The central organ of the cardiovascular system is the heart .
Functions of the heart :
1. Contractile (blood movement)
2. Endocrine (cardiomyocytes secrete cardion, atrial catriolytic factor).
General plan of the structure of the wall of the heart :
1. Endocardium – inner shell
2. Myocardium – middle shell
3. Epicardium – outer shell
There is also a pericardial sac, which is formed by the pericardium. There is a pericardial cavity filled with serous fluid
Embryonic histogenesis : the laying of the heart begins at 3 weeks of embryonic development. Sources of the layers of the heart wall:
1. Paired mesenchymal tubes, merging, form the endocardium.
2. The visceral sheet of the mesoderm forms the myoepicardial plate, from which the myocardium and epicardium are formed.
3. Neuroectoderm forms the nervous apparatus of the heart wall.
4. The parietal sheet of mesoderm forms the pericardium.
The structure of the heart membranes :
1. Endocardium (inner shell) – develops from the mesenchyme and represents 4 layers :
a. Endothelium lying on the basement membrane
b. Subendothelial layer (SRNST)
c. Muscular-elastic layer (smooth myocytes and elastic fibers)
d. Outer connective tissue layer (RVNST + single vessels)
Valves are duplicates of the endocardium. Valve leaflets do not have vessels. There are 2 atrial-ventricular valves and 2 semilunar valves. Fibrocartilaginous base (fibrous rings) are part of the fibrous framework (skeleton of the heart).
Frame functions :
a. Support function for wall structures
b. Cardiomyocytes are attached to the scaffold
c. Valves are attached to the frame
d. Required for valve prosthetics
Frame composition :
a. Four fibrous rings
b. Two fibrous triangles
c. Fibrous tissue of the intracardiac septum
2. Myocardium – a thick shell (10 times thicker than the endocardium and pericardium). The myocardium provides the main function of the heart.
Tissue and structural components of the myocardium :
a. Cardiac muscle tissue, represented by cardiomyocytes (contractible and conductive)
b. RVNST (not much)
c. Adipose tissue (not much in a healthy heart)
d. Nervous tissue (composition of the nervous apparatus)
e. Lots of capillaries and blood vessels.
Types of cardiomyocytes (not capable of mitosis, intracellular regeneration):
1. Typical : Contractile (working) (connected with the help of intercalated discs and anastomoses into a single morphofunctional complex – syncytium) with a set of structural devices:
a. Contractile (myofibrils)
b. Intracellular transport (membrane channels and tubes, T and L tubular systems, Ca ions)
c. Supporting (sarcolemma, telophragms, mesophragms, intercalary discs)
d. Trophic apparatus (core)
e. Nervous apparatus (nerve endings)
2. Atypical : 1) Secretory – localized in the atria. Peculiarities:
a. Reduced contractile apparatus (few myofibrils)
b. Organelles of protein synthesis are developed
c. There are many secretory granules with hormones in the cytoplasm
2) Conducting – cardiomyocytes of the conduction system of the heart. This system provides auto mode of operation (myocardial contractions).
Functions : 1) Autonomous, rhythmic generation of action potential
2) Distribution of biopotentials to working cells
Less differentiation than contractile cardiomyocytes (there is mitosis!)
The intracellular transport system is reduced, there are no T-tubules
Few myofibrils, they are not ordered in space
Few general purpose organelles (mitochondria, ribosomes)
When conducting cardiomyocytes are in contact, intercalated discs are not always formed
Types of conducting cardiomyocytes :
Pacemaker cells (p – cells) – pacemakers of the first order – produce biopotentials at a frequency of 60-90 per minute. These cells are located in the nodes of the conduction system of the heart.
Transitional cells – conduct biopotentials from type I pacemakers within the conducting system. They can generate their biopotentials at a frequency of 40-50 per minute. Normally, their impulsation is suppressed by the impulsation of type I pacemakers.
Purkinje cells – conduct impulses to cardiomyocytes. Among them: small Purkinje cells (intrinsic impulses with a frequency of 30-40 per minute, suppressed) and large Purkinje cells – pacemakers of the IV order (intrinsic impulses 20-30 per minute).
Scheme of the topography of the main structural elements of the conduction system of the heart:
Sinoatrial node: A) Capsule of connective tissue
B) There are many pacemaker cells in the center
B) transitional cells
D) blood vessels
Atrioventricular node: A) Connective tissue capsule
B) Transitional cells
C) Pacemaker cells are few
D) blood vessels
Interventricular bundle of His: A) Capsule of connective tissue
B) Small Purkinje cells
B) blood vessels
Bundle of His bundle (Purkinje fibers): Large Purkinje cells
Diffuse (silent) pacemakers (suppressed)
The epicardium and pericardium are typical serous membranes, thin, transparent, forming the pericardial cavity. From the side of the pericardium, they are completely lined with mesothelium (single-layer squamous epithelium). Under the basement membrane – RVNST + blood vessels + nervous apparatus. Fluid source: blood capillaries + secretory mesothelial cells.
Vascularization of the heart:
Coronary arteries (muscular type)
ICR vessels (many capillaries)
Coronary veins (low muscle type with well defined elastic membranes)
Innervation of the heart : carried out by the sympathetic and parasympathetic autonomic nervous system. The contraction of the heart is an automatic process, independent of innervation. Innervation is needed to regulate the mode of operation of the heart in different conditions.
Age-related changes in the heart:
Differentiation period (from birth to 16-20 years): working LV hypertrophy, partial RV atrophy
Stabilization period (20-30 years)
The period of involution (reverse development) (over 40 years): proliferation of the connective tissue of the myocardium, a decrease in the density of vascularization and innervation, a decrease in sympathetic innervation (sudden cardiac arrest).
Endocardium and epicardium – intracellular regeneration, mitosis.
Connective tissue – the action of fibroblasts
Myocardium – intracellular regeneration of cardiomyocytes
There is no reparative regeneration, defects are replaced by PST – a scar.
Working hypertrophy – restoration of myocardial function