Conclusion - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 800

ventricular postload (heightening vascular bore and attenuating flow resistance). 1
The cardiac chambers may then receive an amount of blood during inspiration
slightly greater than that during expiration. However, the aid given to the right
cardiac pump by a low intrathoracic pressure may not significantly influence the
left cardiac pump, which works against a much higher outlet pressure range.
In addition, the natural respiratory sinus arrhythmia defines a variation in cardiac
frequency during a breathing cycle. The heart beat rate increases during inspiration
and decreases during expiration. The cardiac frequency is controlled by the nucleus
ambiguus among the centers in the medulla oblongata. The nucleus ambiguus in-
creases parasympathetic nervous system inputs to the heart via the cardioinhibitory
vagus nerve. During expiration, neurons in the nucleus ambiguus are activated and
the cardiac frequency slows. Conversely, inspiration triggers inhibitory signals to
the nucleus ambiguus and consequently the vagus nerve remains in a resting state.
In summary, the heart beats more slowly during expiration than inspiration.
Stepwise inspiration and expiration provoke an increase in cardiac frequency
followed by a rapid decrease [ 1620 ]. The fastest and slowest heart rates are
obtained in response to inspiration with respect to expiration. Changes in breathing
frequency (from 0.05 to 0.2 Hz) also cause cardiac frequency variations, especially
in a respiration frequency range of 0.1 to 0.12 Hz for which heart rate variation
is maximal. Heart rate variability with deep breathing serves as a measure of
parasympathetic function.
Many factors influence heart rate variability [ 1621 ]: (1) age, as the variability
decreases with advancing age; (2) body position, as the variability is maximal when
the patient is lying supine; (3) respiration frequency, as the variability is maximal
when the breathing rate equals 0.08 to 0.1 Hz; and (4) respiration amplitude, as the
variability is maximal when the tidal volume is approximately 1.2 l for an average
adult.
Both breathing and blood circulation adapt to the body's needs. Blood contents of
oxygen and carbon dioxide primarily control the breathing and cardiac frequencies.
Both the body's ventilation and blood convection are controlled by the autonomic
nervous system.
In the central nervous system, four main centers regulate respiration (inspiratory,
expiratory, pneumotaxic, and apneustic centers; Table 14.1 ). Inspiration results from
a sudden, ramped increase in motor discharge to the inspiratory muscles. The
inspiratory center (dorsal group of respiratory neurons) formed by neurons of the
nucleus of tractus solitarius situated in the upper region of the medulla oblon-
gata controls the diaphragm and intercostal muscles responsible for inspiration.
These inspiratory muscles are connected to the nervous centers via spinal nerves.
Contraction of the diaphragm is triggered by 2 phrenic nerves that emanate from
cervical spinal nerves 3 to 5. Eleven pairs of intercostal nerves, the anterior branches
(rami anteriores) of the thoracic T1 to T11 spinal nerves regulate the activity
of intercostal muscles. Before the end of inhalation, motor discharge declines.
Exhalation is usually passive, except at high ventilation rates. At rest, expiration

1 On the other hand, when arterial blood pressure rises, left ventricular postload elevates.