Cardiovascular System Histology Flashcards

Cardiovascular System Histology

Overview

  • Background and functions of the Cardiovascular System (CVS)
  • The Heart:
    • Endocardium
    • Myocardium
    • Epicardium
  • Vessels:
    • Arteries (elastic, muscular, arterioles)
    • Capillaries (continuous, fenestrated, discontinuous/sinusoids)
    • Venules & Veins
    • Lymphatics
  • CVS = Cardiovascular System

Background

  • The circulatory system consists of:
    • Blood
    • A central pump (the heart)
    • Blood distribution (arterial) and collection (venous) networks
    • A system for exchange of nutrients and waste products between blood and extravascular tissue (microcirculation).
  • A network of vessels (lymphatics) parallels the veins, draining fluid from extravascular spaces into the blood vascular system.

Functions of the Cardiovascular System

  • Maintenance of adequate blood flow (cardiac output)
  • Delivery of oxygen, nutrients, hormones, electrolytes, and water to peripheral tissues
  • Removal of carbon dioxide (CO2)(CO_2) and other metabolic waste products
  • Maintenance of normal thermoregulation
  • Maintenance of normal glomerular filtration rate (GFR) and urine output

Heart

  • The right heart propels unoxygenated blood through the pulmonary circulation.
  • The left heart propels oxygenated blood through the systemic circulation.
  • RA = Right atrium, RV = right ventricle, LA = left atrium, LV = left ventricle
  • First organ to form in the embryo.
  • Mammalian and bird hearts have 4 chambers:
    • Left & right atria (LA, RA)
    • Left & right ventricles (LV, RV)
  • The heart is composed of 3 layers (from inside to outside):
    1. Endocardium (includes valves)
    2. Myocardium (heart muscle)
    3. Epicardium (visceral pericardium)
Endocardium
  • Forms the inner lining and the valves.
  • Equivalent to the tunica intima of blood vessels (BV).
  • Endothelial cells on the innermost surface are in direct contact with blood.
  • Important in hemostasis.
  • 3 layers:
    1. Endothelium
    2. Basal lamina (connective tissue)
    3. Subendocardium (subendocardial layer): subendothelial connective tissue; also contains part of the conductive system and Purkinje fibers.
  • Purkinje fibers are found in the subendocardium (subendocardial layer).
  • Modified cardiomyocytes that function in impulse conduction.
  • Larger than cardiac muscle cells.
  • Fewer myofibrils, located in the periphery.
  • Lots of glycogen and mitochondria.
  • No T-tubules.
  • Connected by desmosomes and gap junctions.
  • Intercalated discs are variable in appearance and number depending on their location compared to myocardiocytes.
  • Specialized conducting fibers extend from the interventricular septum to the papillary muscles and up the lateral walls of the ventricles.
  • Two nodes generate the impulses for myocardial contraction. Both are small masses of specialized cardiac muscle fibers and associated connective tissue, supplied by nerve fibers from the autonomic nervous system that speed up and slow down the heart rate.
    • First impulses are generated by the sinoatrial node (SA) (in the wall of the right atrium) - sending a wave of depolarization around the atria via gap junctions between the muscle fibers.
    • Next the atrioventricular node (AV) (located in the interatrial septum) starts impulse generation around the ventricles.
    • Impulses are sent from it to the AV bundle (bundle of His), which branches to form Purkinje fibers, which lie in the deepest layer of the endocardium and supply the papillary muscles
    • The apex of the heart contracts 1st then the papillary muscles then the wave of depolarization spreads up the walls of the ventricles from the apex upwards
  • 4 dense bands of fibrous connective tissue encircle the base of the pulmonary trunk, aorta, and the AV valves – provides structural support to the heart.
  • A triangular mass of fibrous connective tissue – the fibrous trigon, connects the aortic ring and the L and R atrioventricular rings.
  • This area undergoes osseous differentiation and forms the “Os Cordis” – primarily seen in cattle.
  • The fibrous skeleton of the heart consists of masses of dense connective tissue in the endocardium which anchors the valves and surrounds the two atrioventricular canals, maintaining their proper shape.
Myocardium
  • Left ventricular myocardial thickness is ~2 to 4 times thicker than the right in adults due to higher pressure on the left side.
  • Involuntary striated muscle.
  • Cross-striated, arranged in sarcomeres
  • Central single nucleus
  • Sarcoplasmic reticulum
  • Many mitochondria (up to 20% cell volume, requires a lot of $O_2$)
  • Low regeneration capacity (mitosis), close to zero once the heart is fully developed.
  • Branched fibers connect via intercalated discs.
  • Intercalated disks:
    • Gap junctions
    • Anchoring junctions
  • Lipofuscin (wear and tear)
Epicardium and Pericardium
  • Outer surface of the heart = visceral pericardium.
  • Surface is covered by mesothelium (simple squamous epithelium), a thin layer of dense connective tissue, and a variably thick layer of adipose tissue with blood vessels (coronary arteries and veins).
  • Is contiguous with the endocardium at the level of the endocardial cushion.
  • Single layer of flattened epithelial cells → the mesothelium (simple squamous epithelium)
  • Supported by connective tissue, including fat.
  • Similar mesothelial layer lines the opposing parietal surface of the pericardial sac.
  • Mesothelial cells secrete a small amount of serous fluid that lubricates the movement of the epicardium on the opposite parietal pericardium.
  • The epicardium represents the visceral layer of the pericardial sac.

Heart Structure Summary

  • ENDOCARDIUM: Endothelial lining of the heart chambers surface and covers the surface of the valves. The subendocardium contains a thin layer of connective tissue and the Purkinje fibers.
  • MYOCARDIUM: Cardiac muscle mass – involuntary striated muscle.
  • EPICARDIUM: Single layer of flattened epithelial cells the mesothelium (simple squamous epithelium) supported by connective tissue, including fat. Similar mesothelial layer lines the opposing parietal surface of the pericardial sac. Mesothelial cells secrete a small amount of serous fluid that lubricates the movement of the epicardium on the opposite parietal pericardium. The epicardium represents the visceral layer of the pericardial sac.

Vessels

  • Arteries
    • Elastic arteries
    • Muscular arteries
    • Arterioles
  • Capillaries
    • Continuous
    • Fenestrated
    • Discontinuous /sinusoids
  • Veins
    • Venules
  • Lymphatic vessels
Tunics of Vessels
  • TUNICA INTIMA
    • Innermost layer
    • Endothelium, internal elastic membrane, subendothelial connective tissue
  • TUNICA MEDIA
    • Middle layer
    • Smooth muscle and elastic lamellae/fibers
  • TUNICA ADVENTITIA / EXTERNA
    • Outermost layer
    • Collagen, may contain blood vessels, nerves, capillaries
  • VASA VASORUM: Small blood vessels that supply nutrients and oxygen to the tunica adventitia (externa) of large arteries and veins.
  • NERVI VASORUM: Vascular nerves which innervate arteries and veins. These nerves control vasodilation and vasoconstriction, regulating temperature and homeostasis.
Vascular Endothelium
  • Role in hemostasis
  • Modulates perfusion
  • Role in inflammation
  • Hemostasis is a physiological response to vascular damage, providing mechanisms to seal an injured vessel to prevent blood loss. It is the result of a complex and well-regulated process which maintains blood as a flowing fluid within the cardiovascular system.
Arteries
  • Tunica intima: Endothelium + subendothelial layer + internal elastic lamina
  • Tunica media: Smooth muscle cells producing elastic, reticular, and collagenous fibers
  • Tunica adventitia: Loose connective tissue, blood vessels (vasa vasorum), lymphatics, and nerves (nervi vasorum)
Elastic Artery (e.g., Aorta)
  • Tunica intima: Endothelium + loose connective tissue
  • Tunica media: Largely repeating ELASTIC LAMELLAE
  • Tunica adventitia
  • Location: Closer to the heart.
  • Structure: Contain more elastic tissue
  • Function: Use elastic recoil to store energy from the heart's contraction.
  • Examples: Aorta, pulmonary arteries, brachiocephalic artery, carotid arteries, subclavian arteries, iliac arteries.
Muscular Arteries (e.g., Femoral Artery)
  • Tunica intima: Endothelium + loose connective tissue
  • Tunica media: Thickest tunic, primarily SMOOTH MUSCLE
  • Tunica adventitia: Contains collagen, elastin fibers, and nerves.
  • Generally, they have a round appearance.
  • Structure: Contain more smooth muscle.
  • Function: Muscular arteries contract and expand to control blood flow.
  • Examples: Brachial artery, radial artery, femoral artery, splenic artery.
  • Tunica Media
    • Smooth muscle cells are circumferentially arranged within the tunica media
    • Regulates DIAMETER and TONE (vasodilation/vasoconstriction)
    • Vascular smooth muscle regulates blood flow
Arterioles
  • 1-3 layers of smooth muscle cells
  • Greatest effect on blood pressure
  • Nuclei bulge into the lumen
  • Round appearance of vessel
  • No internal elastic membrane in the smallest arterioles with one smooth muscle cell
  • Metarteriole is a terminal vessel with precapillary sphincters that regulate flow to the capillary bed
Capillaries
  • Thin-walled tubules of mesenchymal origin; in cross-section, made of only one endothelial cell rolled into a tube.
  • Represent the site of exchange between blood and surrounding tissue.
  • Diameter is 7–9 µm, length 0.25 – 1mm; in some organs (adrenal cortex, kidney medulla) can be up to 5-10 cm long.
  • Composed of simple squamous cells which roll to produce a tube.
Pericytes (Rouget Cells)
  • Mesenchymal-like cells that wrap around the endothelium of capillaries and venules.
  • Communicate with endothelial cells by physical contact and paracrine signaling.
  • Help to maintain homeostatic and hemostatic functions in the brain and also sustain the blood–brain barrier.
  • Proliferate after injury.
  • Stem cell source.
  • Important in angiogenesis (new vessel formation).
Classification of Capillaries
  1. Continuous: Most common. Found in muscle, brain, bone, lung, etc. Examples include the blood-brain and blood-testis barriers.
  2. Fenestrated (gaps 10-100 nm): In tissues with substantial fluid exchange, e.g., intestinal villi, choroid plexus, ciliary process, glomerular capillaries.
  3. Discontinuous (Sinusoidal): Hepatic and splenic sinusoids, allowing larger molecules to exit (e.g., RBCs in the spleen).
Continuous Capillaries
  • Consist of a tube of endothelium.
  • Features include:
    1. Nucleus of endothelial cell
    2. Pinocytotic vesicles
    3. Tight junctions
    4. Basement membrane/lamina
Fenestrated Capillaries
  • Homogenous pores around the membrane.
  • Found in the renal corpuscle (glomerulus), a tightly coiled network responsible for the filtration of plasma.
Podocytes
  • Lining the Bowman's capsules (nephrons).
  • Help prevent proteins and other large molecules from being filtered.
  • Their foot processes (pedicels) extend and wrap around the capillaries of the glomerulus to form the filtration slits.
  • The pedicels increase the area of the cells enabling efficient ultrafiltration.
Discontinuous Capillaries (Sinusoids)
  • Lumen is enlarged and irregular; diameter 20 - 40 μm.
  • Lining endothelium is discontinuous and fenestrated.
  • Basal lamina may be absent (discontinuous).
  • Found in bone marrow.
Venules
  • Called POSTCAPILLARY VENULES
  • Very ‘leaky’ vessels
  • NO SMOOTH MUSCLE
  • LEUKOCYTE diapedesis possible here
  • 5mm Hg pressure in vessel.
  • Smallest blood vessel in the venous system, blood pressure is very low
Veins
  • Valves are projections into the lumen of the tunica intima; covered by endothelial cells and have a core of elastic fibers.
  • Larger veins can have nervous vasorum & vasa vasorum
  • A distinct internal elastic lamina is not seen
  • The muscular tunica media of veins is thinner than that of arteries; smooth muscle fibers have an irregular, approximately circular orientation.
Vein vs. Artery
  • Large, wide lumen, thin walls in comparison to same-size arteries.
  • Valves present.
  • Thin tunica media.
  • The tunica adventitia is the thickest tunic.
  • Large veins may have vasa vasorum and nervi vasorum too.
  • May be collapsed in histological sections.
Lymphatic Vessels
  • Complementary to the cardiovascular system
  • Transport lymph throughout the lymphatic system
  • Valves are present
  • Freely anastomose with one another
  • Lined by endothelial cells
  • Thin layer of smooth muscles
  • Adventitia that binds the lymph vessels to the surrounding tissue
  • Very thin wall, very low pressure, may contain valves.
  • No RBCs in lymph, they seem “empty” or clear.