General Heart Structure

• The two-sided heart evolved from the two-chambered heart of bony fish through the insertion of a septum.
• The swim bladder, originally ventral to the heart, evolved into lungs.
• The carotid artery divides into the internal and external carotid arteries.

General Heart Structure

• Four vessels enter the right atrium:

  • Cranial vena cava
  • Caudal vena cava: The fossa ovalis is a remnant of the foramen ovale that allowed connection between the right and left atria in fetal life.
  • Azygos vein: Drains the dorsal thorax and drops into the right atrium in horses, dogs, and cats
  • Coronary sinus: Drains blood from the heart itself

• The inter-venous crest between the cranial and caudal vena cava diverts blood flow to prevent collisions.

• Both atria have an 'out pouching' called the auricle (ear), located cranially and to the left, which increases the atrium's volume.

• The main part of the atrium consists of smooth muscle, while the auricle has ridges in the wall called pectinate muscles.

• The right atrium pumps blood into the right ventricle via the atrio-ventricular valve (tricuspid), with cusps anchored by chorda tendinae (tendinous cords) that attach to papillary muscles and allow the valve to open downwards only.

• The internal surface of the ventricle has ridges called trabeculae carneae, which may help smooth blood flow.

• The right ventricle pumps blood around the cranial side of the heart.

• Coronary vessels are situated in the coronary groove.

• The left atrio-ventricular valve is bicuspid/mitral.

• Moderator bands, or trabecula septomarginalis, are bands of heart muscle found in both ventricles between the septum and ventricular wall, serving an electrical rather than physical function.

Heart Base

• Inflows and outflows include the aorta, pulmonary artery, and vena cavae.
• The ductus arteriosus in the fetus bypasses the pulmonary artery and aorta, closing within the first three days of life and remaining as the vestigial ligamentum arteriosum.
• The fibrous skeleton contains valves and vessels and provides insulation.
• Mineralization of two regions forms ossa cordis (bones of the heart) in large ungulates.
• Coronary circulation accounts for approximately 15% of cardiac output.
• Major vessels are positioned outside the myocardium to prevent occlusion from heart pressure.
• Venous drainage occurs into the atria via foramina venarum minimarum and into the ventricles via venae cordis parvae.
• In species with a left azygos vein (pigs, ruminants), it drains diagonally across to the coronary sinus on the right.
• Pigs typically have a left azygos vein, while ruminants may have both.

Pacemaker and Impulse

• The primary pacemaker of the heart is the sinuatrial node (not sinoatrial node) near the intervenous crest in RA.
• The impulse is transferred across the heart muscle in carnivores, and a specialized bundle of His-like bundle transmits the electrical impulse to the atrio-ventricular node near the coronary sinus opening in larger animals.
• The impulse travels down through the bundle of His, which divides into two crura, reaching the apex and moving upwards to cause a wave-like contraction.
• Moderator bands may allow the impulse to jump from the septum to the wall more quickly.

Innervation

• Parasympathetic innervation arises from the vagus nerves (cranial nerve X), descending in the vago-sympathetic trunk (VST) along with the common carotid artery, and innervates the SAN and AVN, reducing heart rate through cardiovagal fibers.
• Sympathetic innervation originates from the thoracic outflow from stellate and middle cervical ganglia, innervating the myocardium and vessels, and cardiosympathetic fibers increase heart rate and force.