How the kidneys balance water and remove urea, ADH, dialysis and transplants, auxin and tropisms, and the seedling growth required practical.
What you will learn
- Describe filtration and selective reabsorption and the role of ADH (Higher)
- Evaluate dialysis against a kidney transplant
- Explain phototropism and gravitropism with auxin
- Plan and analyse the seedling growth practical
The kidneys
Excess amino acids are broken down in the liver into ammonia, which becomes urea.
The kidneys filter the blood, then selectively reabsorb all the glucose, the ions the body needs and the right amount of water. Urea, excess ions and excess water leave as urine. Protein molecules are too large to be filtered.
Higher: when the blood is too concentrated, the pituitary releases more ADH, which makes the kidney tubules (the collecting ducts) more permeable, so more water is reabsorbed and the urine is small in volume and concentrated. This is negative feedback.
Kidney failure is treated by dialysis (blood flows past a partially permeable membrane and urea diffuses out into dialysis fluid, several hours, several times a week) or by a transplant (a donor kidney with a close tissue match; immunosuppressant drugs for life to stop rejection).
Glucose is filtered out and then all reabsorbed. Protein is never filtered.
The rest of this lesson and its 13 practice questions are locked.
In this lesson
- The kidneys
- Plant hormones
- Required practical: seedling growth
- Try it: A cost comparison
13 practice questions, marked as you go; about 25 minutes in all.
Key terms
Urea, Selectively reabsorb, ADH, Dialysis, Transplant, Auxin
In this unit: Homeostasis and response
How the nervous and hormonal systems keep the body's internal conditions steady: reflexes and reaction time, blood glucose and diabetes, reproductive hormones and contraception, the brain, eye, temperature and kidneys, and plant hormones.