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.

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 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.

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.

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.

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.

Marcos, 14 years old, trains in track and field and has decided to drink protein shakes that quadruple the recommended protein intake for his age. After two months, his lab test shows:

• Urea concentration in blood and urine well above reference values.

• Daily urine output of $2.8\ \text{L}$ (usual: $1.2\ \text{--}\ 1.5\ \text{L}$), accompanied by constant thirst.

1. a) Justify the origin of this excess urea: explain what metabolic process excess amino acids undergo in the human body and why the organism cannot store them as reserves the way it does with fats or sugars.
2. b) Marcos proudly comments: "The more urine I produce, the more toxins I clear out and the healthier my kidneys are." Refute his argument by explaining why eliminating urea forces the loss of large volumes of water (osmotic diuresis) and predict what consequences this prolonged renal overload may have.

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.

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.

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.

Before donating a kidney to her brother, a 35-year-old woman undergoes testing: her two kidneys together filter $120\ \text{mL/min}$ of plasma ($60\ \text{mL/min}$ each). Six months after the surgery, her remaining kidney has slightly increased in size and achieves a filtration rate of $85\ \text{mL/min}$.

1. a) Interpret this result: how is it possible for a single kidney to largely compensate for the loss of the other without creating new nephrons? Explain what happens at a cellular and functional level in the remaining nephrons.
2. b) A relative advises her: "Since your remaining kidney performs almost like two, don't worry about consuming excessive salt, meat, or anti-inflammatory drugs". Refute this advice by explaining the vulnerability of solitary kidney individuals and the importance of protecting their renal functional reserve.

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 leaflet states the time that must pass between doses.

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 (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.

In a school laboratory, $1\ \text{mg}$ of calcium oxalate salts is added to two beakers containing water at $37\ ^\circ\text{C}$ (body temperature):

• Beaker A: contains $50\ \text{mL}$ of water.

• Beaker B: contains $250\ \text{mL}$ of water.

After a while, solid crystals precipitate at the bottom of beaker A, whereas in beaker B the solution remains clear.

1. a) Apply this solubility phenomenon to kidney function: predict and justify which of the two beakers simulates the renal pelvis of a person who drinks less than $1\ \text{L}$ of water per day, and how this is related to the formation of renal colic (kidney stones).
2. b) Design a plan with three daily guidelines (nutritional and habit-related) for a patient who has already passed a calcium oxalate stone, scientifically justifying how each one helps prevent the recurrence of crystalline sediments.

Analyze this diagram describing the relationship between arterial hypertension and kidney disease:

Constant high blood pressure $\rightarrow$ Damage and hardening of glomerular capillaries $\rightarrow$ Drop in renal filtration $\rightarrow$ Retention of water and sodium in the body $\rightarrow$ Increase in circulating blood volume $\rightarrow$ Further increase in blood pressure.

1. a) Justify why the retention of water and salts due to insufficient filtration immediately worsens the patient's arterial hypertension.
2. b) In a routine check-up test of a long-term hypertensive patient, the abnormal presence of plasma proteins (such as albumin) is detected in the urine. Deduce what physical alteration the glomerular filtration barrier has undergone due to continuous pressure and explain why this finding confirms kidney impairment.

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.

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.

The blood urea concentration of two patients is recorded over ten days:

• Patient 1 (healthy kidneys): the concentration remains stable around $25\ \text{mg/dL}$ every day.

• Patient 2 (chronic kidney disease, treated with hemodialysis on Mondays, Wednesdays, and Fridays): urea drops to $20\ \text{mg/dL}$ after each four-hour session, but gradually rises during the intervening days, reaching its maximum value ($90\ \text{mg/dL}$) on Monday morning before the first session of the week.

1. a) Interpret patient 2's progress: justify why the weekend poses the greatest life-threatening risk and why this patient must strictly restrict fluid intake and potassium-rich foods during those days.
2. b) Compare both filtration models (natural continuous vs. artificial intermittent) and predict what harmful effects the sharp fluctuations in solutes and hydration experienced by the hemodialysis patient cause in the body's cells.