The cardiovascular system is derived from embryonic mesoderm and is comprised of the following components:
The arteries, capillaries, and veins are lined with a layer of epithelial cells refered to as the endothelium.
Agnatha and primitive gnathostomes possess a simple two-chambered heart that pumps a single stream of oxygen depleted blood thru the heart and into the aortic arches, via the ventral aorta, where it is oxygenated and preceeds directly to the head and body, and then circulates back to the heart depleted of oxygen.
All vertebrates possess sino-atrial and atrio-ventricular valves to prevent the blood from backflowing into the preceeding chambers.
When tetrapods make the transition from water to land the swim bladder is employed as an oxygen resevoir, eventually giving rise to the lungs and pulmunary loop.
The pulmunary loop is not on the main path of blood flow and results in the mixing of oxygenated with deoxygenated blood in the three chambered heart of riphidistian fishes, amphibians and reptiles (ectotherms).
Birds and mammals have independently developed the four chamber heart and reestablished the flow of fully oxygenated blood flow (endotherms).
Agnatha possess a set of semilunar valves in the conus arteriosus as well.
The sinus venous recieves unoxygenated blood via three veins:
The heart of primitive gnathostomes resembles that of agnathans except that:
The development of a buoyancy or swim bladder appears to immediately follow the of divergence of Osteichthyes (bony fishes) from Chrondrichthyes (sharks, rays, etc...).
Some sarcopterygii have co-opted the swim bladder and developed a vascular network around the bladder for use as a air reservoir under hypoxic conditions.
Crossopterygians can be considered "double breathers" using both the gills and the swim-bladder/lung as oxygenation sources for the blood.
Rhipidistian fish may have possessed a three chamber heart where a partition (the interatrial septum) divides the atrium into left and right chambers, similar to that of it's crossopterygian cousin, the Dipnoi (lung fish).
In Dipnoi, the ventricle is also partially divided by a partition (the interventricular septum).
Unoxygenated blood from the sinus venous empties into the right atrial chamber.
Oxygenated blood flows into the left atrial chamber via the pulmonary vein (6th aortic arch) from the lungs.
This stage represents the beginning of a evolutionary transition between single-type and dual-type circulatory systems.
The heart of primitive tetrapods resembles that of the rhipidistians except that:
Amphibians rely on one and/or more of three possible modes of respiration:
Most amphibians rely on a combination of all three respiratory modes.
In amniotes, where primary respiration occurs via the lungs (pulmunary breathing) rather than the pharyngeal gills, it becomes increasingly necessary to cooridinate and synchronize the cardiac and respiritory pulmunary rhythms so as to obtain a cardiopulmunary rhythm that efficiently integrates the two physiological functions into an adaptive whole.
A cardiopulmonary oscillator network in the brainstem cooridinates heart rate and breathing rhythms.
The sinus venous has become vestigial or lost in reptiles, mammals, and birds.
The conus arteriosus is modified to form the pulmonary trunks and aortic trunks.
A network of interneurons located in NTS and NA generate the respiritory rhythm and communicate with the motor neurons that control respiratory, laryngeal, and cardiac function.
Respiration results in rhythmic fluctuations of heart rate that are refered to as respiratory sinus arrhythmia (RSA).
RSA is due to the actions of NA projections to the sino-atrial node of the heart
RSA reflects the influence of respiration on the flow of sympathetic and DVC/DMNX-vagal impulses to the sinoatrial node of the heart.
VVC/NA-mediated vagal activity is inhibited by inspiration causing heart rate to accelerate in response to the removal of inhibition on the sympathetic input to the sino-atrial node of the heart.
VVC/NA-mediated activity resumes upon expiration and heart rate slows as inhibition of sympathetic input is reinstated.
In mammals the ribcage has become shortened relative to that of reptiles and other vertebrates.
In mammals the diaphragm becomes important for facilitating the type of breathing required by real-time foraging tetrapods.
There are at least two respiration-related rhythm generators in the medulla: the pre-Bötzinger complex, which produces inspiratory neuron bursts, and the parafacial respiratory group, which produces predominantly pre-inspiratory (Pre-I) neuron bursts.
The preBotzinger complex appears to play an important role in the respiratory rhythm of mammals providing control of the diaphragm in response to sensory feedback.
The pre-inspiratory parafacial neurons appear to establish a stable respiratory rhythm which can be modulated by the inspiratory preBotzinger complex.
"Humans and other mammals are the only vertebrate species to possess a diaphragm.(?) This muscle played a key role in our ascending the evolutionary ladder by letting us take in more oxygen to feed our bigger brains. We think that the preBotzinger Complex also may have evolved to control the diaphragm."