Homeostasis (Edexcel A Level Biology (A) SNAB): Flashcards
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Define homeostasis.
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Define homeostasis.
Homeostasis is the maintenance of a relatively constant internal environment within restricted limits.
Why is homeostasis important for enzymes?
It maintains optimal conditions for enzyme action.
For example, keeping temperature below the level at which enzymes denature.
Describe how vasodilation increases heat loss.
Arterioles supplying the skin dilate, so more blood flows through skin capillaries.
More heat is lost from the blood to the environment by radiation.
How does sweating cool the body?
Sweat evaporates from the skin.
This takes heat energy from the body to convert liquid water to vapour.
How does shivering warm the body?
Muscles contract rapidly and regularly.
The metabolic reactions powering this generate heat that warms the blood.
What is the role of the hypothalamus in thermoregulation?
It monitors blood temperature and receives input from skin thermoreceptors.
It sends impulses to effectors to bring about warming or cooling responses.
Define negative feedback.
Negative feedback is a control mechanism that reverses a change in a factor, returning it to its normal level.
What three components are involved in a negative feedback loop?
A receptor detects the change.
A coordination system (nervous or hormonal) transfers information.
An effector carries out the response.
Define positive feedback.
Positive feedback is a mechanism where a response causes the factor to deviate even further from normal, enhancing the original stimulus.
Give an example of positive feedback in the body.
The dilation of the cervix during labour (via oxytocin).
Blood clotting, where activated platelets activate more platelets.
True or False?
Positive feedback is a form of homeostasis.
False.
Positive feedback moves a factor away from normal, so it is not homeostasis.
How do steroid hormones such as thyroxine act inside a cell?
They are lipid-soluble, so they cross the membrane and enter the nucleus.
They bind to transcription factors to switch genes on or off.
How do protein hormones such as adrenaline act on a cell?
They cannot cross the membrane, so they bind to surface receptors.
This activates a second messenger (e.g. cAMP), triggering a cascade of reactions.
Adrenaline binds to a cell surface receptor and activates a second messenger called .
Adrenaline binds to a cell surface receptor and activates a second messenger called cAMP.
Why does vasoconstriction reduce heat loss?
Arterioles to the skin constrict, so less blood flows through skin capillaries.
Blood is diverted deeper, losing less heat to the environment.
Define myogenic.
Myogenic means the heart generates its own heartbeat without any external stimulus.
What is the role of the sinoatrial node (SAN)?
It is the heart's pacemaker.
It initiates a wave of depolarisation that makes the atria contract.
Why is there a band of non-conducting tissue between the atria and ventricles?
To stop the depolarisation spreading straight to the ventricles.
This ensures the atria contract before the ventricles.
What is the role of the atrioventricular node (AVN)?
It receives the wave of depolarisation from the atria.
After a slight delay, it passes it to the bundle of His.
Why is the delay at the AVN important?
It ensures the ventricles contract after the atria.
This lets the atria empty into the ventricles first.
What do the bundle of His and Purkyne fibres do?
The bundle of His carries the impulse down the septum.
The Purkyne fibres spread it through the ventricle walls, causing them to contract.
Why do the ventricles contract from the apex upwards?
The Purkyne fibres carry the impulse to the apex first.
Contracting from the bottom up forces blood upwards into the arteries.
The heart's natural pacemaker is the node, found in the wall of the right atrium.
The heart's natural pacemaker is the sinoatrial node, found in the wall of the right atrium.
True or False?
The heart needs a nerve impulse from the brain to start each beat.
False.
The heart is myogenic — it generates its own beat; nerves only adjust the rate.
In which part of the heart is the bundle of His located?
In the septum (the middle wall dividing the two sides of the heart).
Put the conducting structures of the heart in the correct order.
SAN → AVN → bundle of His → Purkyne fibres.
What causes atrial contraction?
The wave of depolarisation from the SAN spreading across the atria.
Define electrocardiogram (ECG).
An electrocardiogram is a trace of the heart's electrical activity, recorded by electrodes placed on the skin.
What does the P wave of an ECG represent?
Depolarisation of the atria, which causes atrial contraction.
What does the QRS complex represent, and why is it the largest?
Depolarisation of the ventricles, causing ventricular contraction.
It is largest because the ventricles have the greatest muscle mass.
What does the T wave represent?
Repolarisation of the ventricles, which causes ventricular relaxation (diastole).
Define tachycardia.
Tachycardia is a resting heart rate that is too fast — over 100 bpm.
Define bradycardia.
Bradycardia is a resting heart rate that is too slow — below 60 bpm.
What is an ectopic heartbeat?
An early heartbeat followed by a pause.
It is common and usually needs no treatment.
Define fibrillation.
Fibrillation is an irregular heartbeat in which the atria or ventricles stop contracting properly; severe cases can be fatal.
Why is a larger wave on an ECG significant?
It shows greater electrical activity.
This results in a stronger contraction of that part of the heart.
The complex on an ECG is caused by depolarisation of the ventricles.
The QRS complex on an ECG is caused by depolarisation of the ventricles.
True or False?
A resting heart rate below 60 bpm is always dangerous.
False.
Many fit athletes have low resting rates (bradycardia) that are usually not dangerous.
How can an ECG help diagnose heart problems?
Certain conditions produce characteristic wave shapes or rhythms.
Comparing an ECG with a healthy trace can reveal problems such as fibrillation.
Define cardiac output.
Cardiac output is the volume of blood pumped by the heart (a ventricle) per unit of time.
Define stroke volume.
Stroke volume is the volume of blood pumped out of the left ventricle in one cardiac cycle.
Define heart rate.
Heart rate is the number of times the heart beats per minute (cardiac cycles per minute).
State the equation for cardiac output.
cardiac output = heart rate × stroke volume.
A heart has a rate of 50 bpm and a stroke volume of 75 cm³. Calculate the cardiac output.
CO = heart rate × stroke volume
= 50 × 75 = 3750 cm³ (3.75 dm³)
An athlete has a cardiac output of 9800 cm³ and a heart rate of 110 bpm. Calculate the stroke volume.
stroke volume = cardiac output ÷ heart rate
= 9800 ÷ 110 = 89.09 cm³
Why does cardiac output increase during exercise?
To match the increased metabolic demands of the cells.
More blood is pumped to supply oxygen and glucose.
Why do fitter people often have a higher cardiac output?
They have thicker, stronger ventricular muscle.
This pumps a larger stroke volume with each beat.
How would you find heart rate if one cardiac cycle takes 1.2 seconds?
heart rate = 60 ÷ time for one cycle
= 60 ÷ 1.2 = 50 bpm
Cardiac output equals heart rate multiplied by volume.
Cardiac output equals heart rate multiplied by stroke volume.
True or False?
1 dm³ is equal to 1000 cm³.
True.
Remember to convert units to match before doing cardiac output calculations.
How would you rearrange the cardiac output equation to find heart rate?
heart rate = cardiac output ÷ stroke volume.
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