CV1 ANATOMY 1 Notes PDF

Lecture CV 1: Cardiac anatomy, histology and function 1 Lecturer: Yolanda Martinez Pereira (notes by Thalia Blacking) Learning objectives: 1. Understand the basic structure, location and anatomical relationships of the mammalian heart. 2. Describe the pericardium and its layers, the myocardium an...

Lecture CV 1: Cardiac anatomy, histology and function 1 Lecturer: Yolanda Martinez Pereira (notes by Thalia Blacking) Learning objectives: 1. Understand the basic structure, location and anatomical relationships of the mammalian heart. 2. Describe the pericardium and its layers, the myocardium and its arrangement, and the endocardium. 3. Understand the term “skeleton” as it pertains to the heart, and understand its functional relevance. 4. Identify the external features of the heart. 5. Identify the salient features of individual cardiac chambers. 6. Recognise the arrangement of the great vessels and understand how they convey blood between the heart and pulmonary & systemic circulations. 7. Describe the valves of the heart. 8. Appreciate the general layout of the coronary circulation. 9. Describe the innervation of the heart. Basic structure: Muscular 4-chambered pump (functionally, 2 pumps in series - left and right sides are separated). Two chambers on each side – one atrium, one ventricle - separated by valves. Right side (deoxygenated) – input from systemic circulation, delivers blood to lungs. Left side (oxygenated) – blood received from pulmonary circulation and pumped into the systemic circulation i.e. around the body. *Muscle mass L>R (though note that the volume of blood pumped by each side has to be equal). Shape: Varies with species – more ovoid in carnivore, more conical in horse / ruminant Location: Thorax, approximately midline though lies farther to left and apex contacts left thoracic wall (“apex beat” is palpable); sternal contact (±fat). Relationships: Mediastinal structure – thymus is cranial, lungs / phrenic nerves are lateral, and trachea / bronchial bifurcation / oesophagus lie dorsal to heart. N.B. Though divided “left” and “right”, the orientation of the heart within the thorax means that the right side is also more cranial. Note terminology – “base” (dorsal) and “apex”. Size: Generally ∼1% bodyweight (roughly 2/3 width / height of the thorax in carnivores). Form/Structure: Pericardium (literally – “around the heart”): double layered sac in which the heart is enclosed. The nomenclature can be confusing. Functionally, its layers are referred to as serous and fibrous – think of the serous pericardium as closed mesothelial sac wrapped around the  heart (left). The inner layer (the visceral pericardium) has become firmly attached to the heart itself (the epicardium). The outer layer of this sac (the parietal pericardium) is firmly attached to a tough fibrous covering, the fibrous pericardium. This blends with the connective tissue adventitia around the great vessels at the heart base. A small amount of serous fluid acts as lubricant within the pericardial space (between the two serous layers), though it is essentially a virtual space. Several important clinical conditions involve the accumulation of fluid (transudate, blood, pus) within the pericardial space. These are life threatening, particularly as the indistensible nature of the fibrous pericardium leads to cardiac tamponade – the heart (particularly the right ventricle) is unable to function properly. Myocardium (myo – muscle) – The intermediate layer (and most of the mass of the heart). Predominantly comprises contractile muscle cells – cardiomyocytes. It also includes specialised pacemaker and conducting cells, interstitial connective tissue and blood vessels / nerves supplying the heart wall. Cardiac muscle shares many features with skeletal muscle, but there are important distinctions (see L5). The muscle fibres of the myocardium are arranged in concentric, interconnected layers with some adopting a spiral / figure of eight orientation. Endocardium (endo- “within”) – inner endothelial layer (continuous with that of great vessels): Single layer of flattened endothelial cells (simple squamous epithelium) with a basement membrane. The endocardium includes a subendothelial layer of connective tissue - permits myocardial movement without endothelial damage. Blood vessels, nerves and specialised conducting tissue branches run within this subendothelial layer. “Skeleton of the heart” – A platform of fibrous tissue separates the atrial and ventricular parts of the heart. Each heart valve is surrounded within this platform by a ring of fibrous tissue, the annulus fibrosus. A fibrous triangle between the rings ossifies in some species (e.g. ox) – os (pl. “ossa”) cordis. The atrioventricular node is the only path for conduction of electrical impulses through this platform (from atria to ventricles). This is critical for co-ordination of cardiac contraction. External features of the heart: Atria and auricles (“little ears”, blind-ending appendages to the atria) form the heart base; they are divided from the ventricles by the coronary groove. The ventricles are demarcated externally by grooves that run towards the apex. Note that it is the left ventricle which forms the apex. Major features of the chambers of the heart: Right Atrium: Entrance of cranial and caudal venae cavae which carry deoxygenated blood, separated by intervenous tubercle (directs blood flow towards the right A-V valve & ventricle). Terminal crest at entrance of CrVC – location of the sinoatrial node. Coronary sinus (carries venous blood from coronary circulation). Fossa ovalis – Remnant of the embryonic foramen ovale (closes at birth). The azygous vein drains lumbar and intercostal veins and enters the heart via the right atrium or the cranial vena cava. In most species it is unpaired (horses and carnivores have a right azygous, but ruminants have a left or sometimes both L and R). Left Atrium: Entrance of pulmonary veins (2 or 3 sites). Auricle. The fossa ovalis can also be appreciated from the left. Features of the Ventricles: Cross section – Note the different wall thicknesses, and how the right “wraps around” the left. The “powerhouse” of cardiac contraction is the left ventricle – it has to overcome the resistance of the arterial / systemic circulation. Thus, it must generate higher pressures than the right in order to do its job, and has a considerably thicker wall (∼3x RV). Interior walls – Papillary muscles are insertions for chordae tendineae of atrioventricular valves. Right ventricle: The trabecula septomarginalis – a muscle band crossing the ventricle (linking the septum to the ventricular wall) – may provide physical stabilisation as well as a short cut for conduction Left ventricle: Note much greater wall thickness. Great vessels – At the heart base, the cranial & caudal venae cavae lie towards the right (→RA). The pulmonary trunk emerges cranial/left of aorta to divide into the pulmonary arteries, taking deoxygenated blood to lungs. Pulmonary veins carry oxygenated blood –moving blood towards the heart, into the LA. Aorta exits most centrally, arching craniodorsally, then caudally to run along left dorsal aspect of thoracic cavity (thoracic aorta). After the coronary arteries (which supply the heart itself) the first vessels to originate from aorta are paired subclavian and paired common carotid arteries. In ruminant and horse, these emerge together as the brachiocephalic trunk, whereas in dog and pig the left subclavian has a distinct, more distal origin. Valves – Two pairs – each side of the heart has an atrioventricular and a semilunar valve. Atrioventricular valves (right – tricuspid, left – mitral or bicuspid). As their names suggest, the tricuspid has three cusps in humans, and mitral two. Numbers may be more varied in our species, however. Each cusp is attached to papillary muscles by chordae tendineae, promoting co-ordinated closure during systole. The valves are fine endothelium-covered structures but have strong fibroelastic support. They prevent blood from moving back into the atria during ventricular systole. Semilunar valves (right – pulmonic, left – aortic) – both have three cusps (and no chordae tendineae). The half-moon-shaped cusps fit together closely to prevent retrograde flow of blood during ventricular diastole. Coronary Circulation – Arteries: The heart receives approximately 5% of cardiac output (subject to variation). Left and right coronary arteries arise from the aorta (just distal to aortic valve) and take separate courses to the coronary groove between the atria and the ventricles. Interventricular branches run in the interventricular grooves, taking blood towards the apex. Coronary veins: Run parallel to arteries, to return blood to the RA at the coronary sinus. Innervation – Autonomic supply, both sympathetic and parasympathetic. Sympathetic – largely via major local sympathetic ganglia (stellate, middle cervical). Parasympathetic supply is from the vagus nerve (some via the recurrent laryngeal branch), and innervates primarily the atria, particularly the sinoatrial node. The recurrent laryngeal nerve is noteworthy for its circuitous course (particularly the left) - from brain, down into thorax, around aorta (left) or right subclavian artery (right) and back up the neck to innervate the larynx. In the horse, the LRLN may be 2.5m long (Cole, 1946), >10% longer than the right – associated with dysfunction (whistling / roaring). Note the phrenic nerve (Ce 5,6,7 to diaphragm) crosses the pericardium – e.g. thoracic surgery. References / Further Reading (lectures 1 and 2): Textbook of Medical Physiology, 14th Ed (Guyton AC, Hall JE; Elsevier Saunders; 2020) Textbook of Veterinary Anatomy (Dyce, KM, Sack, WO, Wensing CJG; WB Saunders & Co.; 2017) Textbook of Veterinary Histology, 6th Ed. (Dellmann HD, Eurell J; Williams & Wilkins; 2006) Textbook of Veterinary Physiology, 6th Ed. (Ed. Cunningham JG; WB Saunders & Co.; 2019) Sisson & Grossman’s The Anatomy of the Domestic Animals, 5th Ed. (Getty, R; WB Saunders & Co.; 1975) Wheater’s Weather’s Functional Histology, 7th Ed. (O’Dowd, G., ell S., Wright S.; 2023)