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.

Name the four organs of the urinary system and state which of them filters the blood and which one expels urine to the outside.

State whether these statements are true or false and justify your answer: a) urine is made in the bladder; b) each kidney contains about one million nephrons; c) the amount of water we drink influences the volume of urine we produce.

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

Explain what a nephron is and why it is said to be the functional unit of the kidney.

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.

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.

Blood enters the kidney through the renal artery and leaves through the renal vein. Explain how the blood that leaves differs from the blood that entered, which substances have been removed, and where they have ended up.

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.

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

Urea is one of the waste substances eliminated in urine. Explain where this substance comes from inside our body and why it is necessary to eliminate it.

In a healthy person, the glucose filtered in the nephrons is completely reabsorbed and does not appear in the urine. If a test detects glucose in a person's urine, reason what may be failing and why it is a warning sign for the doctor.

Medicines taken by mouth end up being eliminated largely in the urine. Follow their path from the intestine to the kidney and explain why the package insert states the time that must pass between doses.

The fluid filtered at the start of the nephron contains water, salts, glucose, amino acids and urea; the final urine contains mainly water, salts and urea. Compare both compositions and explain which process in the nephron causes the differences and why it is beneficial that it occurs.

Besides the kidneys, other organs eliminate substances from the body. State what the lungs eliminate and what the skin eliminates, and explain why they are also said to take part in excretion.

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.

After running a race in summer, urine is scarce and very concentrated even if the person has drunk as normal. Relate sweat, the water in the blood and the work of the nephrons to explain this change.

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 (blood volume) 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.

Unit challenge. Become a physiologist for a day! You are going to investigate at home how your four nutritional systems work together synchronously in response to an energy demand.

1. Personal data collection: Measure and record at rest your heart rate (beats per minute) and breathing rate (inhalation/exhalation cycles per minute). Perform $3$ minutes of moderate exercise (such as jumping jacks or climbing stairs) and measure them again immediately. Also record visible bodily changes (sweat, thirst, skin temperature).

2. Analysis and integration: Prepare a comparative data table (rest vs. exertion) and write a short report explaining the coordinated action: how the respiratory and circulatory systems increase the supply of $O_2$ to cells, the digestive system supplies previously absorbed nutrients (such as glucose), and the excretory system dissipates heat and toxins through sweat.

3. Recovery proposal: Design a reasoned post-exercise snack (intake of carbohydrates, salts, and water volume, for example $0.4\text{ L}$), justifying how it will help restore the internal environment and renal balance.

Assessment criteria:

• Thoroughness in obtaining and presenting personal physiological data in a table.

• Quality of the scientific explanation that explicitly connects the four organ systems.

• Consistency and biological justification of the proposed snack and rehydration.