The structural difference between a protonephridium and a metanephridium can be seen in the diagrams:
The functional differences are:
The similarities between a protonephridium and a metanephridium are:
• Reabsorption of valuable solutes and ions
• Addition of waste solutes to the fluid in the tubules
• Basic process of urine formation is the same where fluid enters a tubule and continuously flows through it.
The strategy used by a kangaroo rat that allows it to survive in the desert without drinking any water is that it reduces the amount of moisture lost from its body and retains the water gained from food and metabolism.
The ways in which it does it are:
• It produces concentrated urine with the help of an extra long loop of Henle in the kidneys.
• It also produces dry feces.
• It also keeps its nasal passages cooler so that respiratory moisture condenses forming water which is reabsorbed back into the body.
• As kangaroo rats have sweat glands only in their feet this reduces evaporative cooling (sweating), because the animal can radiate heat.
The descending loop of Henle is permeable to water but impermeable to sodium chloride. On the other hand, the ascending loop of Henle is impermeable to water and permeable to sodium chloride. NaCl pumps are also present which pump out NaCl out of the ascending loop of Henle. This process increases the concentration of NaCl outside the tubule which increases the osmotic concentration in the interstitial fluid of the medulla.
The collecting duct is permeable to urea. This raises the osmotic concentration of the interstitial fluid of the medulla.
This high osmotic concentration created by the movement of NaCl and urea out of the tubules creates an osmotic gradient for the controlled reabsorption of water from the collecting duct.