What is the primary difference between filtration and selective reabsorption in the kidney?

Filtration is a passive process driven by high pressure that pushes all small molecules out of the blood. Selective reabsorption is an active process where the kidney specifically pumps useful substances like glucose and some ions back into the blood.

How does a kidney transplant differ from dialysis in terms of long-term patient health?

A transplant provides continuous, natural regulation of blood chemistry and allows a less restrictive diet. However, it requires lifelong immunosuppressant drugs to prevent the immune system from attacking the foreign organ antigens.

Why is the concentration of glucose in dialysis fluid kept at the same level as healthy blood?

This prevents a concentration gradient from forming, ensuring that glucose does not diffuse out of the patient's blood into the dialysis machine, as the body needs to retain its primary energy source.

What is a common misconception regarding how urea leaves the blood during kidney filtration?

Students often wrongly assume urea leaves via diffusion. In reality, it is forced out by high-pressure mass flow along with water and other small solutes through the pores of the capillary filters.

What error occurs in logic when students describe the effect of 'less ADH'?

Students often forget that less ADH makes the kidney tubules *less* permeable to water. This results in less water being reabsorbed and a larger volume of dilute urine, not more concentrated urine.

Why is it incorrect to say that the lungs 'control' water loss?

While water is lost through the lungs during exhalation, this loss is a byproduct of respiration and gas exchange. The lungs cannot adjust the amount of water they release to maintain homeostasis; only the kidneys provide that control.

Define 'Deamination' and state where it occurs.

Deamination is the removal of an amino group from an excess amino acid molecule. This process occurs in the liver and results in the production of ammonia.

What is ADH and what is its target organ?

Antidiuretic Hormone (ADH) is a hormone produced by the pituitary gland that targets the kidney tubules to regulate their permeability to water.

What is the chemical relationship between ammonia and urea?

Ammonia is a highly toxic byproduct of deamination. The liver converts it into urea, which is much less toxic and can be safely transported in the blood for excretion.

How does the body respond to a state of dehydration via the ADH feedback loop?

The brain detects low water levels and signals the pituitary to release more ADH. This increases tubule permeability, causing more water to be reabsorbed into the blood and producing concentrated urine.

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Homeostasis & Response

Maintaining Water & Nitrogen Balance in the Body

Summary

The human body maintains internal stability through osmoregulation and the excretion of nitrogenous waste. This process involves the liver converting toxic ammonia into urea, and the kidneys filtering the blood to regulate water, ion, and urea concentrations via hormonal feedback loops involving Antidiuretic Hormone (ADH).

1. Definition & Core Concepts

Osmoregulation is the process of maintaining salt and water balance (osmotic balance) across membranes within the body's fluids. It is a critical component of homeostasis because cells must be bathed in an isotonic environment to prevent shriveling (dehydration) or bursting (lysis) due to osmosis.
Excretion refers to the removal of metabolic waste products from the body. In the context of fluid balance, the primary wastes are urea (from nitrogen metabolism), excess ions (like sodium and potassium), and water.
The body loses water through three main routes: the lungs during exhalation, the skin via sweat, and the kidneys through urine. While losses from the lungs and skin are uncontrolled, the kidneys provide the primary mechanism for regulated water balance.

2. Nitrogen Balance: Deamination and Urea Formation

Proteins cannot be stored by the body; excess amino acids from the diet must be processed in the liver through a process called deamination. During this process, the nitrogen-containing amino group is removed from the amino acid.
Deamination initially produces ammonia, which is highly toxic to human cells even in small concentrations. To ensure safety, the liver immediately converts ammonia into urea, a less toxic, water-soluble compound that can be transported in the blood to the kidneys.
The remaining carbon-based part of the amino acid is typically converted into carbohydrates like glycogen or fats, which can be stored or used for energy production in cellular respiration.

3. The Kidney: Filtration and Selective Reabsorption

Diagram showing the flow of blood into the kidney, the filtration of small molecules, the selective reabsorption of glucose and water back into the blood, and the excretion of urine.

4. Hormonal Control: The ADH Feedback Loop

5. Key Distinctions: Dialysis vs. Natural Kidney Function

6. Exam Strategy & Tips