Development and general structural plan of blood vessels
The first blood vessels form at 2–3 weeks of embryogenesis in the mesenchyme of extraembryonic organs—the yolk sac wall and chorion. In the so-called blood islands, peripheral cells flatten and differentiate into endothelium, while central cells round up and give rise to blood cells. Pericytes, fibroblasts, smooth muscle cells, and adventitial cells develop from the surrounding mesenchyme.
The cardiovascular system integrates the heart, blood vessels, and lymphatic vessels, and ensures the transport of blood, lymph, gases, nutrients, metabolic products, and biologically active molecules. Blood vessels include arteries, arterioles, capillaries, venules, veins, and arteriovenous anastomoses. A special variant is portal systems (miraculous nets): capillary networks between two vessels of the same type (artery–artery in the kidney, vein–vein in the liver).
The walls of arteries and veins share a common structural plan consisting of three layers: tunica intima, tunica media, and tunica externa (adventitia).
Arteries: classification and microstructure
Based on the structure of the tunica media, arteries are divided into three types:
- Elastic type—aorta and pulmonary artery. Blood flow velocity is 0.5–1.3 m/s, pressure is high (approx. 120 mm Hg). They perform primarily a transport function.
- Muscular type—most medium and small caliber arteries of the body, limbs, and internal organs. They provide propulsion and regulation of blood inflow to organs.
- Musculoelastic (mixed) type—common carotid and subclavian arteries.
In elastic-type arteries, the intima includes endothelium on a basal membrane, a subendothelial layer of loose connective tissue with stellate cells, and a plexus of elastic fibers. The matrix is rich in glycosaminoglycans and phospholipids, which regulate permeability. The tunica media contains numerous fenestrated elastic membranes with smooth muscle cells between them; aortic myocytes contain vimentin and have lower contractility, which increases resistance to high pressure. The tunica externa is loose fibrous connective tissue with thick collagen and elastic fibers, vasa vasorum, and nervi vasorum.
In muscular-type arteries, the internal and external elastic membranes are well-defined, and the tunica media consists of smooth myocytes arranged in a gentle spiral with thin layers of loose connective tissue. The mixed type combines smooth muscle cells and spirally oriented elastic membranes.
Microcirculatory bed
The vessels of the microcirculatory bed include arterioles, capillaries, venules, and arteriovenous anastomoses. They regulate blood filling, transcapillary exchange, and perform drainage-depository functions.
Arterioles—the smallest muscular-type arteries. All three tunics are preserved but thinned: the media contains 1–2 layers of smooth myocytes, and the basal and internal elastic membranes contain perforations that allow contact between endotheliocytes and myocytes.
Capillaries form networks, loops, and glomeruli. Their wall consists of endothelium on a basal membrane, pericytes within its clefts, and sparse adventitial cells with thin collagen fibers. They are classified into:
- Somatic capillaries with continuous endothelium (heart, skeletal muscles, lungs);
- Fenestrated capillaries with pores spanned by a diaphragm (endocrine glands, renal glomeruli);
- Perforated (sinusoidal) capillaries with through-holes lacking a diaphragm and with a discontinuous basal membrane (liver, hematopoietic organs).
The luminal surface of endotheliocytes is covered with glycoproteins and exhibits athrombogenic (negative charge, prostacyclin) and barrier functions. Endotheliocytes synthesize thromboplastin, thromboxane, and factors regulating coagulation and permeability. Pericytes regulate the lumen, while adventitial cells are pluripotent precursors of fibroblasts, osteoblasts, and adipocytes.
Arteriovenous anastomoses are divided into true (shunts) and atypical (semishunts) and provide blood redirection bypassing the capillaries.
Veins
Veins are characterized by poorly developed elastic elements, greater distensibility, and a tendency to collapse. Elastic membranes are absent; elastic fibers are few and arranged longitudinally. The number of smooth muscle cells depends on the direction of blood flow: myocytes are significantly more abundant in the veins of the lower extremities than, for example, in the internal jugular vein.
They are classified into:
- Venules—postcapillary (with numerous pericytes), collecting, and muscular;
- Non-muscular (fibrous) veins;
- Muscular veins with weak, moderate, or strong development of myocytes.
Development and heart tunics
Heart morphogenesis begins at the beginning of the 3rd week as bilateral paired clusters of cardiogenic mesenchyme located between the visceral leaves of the splanchnopleuric mesoderm and the endoderm. They form two endothelial tubes that give rise to the endocardium. Adjacent areas of the visceral leaves form the myoepicardial plates, from which cardiomyocytes and the epicardium develop. As the tubes approach each other, a single unpaired heart tube is formed.
Endocardium is structured similarly to a blood vessel: endothelium on a basal membrane, a subendothelial layer, a musculoelastic layer, and a subendocardial layer of loose connective tissue. The endocardium forms valves—thin fibrous plates covered with endothelium, devoid of blood vessels. The left atrioventricular valve is bicuspid (mitral), and the right is tricuspid. The atrial side of the cusps is smooth, while the ventricular side features projections for the attachment of chordae tendineae.
Epicardium—the visceral layer of the pericardium: a thin connective tissue plate tightly fused with the myocardium and covered by mesothelium. It contains four layers of collagen-elastic fibers.
Myocardium and conduction system
Myocardium is formed by striated muscle cells—cardiomyocytes—connected by intercalated discs. Intercellular spaces contain layers of loose connective tissue with vessels and nerves. There are four types of cardiomyocytes: contractile, secretory, pacemaker, and conductive.
- Contractile cardiomyocytes are cylindrical, rectangular in longitudinal sections, and covered by a sarcolemma; the basal membrane is rich in glycoproteins that bind Ca²⁺, which is necessary for contraction.
- Secretory cardiomyocytes are located predominantly in the right atrium and auricles. They are smaller, branched in shape, with a well-developed rough endoplasmic reticulum and Golgi apparatus; their granules contain natriuretic peptide. The T-tubule system is poorly developed.
- Pacemaker cells of the sinoatrial node center are small and polygonal, capable of spontaneous depolarization. Transitional cardiomyocytes lie at the periphery of the node—they are more elongated, with better-developed myofibrils.
- Conductive cardiomyocytes of the bundle of His, its branches, and Purkinje fibers are large, with a sparse network of myofibrils and eccentrically located nuclei; they are surrounded by loose connective tissue.
The conduction system includes the sinoatrial node, atrioventricular node, bundle of His, its right and left branches, and Purkinje fibers, ensuring automaticity and coordinated impulse propagation.
Key points
- Blood vessels develop from the mesenchyme of blood islands during the 2–3rd weeks of embryogenesis.
- The wall of arteries and veins consists of the tunica intima, media, and externa with vasa vasorum and nervi vasorum.
- Arteries are classified into elastic (aorta, pulmonary), muscular, and musculoelastic (carotid, subclavian).
- Capillaries can be somatic, fenestrated, and perforated (sinusoidal).
- Pericytes regulate microvessel lumen, while endotheliocytes control coagulation and permeability.
- Veins feature a weak elastic framework and longitudinal muscle bundles to facilitate venous return.
- The heart has three tunics: endocardium, myocardium, and epicardium—the visceral layer of the pericardium.
- Cardiomyocytes are divided into contractile, secretory (natriuretic peptide), pacemaker, and conductive.