Identify and describe Role of the Kidneys in Homeostasis, What is ADH? and When Blood Becomes Too Concentrated in the context of maintaining water and nitrogen balance in the body (biology only).
BEEZYREVISOR
GCSE Biology Revision
Learn it. Recall it. Revise it.
GCSE Biology revision
Maintaining water and nitrogen balance in the body (biology only)
Hormonal coordination in humans
Your specification
AQA student objectives
Learning pathway
All · Most · Some
Explain the relationships between Role of the Kidneys in Homeostasis, What is ADH? and When Blood Becomes Too Concentrated and how they contribute to maintaining water and nitrogen balance in the body (biology only).
Apply and analyse knowledge of Role of the Kidneys in Homeostasis, What is ADH? and When Blood Becomes Too Concentrated to interpret unfamiliar information about maintaining water and nitrogen balance in the body (biology only).
Revision summary
Key knowledge
Read on screen, then print for Cornell-style active revision.
Role of the Kidneys in Homeostasis
- The kidneys remove the waste product urea from the blood and adjust the levels of ions and water to maintain a stable internal environment.
- During filtration, water, urea, and ions pass out of the blood into tiny kidney tubules, before useful substances are selectively reabsorbed back into the blood.
What is ADH?
- ADH (antidiuretic hormone) is a hormone released by the pituitary gland that controls the water level in the blood.
- ADH travels in the bloodstream to the kidneys, where it causes the kidney tubules to become more permeable to water, allowing more water to be reabsorbed back into the blood.
When Blood Becomes Too Concentrated
- If the water level in the blood falls (e.g. during exercise on a hot day due to sweating), the blood becomes too concentrated.
- The pituitary gland detects this and releases ADH into the bloodstream.
- ADH causes the kidney tubules to reabsorb more water back into the blood, producing less urine, until water levels return to normal.
When Blood Becomes Too Dilute
- If a person drinks a large amount of water, the concentration of water in the blood rises and the blood becomes too dilute.
- The pituitary gland stops releasing ADH, so the kidney tubules become less permeable to water and reabsorb less water.
- More urine is produced, removing excess water until the blood water concentration returns to normal.
Negative Feedback Control
- The regulation of blood water levels by ADH is an example of a negative feedback cycle, where a change from the normal level triggers a response that reverses that change.
- Once the blood water level returns to normal, the stimulus is removed and the pituitary gland stops releasing ADH, switching off the corrective response.
Kidney Dialysis: How It Works
- Kidney dialysis is used when the kidneys fail, to artificially adjust the levels of water, ions, and urea in the blood using a machine.
- During dialysis, the patient's blood passes over a semipermeable membrane, which allows urea, ions, and water to pass through but prevents larger molecules such as proteins and blood cells from crossing.
- The dialysis fluid on the other side of the membrane contains normal concentrations of water and ions but no urea, creating a concentration gradient so that urea diffuses from the blood into the dialysis fluid.
- The dialysis fluid is constantly refreshed to maintain a large concentration gradient, ensuring efficient removal of urea by diffusion.
Advantages and Disadvantages of Kidney Dialysis
- An advantage of dialysis is that there is no shortage of dialysis machines, making it widely available to patients with kidney failure.
- A disadvantage is that patients must visit hospital several times a week for treatment, which is inconvenient and disruptive to daily life.
- Dialysis patients must also follow a controlled diet to avoid producing too much urea, and the treatment is expensive and required long-term.
Kidney Transplants: How They Work
- In a kidney transplant, a diseased kidney is replaced with a healthy kidney donated by a donor.
- A major risk is that the patient's immune system may recognise the donated kidney as foreign and reject it.
- To prevent rejection, patients must take immunosuppressant (anti-rejection) drugs for the rest of their lives.
Comparing Dialysis and Kidney Transplants
- A key disadvantage of kidney transplants compared to dialysis is the shortage of suitable donors.
- Unlike dialysis patients, transplant recipients can lead a relatively normal life without frequent hospital visits or a strictly controlled diet.
- A kidney transplant is only expensive initially, whereas dialysis requires ongoing, long-term costs, making transplants more cost-effective over time.