What causes the sensation of thirst?

The hypothalamus detects that the blood has become too concentrated and produces the feeling of thirst so that water is taken in.

Explain why a badly dehydrated person passes very little urine.

A high level of ADH makes the collecting ducts very permeable, so almost all the water in the filtrate is reabsorbed into the blood and very little is left to be excreted.

Give three signs that a person is becoming dehydrated.

Thirst, dark and concentrated urine, and headache, tiredness or dizziness.

Why can drinking a very large volume of water in a short time be dangerous?

The blood becomes so dilute that water enters the body cells by osmosis and they swell, which is particularly dangerous for the brain cells enclosed in the skull.

OCR GCSE Biology A J247

Cell Level Systems

Topic overview Microscopy, and sub-cellular structures DNA structure: polymer, double helix and nucleotides Protein synthesis and the triplet code Enzyme action, and investigating enzymes Cellular respiration as an exothermic process Aerobic and anaerobic respiration compared Synthesis and breakdown of carbohydrates, proteins and lipids Photosynthesis: producers and the process Photosynthesis as endothermic, and investigating it Limiting factors in photosynthesis

Community Level Systems

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Organism Level Systems

Topic overview The nervous system and coordinated responses The reflex arc The eye and its common defects The structure and function of the brain Investigating and treating the brain Hormonal coordination, thyroxine and adrenaline Hormones in reproduction and the menstrual cycle Contraception and treating infertility Plant hormones and their commercial uses Homeostasis, and the skin in temperature control Insulin and glucagon in blood sugar control Type 1 and type 2 diabetes Osmotic changes in body fluids, and the kidneys The kidney tubule and the effect of ADH Responding to temperature and osmotic challenges

Scaling Up

Topic overview Diffusion, osmosis and active transport Mitosis, the cell cycle and cell differentiation Stem cells in animals and meristems in plants Surface area to volume ratio and exchange surfaces The circulatory system, the heart and blood vessels Red blood cells and plasma Root hair cells, transpiration and translocation Xylem and phloem Factors affecting water uptake, and the potometer

Organism Level Systems

Responding to temperature and osmotic challenges

3 things to know
Paper 3

Overview

How ADH, urine output and thirst respond to heavy sweating, dehydration, drinking too much water and a salty meal, the signs of dehydration, why drinking a very large volume of water is dangerous and salty drinks help after exercise, how the body responds to a falling core temperature, and why sweating challenges both temperature control and water balance.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Describe how the body responds after several hours of heavy sweating in the heat.

Key term

Explain the body's response to drinking far more water than it needs.

Key term

Explain what happens in the body after a very salty meal.

Questions from this subtopic

Each one with the model answer underneath.

1
Describe how the body responds after several hours of heavy sweating in the heat.
Model answer
The blood becomes more concentrated, so more ADH is released, the kidneys reabsorb more water and produce a small volume of concentrated urine, and the person feels thirsty and drinks.
2
What causes the sensation of thirst?
Model answer
The hypothalamus detects that the blood has become too concentrated and produces the feeling of thirst so that water is taken in.
3
Explain why a badly dehydrated person passes very little urine.
Model answer
A high level of ADH makes the collecting ducts very permeable, so almost all the water in the filtrate is reabsorbed into the blood and very little is left to be excreted.
4
Give three signs that a person is becoming dehydrated.
Model answer
Thirst, dark and concentrated urine, and headache, tiredness or dizziness.
5
Explain the body's response to drinking far more water than it needs.
Model answer
The blood becomes too dilute, so the pituitary releases less ADH, the collecting ducts become less permeable and a large volume of dilute urine is produced until the concentration is back to normal.
6
Why can drinking a very large volume of water in a short time be dangerous?
Model answer
The blood becomes so dilute that water enters the body cells by osmosis and they swell, which is particularly dangerous for the brain cells enclosed in the skull.

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Cell Level Systems

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Genes, Inheritance and Selection

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AThe blood becomes too dilute.

BLess ADH is released.

CThe collecting ducts become less permeable.

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