A-Level Biology Organisms Respond To Changes In Their Internal And External Environments

IV. Control of Heart Rate
The heart is a myogenic muscle which means it can contract and relax without nervous stimulation.
However, the rate at which the heart beats is influenced by the nervous system and other factors.
The control of heart rate is crucial in regulating blood pressure, and ensuring adequate blood supply
to the body's tissues.
Myogenic Stimulation
 The cardiac muscle of the heart is stimulated by electrical impulses that originate from the
sinoatrial (SA) node, the natural pacemaker of the heart.
 The SA node generates regular impulses at a rate of about 60-100 beats per minute.
 These impulses are conducted to the atrioventricular (AV) node, which is located in the floor
of the right atrium. The AV node delays the impulses before passing them onto the
ventricles.
 The impulses are then transmitted via the Purkinje fibers to the ventricular muscle cells,
causing them to contract.
Roles of the SA and AV Nodes
 The SA node is responsible for initiating the impulses that regulate the heartbeat.
 The AV node slows down the impulses generated by the SA node to allow the atria to
contract and fill the ventricles with blood before the ventricles contract.
 The AV node also acts as a secondary pacemaker in case the SA node fails to generate
impulses.
Chemoreceptors
 The heart rate is also influenced by chemoreceptors located in the aortic arch and carotid
arteries.
 These chemoreceptors are sensitive to changes in oxygen and carbon dioxide levels in the
blood.
 When the oxygen level in the blood decreases or carbon dioxide level increases, the
chemoreceptors signal the medulla oblongata in the brain to increase the heart rate.
Overall, the control of heart rate is an important process that involves both myogenic and nervous
stimulation. The SA and AV nodes play crucial roles in regulating the heart rate, while
chemoreceptors respond to changes in the blood oxygen and carbon dioxide levels.
I. Introduction
 Importance of studying this topic in biology
II. Survival and Response
 Types of stimuli
 Tropisms and taxes
 Reflexes
III. Receptors
 Types of receptors
 Structure and function of receptors
IV. Control of Heart Rate
 Structure and function of the heart

,  The cardiac cycle
 Regulation of heart rate
V. Nervous Coordination
 Structure and function of the nervous system
 Neurons and nerve impulses
 Reflex arc
VI. Synaptic Transmission
 Structure and function of synapses
 Neurotransmitters and their function
 Types of synapses
VII. Skeletal Muscles are Stimulated to Contract by Nerves and Act as Effectors
 Structure and function of skeletal muscles
 Sliding filament theory
 Neuromuscular junction
VIII. Homeostasis is the Maintenance of a Stable Internal Environment
 Definition of homeostasis
 Negative feedback and positive feedback
 Importance of maintaining homeostasis
IX. Control of Blood Glucose Concentration
 Role of insulin and glucagon
 Regulation of blood glucose concentration
X. Control of Blood Water Potential
 Role of ADH
 Regulation of blood water potential
XI. Other responses to the environment
 Plant responses to light and gravity
 Animal responses to day length and seasonal changes
XII. Conclusion
 Summary of key points
 Importance of organisms responding to changes in their internal and external environments
in maintaining survival and reproduction.