Histology of the cardiovascular system

Blood

Blood is composed of 45% cells such as erythrocytes, leukocytes and platelets, and 55% blood plasma. Blood plasma consists mainly of water and contains proteins, antibodies, electrolytes and other substances.

Erythrocytes, also known as red blood cells, transport oxygen throughout the body. Leukocytes, or white blood cells, play a role in the immune system, while thrombocytes, also known as platelets, are responsible for blood clotting. Other tasks of the blood are the transport of nutrients, hormones and heat as well as maintaining the acid-base balance.

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Bone marrow

Bone marrow is one of the primary lymphatic organs and is located in the internal cavities of bones. It is divided into red and yellow bone marrow. Blood formation actively takes place in the red bone marrow, which is why it appears red due to the abundance of erythrocytes. In addition to hematopoietic cells and macrophages, there are also reticular cells that synthesize the reticular connective tissue of the extracellular matrix.

In adults, yellow bone marrow predominates, which consists mainly of fat cells and therefore has a yellow appearance. There is no longer any active blood formation taking place here.

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Blood vessels

Blood vessels are hollow organs that transport blood throughout the body and supply it with oxygen and nutrients. The inner cavity in which the blood flows is called the lumen and is typically surrounded by one or more layers, depending on the type of blood vessel.

The wall of larger vessels such as arteries and veins consists of three layers. The innermost layer is the tunica intima, which is made up of endothelial cells and a subendothelial connective tissue layer. The middle layer, the tunica media, consists of smooth muscle cells and partly elastic and collagenous connective tissue. Arteries have a more pronounced tunica media which gives them less compliance than veins. The outermost tunica externa or adventitia is a layer of connective tissue that anchors the vessel in its surroundings.

Small vessels such as arterioles, venules and capillaries all contribute to microcirculation. These vessels have thinner wall layers, which, for example, in capillaries only consist of a layer of endothelial cells, the basal lamina and pericytes.

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Heart

The heart is a hollow muscular organ with three layers. The endocardium lies inside and also forms the atrioventricular and semilunar valves. It consists of single-layered squamous epithelium (endothelium).

The myocardium, the cardiac musculature, is the thickest layer of the heart wall. It consists of striated muscles, connective tissue, blood vessels and nerve fibers. The heart muscle cells (cardiomyocytes) are connected via adhesive contacts and gap junctions (shiny stripes) and form a functional syncytium. The myocardium also contains specialized cardiomyocytes, which form the conduction system of the heart.

The epicardium envelops the heart and corresponds to the visceral sheet of the serous layer of the pericardium.

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Related articles

Articles within this topic:

• Arteriole
• Atherosclerosis
• Blood
• Blood vessel histology
• Bone marrow histology
• Capillaries
• Cells of the blood
• Erythrocytes
• Heart histology
• Hematopoiesis: Definition, embryology and cell lines
• Struggling with histology? Try these histology slide quizzes