Exercises: The excretory system

Name in order the structures of the excretory system through which urine passes from the moment it is formed in the kidney until it leaves the body, and state which of them temporarily stores it.

In the diagram of a nephron, several structures have been labelled. Write the name of the structures labelled with letters A, B, and C, and briefly explain what happens in each during urine formation.

Explain which substances are mainly eliminated in urine and where these substances come from within our body.

Describe the two major processes that take place in the nephron to produce urine (filtration and reabsorption), and explain what happens to the blood and to useful substances in each of them.

A person drinks little water on a hot day and notices that their urine is sparse and dark in color. Explain, taking into account the function of the kidneys, why the volume and color of the urine change and what the body is trying to keep constant.

Relate the excretory and circulatory systems: explain why excretion requires a continuous blood supply to the kidneys and what would happen if a renal artery were blocked.

In a laboratory analysis, three samples from a healthy person are compared: blood plasma, initial glomerular filtrate (primary urine), and final excreted urine. The results show:

• Glucose: $1.0\ \text{g/L}$ in plasma, $1.0\ \text{g/L}$ in primary urine, and $0.0\ \text{g/L}$ in final urine.

• Urea: $0.3\ \text{g/L}$ in plasma, $0.3\ \text{g/L}$ in primary urine, and $18.0\ \text{g/L}$ in final urine.

• Volume: $180\ \text{L}$ per day of primary urine are produced, but only $1.5\ \text{L}$ of final urine are eliminated.

1. a) Interpret these data, explaining what processes have taken place in the nephron tubules for glucose to completely disappear while the urea concentration increases 60-fold.
2. b) Predict what would happen to the volume and composition of the urine if a toxic substance selectively blocked the tubular reabsorption of water, and justify the life-threatening risk this would pose in a matter of hours.

In a clinic, hemodialysis is performed on a patient with kidney failure by connecting their blood through a semipermeable membrane to a circuit containing dialysis fluid.

1. a) Justify what urea concentration the dialysis fluid should initially have (zero, or similar to the patient's blood?) so that urea moves out of the body. Predict what would happen if, by mistake, a fluid with a higher urea concentration than that of the patient were introduced.
2. b) A classmate says: "The dialysis machine and a transplanted kidney do exactly the same thing, so in terms of health it makes no difference whether you are connected to the machine for life or receive a transplant". Refute this statement by explaining two fundamental limitations of dialysis compared to the complete, continuous activity of a healthy transplanted kidney.

Two $60\ \text{kg}$ athletes go on a three-hour mountain hike at $28\ ^\circ\text{C}$. Both sweat approximately $1.5\ \text{L}$. Athlete A drinks water regularly, while athlete B forgot his canteen and drinks no fluid. At the end, their urine is analyzed:

• Athlete A: produces $300\ \text{mL}$ of light-colored, dilute urine.

• Athlete B: barely produces $60\ \text{mL}$ of very dark, concentrated urine.

1. a) Interpret how athlete B's nephrons have acted to try to keep blood volume (volemia) constant despite heavy sweating.
2. b) Predict what would happen to the concentration of salts in the blood plasma and to the ability to excrete waste products if athlete B continued hiking for another three hours without drinking. Justify why the kidney cannot reduce urine production to $0\ \text{mL}$ indefinitely.