The Cardiovascular System

Greater (Systemic) Circulation and Lesser (Pulmonary) Circulation

The cardiac output is distributed between the greater (systemic) circulation and the lesser (pulmonary) circulation

Sympathetic and Parasympathetic

It increases heart rate

It decreases heart rate

It increases AV nodal conduction

It decreases AV nodal conduction

It increases contractility

It decreases contractility (atria only)

Intercalated discs allow cardiac muscle to function as a syncytium

The main functions of the cardiovascular system are to transport nutrients, gases, waste products, hormones, and heat.

The functional organization of the heart and blood vessels helps in maintaining cardiovascular functions by ensuring unidirectional blood flow, generating and transmitting cardiac impulses, separating the atria from the ventricles, and forming the valves.

The components of the cardiovascular system include the heart, veins, arteries, venules, arterioles, capillaries, and blood vessels.

The three types of blood vessels are veins, arteries, and capillaries.

The special tissues in the heart responsible for generating and transmitting cardiac impulses are the pacemaker (sino-atrial node), conductive system (inter-atrial tracts, inter-nodal tracts, atrio-ventricular node, bundle of His, right and left bundle branches), and Purkinje fibers.

The fibrous tissues in the heart separate the atria from the ventricles and form the valves.

The endocardium covers the inner side of the myocardium, while the pericardium covers the outer side of the myocardium.

The pre-capillary sphincters regulate blood flow into capillaries, allowing for selective perfusion of tissues based on their metabolic demands.

The cardiovascular system transports nutrients (e.g. glucose, amino acids, fatty acids, vitamins), gases (e.g. oxygen, carbon dioxide), waste products, hormones, and heat.

The valves in the heart maintain the unidirectional flow of blood in the cardiovascular system.

Factors that influence rhythmicity in the heart are hypoxia and the levels of ions such as sodium, calcium, and potassium. Hypoxia leads to negative chronotropic effects, while hyponatremia and hypocalcemia also result in negative chronotropic effects. On the other hand, hypernatremia and hypercalcemia cause negative chronotropic effects. Hypokalemia leads to positive chronotropic effects, while hyperkalemia causes negative chronotropic effects.

Excitability in cardiac muscle refers to its ability to respond to stimuli by generating an action potential. The action potential in cardiac muscle differs from that of pacemakers. One key difference is the involvement of different ions, such as potassium (K+), calcium (Ca++), and sodium (Na+). Additionally, the action potential in cardiac muscle is responsible for contraction, while pacemakers are responsible for initiating the electrical impulses that regulate the heart's rhythm.

Nerve action potentials and cardiac muscle action potentials have several differences. In nerve action potentials, there is a rapid influx of sodium ions (Na+) followed by an efflux of potassium ions (K+), leading to depolarization and repolarization. In cardiac muscle action potentials, there is also an influx of sodium ions (Na+), but it is followed by an influx of calcium ions (Ca++) and a slower efflux of potassium ions (K+), resulting in a longer duration and a plateau phase. Additionally, cardiac muscle action potentials do not exhibit hyperpolarization like nerve action potentials.

Pacemaker action potentials and cardiac muscle action potentials have some differences. Pacemaker action potentials have a gradual depolarization phase due to the opening of funny channels (If channels) that allow slow influx of sodium ions (Na+) and calcium ions (Ca++). In contrast, cardiac muscle action potentials have a rapid depolarization phase due to a fast influx of sodium ions (Na+). Another difference is that pacemaker action potentials have a less pronounced plateau phase compared to cardiac muscle action potentials.

The absolute refractory period in cardiac muscle action potentials is prolonged compared to nerve and skeletal muscle action potentials. It spans the entire period of systole and corresponds to depolarization and most of the repolarization. During this period, excitability becomes zero, meaning no stimulus, no matter how strong, can induce contraction. The relative refractory period in cardiac muscle action potentials occurs during part of the diastole and corresponds to the late part of repolarization. A strong enough stimulus can induce contraction during this period.

Several factors can influence cardiac excitability. These include heart rate, body temperature, stimulation of autonomic fibers, and the presence of bacterial toxins and drugs.

The conducting system of the heart consists of various tracts and nodes with different conduction velocities. The inter-atrial tracts have a conduction velocity of 1 m/s, the sino-atrial node has a conduction velocity of 0.05 m/s, the anterior, middle, and posterior inter-nodal tracts have a conduction velocity of 1 m/s, the Purkinje fibers have a conduction velocity of 4 m/s, the AV node has a conduction velocity of 0.01 m/s, and the bundle of His and the right and left bundle branches have a conduction velocity of 1 m/s and 4 m/s, respectively.

Vagal tone refers to the influence of the vagus nerve on heart rate. Although the sino-atrial (SA) node rhythm is approximately 90 discharges per minute, the heart contracts at a rate of about 70 beats per minute due to the effect of the vagus nerve. The vagus nerve decreases heart rate by releasing acetylcholine, which slows down the firing rate of the SA node.

Various factors can affect the conducting system of the heart.