Excretory System Review

1. Draw, label, and annotate (note the functions of the different parts) a kidney.

2. Draw, label, and annotate (note the functions of the different parts) a nephron.

3. Explain the process of ultrafiltration in the glomerulus.Ultrafiltration occurs as molecules move from the glomerulus to the Bowman’s capsule. Small molecules (like urea, glucose, water, salts) can fit through fenestrations (slits) in the glomerulus while large molecules (proteins, blood cells) cannot. The blood enters the glomerulus through the afferent arteriole and leaves through the efferent arteriole. The afferent arteriole is larger than the efferent arteriole so blood comes into the arteriole faster than it can move out, creating high blood pressure in the glomerulus. This pressure pushes the small molecules through the fenestrations into the Bowman’s capsule while big molecules stay in the blood and move out in the efferent arteriole.

4. Explain the process of reabsorption in the convoluted tubule. Why is it necessary? What types of cellular transport are used for the different types of molecules being reabsorbed.The water, glucose, and salts in the filtrate are useful and necessary so they need to be moved back into the blood instead of lost with the urine. Reabsorption occurs mostly in the proximal convoluted tubule. Salt is moved back to the blood from the filtrate in the nephron by active transport, which requires energy. As the salt moves to the blood, the higher concentration of solute in the blood causes water to move to the blood by osmosis. This does not require energy. Glucose is also moved back to the blood by active transport (requiring energy). 100% of the glucose in the filtrate in the kidney is moved back to the blood in a healthy individual.

5. What is osmoregulation and why is it important?Osmoregulation is the regulation of water balance in the body to prevent dehydration or overhydration.

6. Explain the process of osmoregulation in the collecting ducts. How is ADH involved? Where is ADH produced? How does alcohol consumption affect ADH production and urine output?The majority of osmoregulation occurs at the collecting ducts. Water can either stay in the collecting duct with the urine and move to the renal pelvis and then on to the bladder and out of the body in the urine OR it can move out of the collecting duct and back into the blood and be retained by the body. Whether it moves back to the blood depends on if the collecting duct walls are permeable to water (water can move through the walls). ADH (antidiuretic hormone) is produced by the pituitary gland in the brain. When ADH is produced, the collecting duct walls become more permeable to water, so more water moves back into the blood and less water goes out of the body as urine. This decreases urine output and loss of water. If an individual drinks a lot of water, the brain stops producing ADH. The makes the collecting ducts impermeable to water, so water is not able to move back into the blood and must go out with the urine, increasing urine output. Drinking alcohol halts the production of ADH, increasing urine output and leading to dehydration.

7. What substances should or should not be found in the urine of a healthy individual? What does the presence of these substances in the urine indicate?

You should find urea because that is the entire purpose of producing urine – to get rid of urea (which is toxic in high concentrations in your body). Urine should contain water and some salts as well.You should not find:

-Glucose – 100% of glucose should be actively transported back to the blood in the proximal convoluted tubule. There is only glucose in the urine of blood sugar levels are so high that so much glucose is in the filtrate that your kidneys are not able to transport all of it back to the blood. This indicates excessively high blood sugar which indicates diabetes.

-Blood cells and proteins – these are too big to move through a healthy glomerulus which indicates that either the kidney has an infection which is breaking down the glomerulus and making the fenestrations large enough for blood cells and proteins to pass through. Blood cells could also indicated bleeding/infection in tissues that urine travels through after it is produced, like the bladder.

-Drugs – most stay in the blood 2-3 days. Marijuana can be found in the urine for several weeks.

8. What is the purpose of dialysis and how does it work?Dialysis artificially cleans the blood of people whose kidneys are not working well enough. The blood is removed from the patient and passed through a machine that contains a solution. The removal of urea is dependent entirely on diffusion. The solution contains things like glucose and salts so that those substances do not diffuse out of your blood. It is important that the solution does not contain any urea so that the urea in the blood diffuses out into the dialysis solution. The cleaned blood is then returned to the patient.