Answer the following questions on the urinary system: 

1. What are the three functions of the nephron? 
2. Where within the nephron do each of the above functions occur? Be sure to indicate if any of these functions occur only, or to a higher degree, within a specific region of the nephron. 
3. What are the four regions of the nephron? Compare and contrast these regions, with the following in mind. 

• Location within the kidney. 
• Location relative to the other regions. 
• Tissue composition. 
• Function

 

 1. Functions of Nephron:

a. regulates water and soluble substances in the blood by filtering the blood

b. reabsorbing what is needed

c. excretion of body waste in the form of urine.

2. The parts of the nephron where those functions in no.1 occur:

a. The function of regulating water and soluble substances in the blood by filtering the blood is accomplished by the glomerulus and tubules. Each of the nephrons contain a filter called the glomerulus . The fluid that is filtered out from the blood then travels down a tiny tube-like structure called a tubule. The tubule adjusts the level of salts, water, and wastes that will leave the body in the urine. Filtered blood leaves the kidney through the renal vein and flows back to the heart.

b. Reabsorption occurs in the tubules. It takes place mainly in the proximal convoluted tubule of the nephron . Nearly all of the water, glucose, potassium, and amino acids lost during glomerular filtration reenter the blood from the renal tubules. Reabsorption also occurs in the collecting duct. This duct is a long, straight tube where H+ and K+ ions are secreted to maintain the electrolyte balance of the blood. This is also the region where the maximum reabsorption of water takes place to produce concentrated urine.

c. The excretion of urine happens in the collecting duct. The urine or ultrafiltrate, travels down via the collecting duct to the bladder, where it will be stored and released through the urethra.

3.The four regions of the nephron :

Renal Corpuscle

The renal corpuscle is responsible for the filtration of the plasma or blood. It contains two structures: the glormerulus and Bowman's capsule. The glomerulus is a cluster of capillary loops enclosed by Bowman's capsule, which is part of the renal tubule.

Bowman's capsule has two layers:

• The visceral layer is in contact with the glormerulus, and is composed of specialized epithelial cells known as podocytes.
• The parietal layer is the outer layer, and is composed of simple squamous epithelial cells. This layer is continuous with the epithelium of the proximal convoluted tubule.

The space between the two layers is named Bowman's space, and this space contains the ultrafiltrate of plasma. The plasma has to pass through a filtration barrier of three layers to enter Bowman's space: the capillary endothelium, the podocyte layer, and their fused basement membrane. Bowman's space is continuous with the proximal convoluted tubule.

Blood enters the renal corpuscle via afferent arterioles and then leaves via efferent arterioles. The part of renal corpuscle where afferent and efferent arterioles are located is known as the vascular pole. On the opposite end of the vascular pole is where the renal tubule begins and is known as the urinary pole.

Mesangial cells can also be found within the glomerulus. These cells secrete a matrix of basement membrane-like material to support the structure of the glomerulus.

The Proximal Convoluted Tubule

Bowman's capsule gives rise to the PCT(proximal convoluted tubule), which lies adjacent to the glomerulus in the renal cortex. The PCT forms from simple cuboidal epithelium dedicated to the absorption and transport of water, electrolytes, and other particles. These cells are characterized by a brush border of microvilli designed to increase the surface in contact with the glomerular ultrafiltrate, with abundant long, thin mitochondria lining the basal pole of the cell; and numerous vesicles involved in transcellular transport of 60 to 80% of the ultrafiltrate.

The peritubular capillaries surround the PCT. This capillary network is responsible for the blood supply of the tubules as well as the recovery of the reabsorbed free water, ions, and other plasma constituents like amino acids and glucose.

From the 160 to 180 L of ultrafiltrate produced per day, only 1.5 to 2 L of urine is excreted. Reabsorption of 60 to 65% of free water and NaCl occurs in the PCT. Additionally, most of the potassium, phosphate, HCO3, and nearly all nutrients, such as glucose and amino acids, are reabsorbed in this segment. The solute and water reabsorption in the proximal tubule is isotonic, with a minimum change in luminal osmolarity. This site of the nephron is also responsible for active solute secretion, hormone production, and renal gluconeogenesis.

Loop of Henle

The loop of Henle forms a hair-pin structure that dips down into the medulla. It contains four segments: the pars recta (the straight descending limb of proximal tubule), the thin descending limb, the thin ascending limb, and the thick ascending limb. The turn of the loop of Henle usually occurs in the thin segment within the medulla, and the tubule then ascends toward the cortex parallel to the descending limb. The end of the loop of Henle becomes the distal convoluted tubule near its original glomerulus. The loops of Henle run in parallel to capillary loops known as the vasa recta. Recall from Physiology that the loop of Henle serves to create high osmotic pressure in the renal medulla via the counter-current multiplier system. Such high osmotic pressure is important for the reabsorption of water in the later segments of the renal tubule.

The PCT leaves the renal cortex and turns into the thin descending limb (TDL) of the loop of Henle penetrating the renal medulla. The tubule becomes narrower, and the cells become smaller, with few mitochondria and short microvilli often unnoticeable on light microscopy.

The tubule then makes a turn upward towards the cortex, turning into the thick ascending limb (TAL). Here the lining cells become larger with more numerous microvilli and mitochondria to engage in the active transport of sodium to dilute the urine.

Distal Convoluted Tubule

The TAL(thin ascending limb) turns into the DCT(distal convoluted tubule) after returning to the renal cortex near its glomerulus of origin.[ The DCT comprises the nephron segment between the macula densa and the cortical collecting tubule (CCT).The DCT cells are tall cells notable for containing the largest number of mitochondria among other cells in the nephron. They have an extensive basolateral amplification enclosing multiple mitochondria, creating a palisading appearance in the basal part of the cells. Intercalated cells begin to appear in the latter segment of the DCT and remain throughout the connecting and collecting tubules.

The initial segment of the distal convoluted tubule lies right next to the glomerulus and forms the juxtaglomerular apparatus. The juxtaglomerular apparatus is a specialized structure formed by the distal convoluted tubule and the glomerular afferent arteriole. It is located near the vascular pole of the glomerulus. The main function of the apparatus is the secretion of renin, which regulates systemic blood pressure via the renin-angiotensin-alodosterone system. The juxtaglomerular apparatus is composed of:

• The macula densa, a collection of specialized epithelial cells of the distal convoluted tubule. These cells are enlarged as compared to surrounding tubular cells. The cells of the macula densa sense sodium chloride concentration in the tubule, which in turn reflects the systemic blood pressure.
• The juxtaglomerular cells of the afferent arterioles, which are responsible for secreting renin. These cells are derived from smooth muscles cells of afferent arterioles.
• The extraglomerular mesangial cells, which are flat and elongated cells located near the macula densa. Their function is currently unclear.

The terminal portion of the distal tubule empties through collecting tubules into a straight collecting duct in the medullary ray. The collecting duct system is under the control of antidiuretic hormone (ADH). When ADH is present, the collecting duct becomes permeable to water. The high osmotic pressure in the medulla (generated by the counter-current multiplier system/loop of Henle) then draws out water from the renal tubule, back to vasa recta.