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Chapter

BIOC63H3 Chapter Notes -Active Transport


Department
Biological Sciences
Course Code
BIOC63H3
Professor
Ivana Stehlik

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Cardiovascular system: The Heart
Objectives
After the series of lectures on this topic, the students should be able to
1. Discuss the membrane potential, ion channels and pumps of the heart.
2. Discuss the electrophysiology of cardiac muscle and the origin of the heart beat.
3. Describe the physiology of cardiac muscle contraction.
4. Describe the conducting system of the heart.
5. State the differences between slow and fast channels in cardiac muscle, and describe
action potentials in cardiac muscle.
6. Explain the importance of the refractory period in cardiac muscle.
7. Explain the various components of the cardiac cycle, including systole, diastole, and
heart sounds.
8. Define cardiac output (CO) and describe the factors that affect it.
9. Explain the various features of an electrocardiogram (ECG) and the events that those
features reflect.
10. Define the term autorhythmicity, and explain how the sinoatrial (SA) node functions as
the pacemaker.
11. Explain how heart rate is regulated.
12. Describe the differences between sympathetic and parasympathetic stimulation of heart
function.
13. Explain heart failure and its compensatory mechanisms.
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Membrane Potential, Ion Channels and Pumps (Figure 1)
The cell membrane is composed of a lipid bilayer that has an intrinsicially low permeability to
charged ions. Spanning the bilayer, however, are a variety of structures through which ions can
enter or leave the cell. Ion channels allow specific ions to move passively, as though through a
pore. By contrast ions pumps use energy to actively transport ions across the membrane,
normally against the concentration gradient. Ion channels and pumps are fundamental to cell
function. They regulate the ionic gradients across the cell membrane, and determine membrane
potential.
The resting membrane potential
At rest, most active ion channels are of a type selective for K+, so that the membrane is more
permeable to K+ than other ions. Membranes with this property are called semipermeable. The
cell contains large negatively charged molecules (e.g. proteins) that cannot cross the membrane.
The presence of fixed negative charges attracts positively charged ions. As the membrane is
most permeable to K+, this leads to an accumulation of K+, within the cell. However, the
electrical forces attracting K+ into the cell are then counterbalanced by the increased
concentration gradient, which tends to drive K+ out of the cell. An equilibrium is reached when
these two opposing forces exactly balance. In cardiac muscle cells this occurs when intracellular
K+ is 120 mM for an extracellular [K+] of ~ 4 mM. The opposing effect of the
concentration gradient means that slightly fewer positive charges (in this case K+ ions) move
into the cell than there are intracellular negative charges (e.g. proteins). The inside (charge
separation), and as a result a potential develops across the membrane. If the membrane was only
permeable to K+, the potential at equilibrium would be defined entirely by the concentration
gradient for K+ across the membrane. This is the K+ equilibrium potential, and it can be
calculated from the Nernst equation.
The actual resting membrane potential (RMP) is less negative than theoretical K+ equilibrium
potential. This is because other ions (e.g. Na+) can also cross the membrane, although the
membrane permeability for these ions is much less than that for K+. Unlike K+, Na+ has a
concentration gradient that is far from equilibrium, as a result of the activity of the Na+ pump
(Na+-K+ ATPase). This pumps three Na+ ions out of the cell in exchange for two K+ ions into the
cell, using ATP as an energy source. As a result intracellular [Na+] is low (~10mM), even
though extracellular [Na+] is high (~140mM). The equilbrium potential for Na+ (the potential at
which electrical and concentration gradient forces would be exactly balanced) is therefore very
positive about +65 mV.
The RMP in a cardiac muscle cell from a ventricle is approximately 90 mV, close to the K+
equilibrium potential. This attracts Na+ ions into the cell. Whereas this inward electrical
attraction is balanced in the case of K+ by the outward K+ concentration gradient, the
concentration gradient for Na+ is inward due to the Na+ pump. Thus both concentration and
electrical forces draw Na+ into the cell, and the electrochemical gradient (the net effect of
concentration and electrical forces) is inward. The amount of Na+ actually entering the cell is
limited by the low membrane permeability for Na+, and the continuous action of the Na+ pump as
it pumps Na+ out. An equivalent situation is apparent for Ca2+, as the Ca2+ concentration
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