3.2 Cardiovascular and Lymphatic Systems

Cardiovascular and Lymphatic Systems

This topic is 11% of General Anatomy on the official Part I test plan. The blueprint lists heart, arteries, veins, lymphatic vessels and lymphoid tissue (explicitly spleen and thymus), development, and histology. Physiology of the cardiac cycle and ECG belongs to the Physiology domain; here the scoring objects are names, relations, and embryonic remnants.

Heart

The heart sits in the middle mediastinum, two-thirds to the left of midline. Sternocostal (anterior) surface is mostly right ventricle. Diaphragmatic (inferior) surface is left ventricle plus some right ventricle. The base is the left atrium facing the vertebral column and esophagus. The apex is left ventricle, in the left fifth intercostal space near the midclavicular line.

Pericardium: tough fibrous pericardium (fused with the central tendon of the diaphragm and the adventitia of the great vessels) plus serous pericardium (parietal lining the fibrous sac; visceral = epicardium). The pericardial cavity is the potential space between parietal and visceral serous layers. The transverse sinus is a passage behind the aorta and pulmonary trunk (clamp site in cardiac surgery). The oblique sinus is a cul-de-sac behind the left atrium among the pulmonary veins.

Chambers and internal landmarks

Right atrium receives SVC, IVC, and coronary sinus. The crista terminalis separates smooth sinus venarum (sinus venosus origin) from the rough pectinate wall (true atrium). The fossa ovalis on the interatrial septum is the sealed foramen ovale. The triangle of Koch (tendon of Todaro, coronary-sinus ostium, septal leaflet of the tricuspid valve) locates the AV node.

Right ventricle has coarse trabeculae carneae, three papillary muscles to the tricuspid valve, and the septomarginal trabecula (moderator band) carrying the right bundle branch to the anterior papillary muscle. The smooth conus arteriosus (infundibulum) leads to the pulmonary valve.

Left atrium is mostly smooth (pulmonary-vein incorporation) with a pectinate left auricle—a common site of thrombus in atrial fibrillation. Four pulmonary veins enter it.

Left ventricle has a thick myocardium, fine trabeculae, two papillary muscles (anterolateral and posteromedial) to the mitral valve, and a smooth aortic vestibule.

Valves

Right and left aortic sinuses give the right and left coronary arteries. The noncoronary sinus does not.

Coronary arteries and veins

RCA runs in the right AV groove: SA nodal branch in about 60% of hearts, right (acute) marginal, AV nodal branch in about 80%, and—when the heart is right-dominant—the posterior descending (posterior interventricular) artery in the posterior interventricular groove, which also supplies the posterior third of the IV septum.

LCA quickly divides into left anterior descending (LAD, anterior interventricular)—septal perforators and diagonals; anterior two-thirds of the septum, anterior LV, apex—and left circumflex (LCx) in the left AV groove with obtuse marginal branches. In left dominance, LCx gives the PDA.

Dominance is defined by which artery supplies the PDA. Right dominance is the majority pattern in commonly cited series (~85%), with smaller left-dominant and codominant fractions. A proximal LAD occlusion is the classic “widow-maker” because of the anterior wall and septum.

Venous return: great cardiac vein (with LAD) → coronary sinus (in the posterior left AV groove, into RA). Middle cardiac vein travels with the PDA; small cardiac vein with RCA. Anterior cardiac veins drain directly to the RA. Thebesian veins empty into all chambers.

Conduction system

The SA node sits at the SVC–right atrial junction at the superior end of the crista terminalis. Internodal myocardium carries the impulse to the AV node in the triangle of Koch. The bundle of His penetrates the central fibrous body at the membranous septum, then splits into right and left bundle branches. Purkinje fibers are large, glycogen-rich, pale myocytes with few contractile filaments—histology that matches their conduction job. SA-node arterial supply is RCA in ~60% and LCx in the remainder; AV-node supply is usually RCA.

Arteries

Ascending aorta gives the coronaries. The arch gives brachiocephalic trunk (right common carotid and right subclavian), left common carotid, and left subclavian. Descending thoracic aorta gives posterior intercostals (3–11), bronchial, esophageal, and superior phrenic branches, then becomes abdominal aorta at T12 (aortic hiatus with thoracic duct and azygos).

Unpaired abdominal viscera (foregut/midgut/hindgut pattern, reused in the digestive section):

Paired branches: middle suprarenal, renal (L1), gonadal, lumbar. Aorta bifurcates into common iliac arteries at L4 (about the iliac crests / umbilicus).

Celiac details: left gastric (lesser curve, esophageal branch—portosystemic anastomosis). Splenic (pancreatic branches, short gastrics, left gastro-omental; tortuous along the superior pancreatic border). Common hepatic → hepatic artery proper (with right gastric) and gastroduodenal (right gastro-omental and superior pancreaticoduodenal).

Upper limb: subclavian becomes axillary at the first rib, then brachial at the lower border of teres major. Pectoralis minor divides axillary artery into three parts (1 branch, 2 branches, 3 branches: superior thoracic; thoracoacromial and lateral thoracic; subscapular, anterior and posterior circumflex humeral). Brachial artery divides into radial and ulnar; ulnar gives the common interosseous. Superficial palmar arch is mainly ulnar; deep palmar arch is mainly radial.

Lower limb: external iliac becomes femoral under the inguinal ligament, gives profunda femoris (medial and lateral circumflex femoral; perforators), then popliteal after the adductor hiatus. Popliteal divides into anterior tibial (then dorsalis pedis) and the tibioperoneal trunk → posterior tibial and fibular. Posterior tibial divides into medial and lateral plantar arteries.

Veins

SVC forms from the union of the brachiocephalic veins and also receives the azygos arch. IVC forms from the common iliac veins at L5, receives renal and hepatic veins, and passes the diaphragm at T8.

Left renal vein is longer, crosses anterior to the aorta (and posterior to the SMA—nutcracker relation), and receives the left gonadal and left suprarenal veins. Right gonadal and right suprarenal drain directly to the IVC.

Azygos system

The azygos vein (typically from the right ascending lumbar and right subcostal veins, sometimes from the IVC) ascends on the right of the vertebral column, receives right posterior intercostals, hemiazygos, accessory hemiazygos, bronchial, and esophageal veins, then arches over the root of the right lung (about T4) to enter the SVC. Hemiazygos (left lower) usually crosses at T8–T9; accessory hemiazygos (left upper) crosses at about T7–T8. The system is the caval bypass if the IVC is obstructed and is the systemic side of esophageal varices.

Portal vein

The portal vein forms behind the neck of the pancreas from the superior mesenteric vein and the splenic vein. The inferior mesenteric vein typically joins the splenic vein (variable: sometimes the SMV or the confluence). Portal blood is nutrient-rich and deoxygenated; it supplies about 75% of hepatic blood flow. Inside the liver it travels with the hepatic artery and bile duct as the portal triad in each portal canal.

Portosystemic anastomoses (portal hypertension → varices):

Great saphenous vein (medial; travels with the saphenous nerve) empties into the femoral vein at the saphenous opening. Small saphenous vein (posterior calf; with the sural nerve) empties into the popliteal vein.

Lymphatic Vessels and Lymphoid Tissue

Lymph capillaries are blind-ended, lack tight endothelial junctions, and are tethered by anchoring filaments so tissue swelling opens them. Collecting vessels have valves. Lymph nodes have a cortex of B-cell follicles, a paracortex of T cells with high-endothelial venules, and a medulla of cords and sinuses. Afferents enter the capsule; a single efferent leaves the hilum.

Thoracic duct: cisterna chyli (about L1–L2, right of the aorta) → aortic hiatus → posterior mediastinum to the left of the esophagus → left venous angle (junction of left internal jugular and left subclavian veins). It drains everything except the right upper quadrant. The right lymphatic duct drains the right head/neck, right upper limb, and right thorax into the right venous angle.

Spleen

Intraperitoneal in the left upper quadrant, under ribs 9–11, axis along the tenth rib. Gastrosplenic ligament carries short gastrics and left gastro-omental vessels; splenorenal ligament carries the splenic vessels and the tail of the pancreas. Hilum faces the left kidney. Histology: white pulp = periarteriolar lymphoid sheaths (T cells) and B-cell follicles; red pulp = cords of Billroth and venous sinusoids that filter RBCs. Marginal zone sits between them. The spleen is the major site of opsonization of encapsulated organisms and of platelet storage.

Thymus

Anterior superior mediastinum, overlying the great vessels and upper pericardium. Cortex is densely packed developing T cells; medulla is paler and contains Hassall corpuscles (concentric type-VI epithelial cells). The organ involutes after puberty, replaced by fat, but remnants persist. Development: third pharyngeal pouch (with inferior parathyroids). Failure of pouch development is the thymic/parathyroid hypoplasia of 22q11 DiGeorge sequence—anatomy first, immunology later.

Development of the Heart and Great Vessels

The heart tube (cranial to caudal) is truncus arteriosus, bulbus cordis, primitive ventricle, primitive atrium, sinus venosus. It folds in a D-loop so the future RV moves right and anterior. Endocardial cushions split the AV canal into mitral and tricuspid orifices. Septum primum grows toward the cushions (foramen primum), then foramen secundum opens in septum primum; septum secundum forms the fossa ovalis rim and the foramen ovale flap. Neural-crest aorticopulmonary (spiral) septum divides truncus into aorta and pulmonary trunk; failure yields persistent truncus or transposition-spectrum defects.

Fetal circulation: oxygenated umbilical-vein blood → ductus venosus (liver bypass) → IVC → RA → foramen ovale → LA → LV → brain. SVC blood → RA → RV → PA → ductus arteriosus → descending aorta. Umbilical arteries (from internal iliacs) return to the placenta.

Remnants: umbilical vein → ligamentum teres hepatis; ductus venosus → ligamentum venosum; foramen ovale → fossa ovalis; ductus arteriosus → ligamentum arteriosum (left recurrent laryngeal nerve hooks under it); umbilical arteries → medial umbilical ligaments.

Aortic arches: 3 → carotids; 4 → left aortic arch and right subclavian; 6 → pulmonary arteries and left ductus arteriosus. Arches 1–2 largely regress (maxillary, hyoid/stapedial).

Histology of Vessels and Cardiac Muscle

Cardiac myocytes are branched, striated, with central nuclei and intercalated discs (fascia adherens, desmosomes, gap junctions). Capillaries are abundant. Purkinje fibers are paler and larger than working myocytes.

Sinusoidal capillaries (liver, spleen, bone marrow) have discontinuous endothelium and basement membrane—matching the filtering job of red pulp and hepatic plates. Fenestrated capillaries (endocrine glands, gut mucosa, glomerulus) have pores in endothelium.