The Circulatory System

• System consists of blood and lymphatic vascular systems.

   

  Blood Vascular System:

  • The heart -> pumps blood
  • A series of efferent vessels (E = Exit heart)
  • A diffuse network of thin tubules (capillaries)
  • The Afferent vessels (A = Arrive at heart)
• Lymphatic System

  Begins in dead-end tubules (lymphatic capillaries), which gradually anastomose in vessels of steadily incr. size and terminate in the blood vascular system, emptying into large veins near the heart.
• Lymphatic System Function:

  To return the fluid of the tissue spaces to the blood, which on penetrating the lymphatic capillaries, contributes to the liquid part of the lymph.

  By passing through the lymphoid organs, contributes to circulation of lymphocytes and other immunologic factors

  Entire circulatory system is internally lined by simple squamous epithelium = Endothelium
• General Structure

  All blood vessels with lumen above a certain diameter present a no. of structural features in common and are usually composed of:

  1. Tunica intima
  2. Tunica media - chiefly smooth muscle cells - elastin, proteoglycan are intertwined
  3. Tunica adventitia (external)
• Capillaries

  Single layer of endothelial cells, rolled up in the form of a tube

  Avg. 7-9um

  Grouped into 3 types based on structure of endothelial cell walls:

  • Continuous capillary
  • Fenestrated or perforated capillary - presence of pores through the walls of the endothelial cells
  • Sinusoidal capillary:
  • A tortuous path and greatly enlarged diameter (30-40um) which slows the blood circulation
  • Absence of continuous endothelial lining in walls
  • Presence in and around wall of cells with phagocytic activity
  • Absence of a continuous basal lamina
• Arteries classified according to size

  • Arterioles
  • Arteries of med size or muscular arteries
  • Large or elastic arteries, where elastic tissue predominates
• Arterioles


  • Fine, <0.5mm in diameter and have relatively narrow lumens
  • Tunica intima with no sub-endothelial layer and generally lack an internal elastic membrane
  • Media is muscular, composed of 1-5 layers of smooth muscle cells
  • Adventitia is narrow, poorly developed and shows no external elastic limiting membrane
• Small to Medium Arteries (Muscular Arteries)

  • Thick muscular layer with up to 40 layers of smooth muscle cells
  • These cells intermixed with variable amounts of elastin, as well as collagen and proteoglycans
• Larger Arteries (Elastic Arteries)


  • Yellowish colour due to accumulation of elastin in the media
  • The intima, thicker than the tunic of a muscular artery, is lined by endothelial cells
  • The media, consists of a series of concentrically arranged perforated elastic membranes
  • The adventitia does not show an external limiting membrane, is relatively underdeveloped and contains elastic and collagen fibres
• Veins


  • Venules
  • Veins of small, medium and large size
• Venules


  • Small with a diameter of 0.2-1mm
  • Intima composed of endothelium
  • Thin media that may consist of 0 to a few layers of smooth muscle
  • Adventitial layer - thickest and composed of connective tissue
• Small - Medium Sized Veins


  • With exception to main trunks, most veins are small or medium with a diameter of 1-9mm
  • Intima - thin sub-endothelium layer (can sometimes be absent)
  • Media - small bundles of smooth muscle cells intermixed with collagen fibres and a delicate network of elastic fibres
  • Collagenous adventitial layer is well developed
  • Exhibit valves in their interior. These structures consist of 2 semilunar folds of the internal layer of the vessel that projects into the lumen
  • Composed of elastic connective tissue and lined on both sides by endothelium
  • Valves are numerous in the veins of the limbs. Propel the venous blood in direction of heart, thanks to contractions of skeletal muscles that surround the vein
• Large Veins


  • Well developed intima
  • Media - much smaller, few smooth muscle cells and abundant connective tissue
  • Adventitia is thickest layer - largest vessels may contain longitudinal bundles of smooth muscle
• Arteriosclerosis

  Thickening and loss of elasticity of arterial walls.

  3 patterns:

  • Atherosclerosis
  • Medial Calcific Sclerosis
  • Arteriosclerosis
• Atherosclerosis

  • Intimal lesions called atheromas/atheromatous/fatty plaques
  • They protrude into and obstruct vascular lumens and weaken the vessel
• Aneurysm

  • Localised abnormal dilation of a blood vessel or wall of heart
  • 2 types
  • True
  • False
• True Aneurysm

  Aneurysm is bounded by arterial wall components or the attenuated wall of the heart
• False Aneurysm

  A breach in the vascular wall leading to an extravascular haematoma that freely communicates with the intravascular space
• Vascular Tumours

  Tumours of the blood and lymphatic vessels

  Primary tumours of the large vessels are extremely rare - most are connective tissue sarcomas

  • Spectrum from benign
  • Intermediate lesions (locally aggressive but rarely metastasis)
  • Highly malignant
  • Aorta
  • Pulmonary artery
  • Vena cava
  • Haemangiomas
  • Lymphangioma
  • Glomus tumours
  • Kaposi sarcoma
  • Haemangioendothelioma
  • Angiosarcoma
  • Haemangiopericytoma

   
• Blood

  A specialised connective tissue consisting of cells and an abundant extracellular interstitium

  If blood is removed from the system, it will clot. The clot contains formed elements and a clear yellow liquid - serum. Blood serum is same as plasma except it lacks fibrinogen and other protein factors required for the clot to form and contains inc. levels of serotonin

  The formed elements of blood make up about 43% of blood volume. Remaining 57% is plasma
• Erythrocytes

  Normal erythrocytes have no nuclei and are 7um in diameter.

  >9 = macrocytes, <6um = microcytes

  Contain haemoglobin - oxygen carrying protein

  Normal concentration in blood is ~ 4.5-5.0 million/uL

   
• Leukocytes

  Classified into:

  • Granulocytes
  • Agranulocytes

  Granulocytes have irregularly shaped nuclei and cytoplasm contains specific granules - neutrophils, eosinophils and basophils

  Agranulocytes have nuclei with a regular shape, the cytoplasm doesn't contain granules. Depending on appearance of nuclei, and cytoplasmic staining characteristics, the agranulocytes can be classified as lymphocytes or monocytes
• Platelets

  Aka thrombocytes

  Enucleated, disc-like cell fragments 2-5um in diameter

  Originate from budding of giant multinucleated megakaryocytes found in BM