plasma and its' dissolved molecules are transported from the Bowman's capsule into the lumen or inside of the nephron. 

5.Proximal Convoluted Tubule 
At this point in the nephron, the filtrate from the blood contains both waste molecules like urea and useful molecules like amino acids, glucose, and salt. The filtering process in the glomerulus is not selective (except by size) so the proximal convoluted tubule works to actively transport nutrients (glucose, amino acids, and salts) back into the blood capillaries so they are not lost in the urine. The tubule has a brush border with many villi to increase the surface area for this process of selective reabsorption. The cells of the tubule also contain many mitochondria that produce the ATP required for the large amount of active transport taking place throughout the length of the tubule. 

6.Loop of Henle 
The Loop of Henle dips down into the hypertonic environment of the kidney medulla and is responsible for the reabsorption of water from the filtrate. Water is drawn out of the descending limb of the loop, by osmosis, into the hypertonic medulla of the kidney. This water can then move into the capillary network and back into circulation to maintain blood volume. The ascending limb of the loop is impermeable to water and removes salt into the medulla of the kidney adding to the hypertonicity. 

7.Distal Convoluted Tubule 
The Distal Convoluted Tubule is the site of tubular secretion. This process involves the active transport of materials from the blood directly into the tubule. Examples of molecules that are secreted by the distal convoluted tubule are creatinine, drugs like penicillin, and hydrogen ions. Tubular secretion of hydrogen ions along with the reabsorption of bicarbonate ions (a biological buffer) acts as a mechanism to control blood pH. 

8.Peritubular Capillary Network 
The Peritubular Capillary Network acts as the blood supply to the nephron. The capillaries provide the nephron with glucose and oxygen to help with ATP production needed for active transport. The nutrients and water that are reabsorbed by the nephron are taken up by the capillary network surrounding the tubules and carried back to the renal vein and the body. 

9.Collecting Duct 
The collecting duct receives filtrate from several nephrons. The collecting duct will remove varying amount of additional water from the filtrate depending on the hydration state of the individual. This results in a wide range of urine, from very dilute (lots of water remains) to very concentrated (most of the water was reabsorbed). The amount of water reabsorbed is controlled by a hormone called ADH and is the topic of a future lesson. Once in the pelvis of the kidney the filtrate (now urine) contains substances that were pressure filtered and never reabsorbed such as urea, excess salt, water and other substances that were secreted from the blood such as hydrogen ions, drugs, or creatinine. 

formation of urine: 
For the production of urine, the kidneys do not simply pick waste products out of the bloodstream and send them along for final disposal. The kidneys' 2 million or more nephrons (about a million in each kidney) form urine by three precisely regulated processes: 
filtration, reabsorption, and secretion. 

1.FILTRATION: 
Urine formation begins with the process of filtration, which goes on continually in the renal corpuscles . As blood courses through the glomeruli, much of its fluid, containing both useful chemicals and dissolved waste materials, soaks out of the blood the blood through the membranes (by osmosis and diffusion) where it is filtered and then flows into the Bowman's capsule. This process is called glomerular filtration. The water, waste products, salt, glucose, and other chemicals that have been filtered out of the blood are known collectively as glomerular filtrate. 

2.REABSORPTION: 
Reabsorption, by definition, is the movement of substances out of the renal tubules back into the blood capillaries located around the tubules (called the peritubular copillaries). Substances reabsorbed are water, glucose and other nutrients, and sodium (Na+) and other ions. Reabsorption begins in the proximal convoluted tubules and continues in the loop of Henle, distal convoluted tubules, and collecting tubules 

3.SECRETION: 
Secretion is the process by which substances move into the distal and collecting tubules from blood in the capillaries around these tubules