HISTOLOGY FULL-TEXT

William A Beresford MA, D Phil ©
Professor of Anatomy
Anatomy Department, West Virginia University, Morgantown, USA

Chapter l5 CIRCULATORY SYSTEM

A GENERAL FUNCTIONS AND ASPECTS OF THE CIRCULATORY SYSTEM.

Fig. 6               |    GAS EXCHANGE    |
                     |      (Lungs)       |
                     |                    |
                     |                    |
                     |      PUMPING       |
                   /  --------------------- \  
                  /        (Heart)           \         
                 /                            \
                /                              \ 
 HORMONES  __  /                                \ __ FOOD & (Gut)                                           
(Endocrine    |                                  |   WATER
 Glands)      |                                  |
              |                                  |
              |                                  |   
   WATER      |                                  |
              |                                  |
WASTE & HEAT /\                                  |__ STORAGE &     
( Kidneys, Gut \                                 /   PROCESSING
 Lungs, Skin)   \                               /     (Liver)
                 \                             /
                  \                           /
      BLOOD CELLS  \ _______________________ /                       
           &       /          | |       ~     \ CLEANING (Spleen, Liver,
      ANTIBODIES              | |         ~                       Marrow)
      (Red Marrow             | |           ~           
       Lymphoid Organs)       | |             ~
                              | |               ~
                              | |              Lymphatic drainage
                              | |                   ~
                 DETERMINING CELL ENVIRONMENTS        ~
                                                        ~
        1. By forming special fluids,      2. Forming tissue fluids in
           e.g., CSF, aqueous                 extracellular space by
           humour, synovial fluid             more widespread diffusion
                                              and transport, serving, e.g.,
                                              CT, epithelia, muscles.     

l Closed system of tubes, through which blood is forced by the pumping action of the four-chambered, contractile heart.
2 Tubular walls are permeable so that exchange of materials can take place between the system of small blood vessels and their environment - cells, or tissue spaces.
3 Lymphatic system collects fluid and colloids and crystalloids from the tissue spaces and returns them to the bloodstream.
4 There is a balance whereby materials are lost, e.g., from kidneys, lungs, skin, and replenished by the intake of foodstuffs, air and water.
Vessels Powerpoint

B SYSTEMIC BLOOD VESSELS

l Blood capillaries
l Very numerous, anastomosing, delicate tubes of diameter 7-9 µm.
2 Total cross-sectional area of the capillary bed is very great, thus blood flows slowly under low pressure.
3 Wall is made up of curved, thin, plate-like endothelial cells lying on a BL and oriented with the tube's long axis.
4 Type l unfenestrated capillaries have complete endothelial cells, e.g., in muscle and skin: type ll capillaries have endothelial cells with fenestrations/pores through them (not between them), e.g., in kidney and choroid plexus.
5 Endothelial cells have serrated margins where they attach by glycocalyx and gap junctions to each other, and by occluding junctions where more of a barrier is needed, e.g., in the brain. Continuous capillaries have no gaps between the endothelial cells, in contrast to discontinuous capillaries.
6 Transport is controlled by the cells, with diffusion and facilitated transport for small molecules, and transcytotic vesicles or passage through the pores for larger materials.
7 Some capillaries have the occasional pericapillary cell - pericyte - imbedded within the BL, perhaps playing a contractile role.
8 Show transitions at both ends: to arterioles (by acquiring smooth muscle cells), or venules (by widening and taking on more collagen fibrils).
9 Endothelial cells secrete vasoconstrictor, vasodilator, and mitotic agents, and their own BL; they interact with blood, leucocytes and platelets, vary their permeability, and proliferate. Despite their lack of presence in routine light microscopy, they keep very busy, and are specialised for each organ that they serve.
10 Selectins are molecules expressed on the endothelial cells of small vessels, and on white blood cells. They bond intermittently with the sugars of a glycoprotein on the corresponding cell to cause the WBC to roll to a stop attached to the endothelium, before squeezing through the vessel wall into the connective tissues for defence. Sometimes the selectin is on endothelium, the ligand on the WBC, at other times the reverse achieves a similar result.
11 von Willebrand factor (vWF) also has a dual distribution, being present in Weibel-Palade storage granules of endothelial cells and alpha granules of platelets. Vascular injury releases vWF from endothelium to cause platelet activation, aggregation, binding to subendothelial collagen, and blood clotting - processes of haemostasis.

2 Sinusoids
l Have wider, more irregular lumens than capillaries.
2 Some of the lining cells are phagocytic.
3 Basal lamina may deficient or absent so that lining endothelial and phagocytic cells lie directly on reticular fibres and other cells, as in the liver.

3 Sinusoidal capillaries
l Have wide irregular lumens and a continuous, but fenestrated, non-phagocytic lining;
2 are the usual smallest vessel in endocrine tissue.

4 Arteries
l Have three main layers composed of several tissues:

Tunica intima
... (a) Endothelial lining on a BL
... (b) Subendothelial CT
... (c) Internal elastic lamina (fenestrated)

Tunica media
... (d) Smooth muscle cells (tightly spiralling or 'circular') 
... (e) Sparse reticular and elastic fibres

Tunica adventitia
... (f) External elastic lamina (interrupted)
... (g) Collagenous and elastic CT (mostly longitudinal)
2 Arterioles, less than 0.5 mm wide, have (a),(c),(d),(e) and (g) of the above.
3 Small and medium-sized arteries (muscular/distributing) have all elements.

4 Large arteries (elastic/conducting) differ significantly:

Tunica intima
... (a) Endothelium on a BL
... (b) Subendothelial CT
... (c) Innermost fenestrated elastic lamina

Tunica media
... (d) Many fenestrated elastic laminae interspersed with
... (e) smooth muscle cells and collagen fibres

Tunica adventitia
... (f) Collagenous CT with vessels and nerves
The larger arteries and veins have nutrient vessels and nerves (of vessels) in the adventitia - vasa vasorum and nervi vasorum.

In atherosclerosis, the arterial smooth muscle cell (SMC) changes its phenotype from static and contractile to proliferative, migratory, and synthetic. The converted SMC is further delinquent in invading the territory of the intima, where it lays down matrix and encourages the deposition of lipid, which, aside from narrowing the lumen, attracts platelets and macrophages. Their activation carries worse implications for blood flow, clotting, and deterioration of the vessel wall.

5 Veins
l Venules have an endothelial lining, BL and a collagenous outer sheath. Pericytes are numerous. The wall is thin enough to permit transport through it. White blood cells can squeeze between endothelial cells (transmigration/ diapedesis) and escape into the tissues. Lymphocytes may migrate actually through the interior of the endothelial cell.
[Emperipolesis is the migration of a cell into (and out of) another cell, while remaining intact: high endothelial cells, megakaryocytes, and thymic epithelio-reticular cells are hosts for such activity.]
2 Small veins acquire an additional thin media of smooth muscle and a thicker adventitia of collagen and elastic fibres.
3 No distinct elastic laminae are seen, but sparse elastic networks are found throughout the wall.
4 Many veins have valves - leaf-like projections of the intima, usually in a bicuspid form.
5 Large veins (e.g., vena cava) have bundled longitudinal smooth muscle in the CT adventitia and intima, whilst the media is thin or absent.

6 Comparison between a vein and its companion muscular artery
Both are tubes lined by endothelium and may contain RBCs.

Artery                          Vein
 (a) Shape less deformed       (a) Flattened
 (b) Thick wall                (b) Thin wall
 (c) Intima crinkled           (c) Intima smooth
 (d) Three distinct layers     (d) Layering indistinct
     (media prominent)             (media weak)
 (e) Internal elastic lamina   (e) No internal elastic lamina
7 Exceptional vascular structures
l Cerebral, retinal and osseous veins have no valves and no media. Veins in general are very variable in their structure.
2 Cerebral arteries are thin walled and have no external elastic layer.
3 Umbilical vein is very muscular; and the umbilical arteries have little elastic, and a media with distinct longitudinal and circular muscle layers.
4 Arterial intimal cushions are present in arteries to erectile tissue, kidneys, etc.
5 Some vessels have a high protruding endothelium, e.g., fetal stem arteries.

8 Exceptions to the vascular pattern of arteries, arterioles, capillary bed, venules, veins, heart

  1. Arteriovenous anastomoses - bypassing the bed (e.g., in the skin and gut) with thick muscle to close the bypass.
  2. Arterial anastomoses, e.g., circle of Willis to the brain.
  3. In the periphery, arteries and veins run together with nerves bound in CT as a neurovascular bundle. In the brain, arterial and venous distributions are separate.
  4. Vasa vasorum are blood vessels serving the adventitia and media of larger arteries and veins.
  5. Portal systems exist to the liver and pituitary gland, where venous blood drained from one organ is fed as a supply to the sinusoids or capillaries of another.
  6. Sinusoids may take the place of a capillary bed. Thus, for instance, sinusoidal capillaries permit blood to pass slowly by and influence chemosensitive epithelioid cells in the carotid body/glomus caroticum.
  7. Venous sinuses are endothelium-lined CT spaces where blood can collect for purposes other than metabolic exchange, thus, as part of the venous collecting system, e.g, coronary and dural sinuses, and in erectile tissue. (Caution: sinus is also a term for the pocket behind a venous valve - a site causing problems, when veins are grafted to substitute for arteries.)
9 Morphology in relation to physical factors in various vessels of the system
  1. Large elastic or conducting arteries - collagen fibres and elastic laminae predominate for strength, and elastic distensibility provides for elastic recoil during diastole thus damping the pulsatile flow resulting from the intermittent contractions of the heart. Endothelium provides a smooth lining to facilitate flow and prevent clotting.
  2. Muscular distributing arteries - lumens of a controllable size are narrowed by smooth muscular contraction to direct blood flow appropriately for the needs of various regions; mainly muscular media, strong CT adventitia with vasa vasorum, and autonomic nerve fibres to the muscle; elastic tissue limits distension of the lumen, and acts with the muscle.
  3. Arterioles - smooth muscle provides for a great reduction, by vasoconstriction, of the blood flow to a region; they maintain an adequate arterial pressure, but reduce blood pressure to an acceptable level for:
  4. Capillaries - pressure is low so can be thin-walled to permit exchange of gases, minerals, carbohydrates and small molecules + water by: The wall serves to keep back in the capillary most of the colloidal proteins of the plasma; the presence of these then encourages the return of fluids at the venous end of the capillary.
  5. Veins - low, even pressure so have large lumens, thin collagenous walls, and valves to prevent backflow; larger veins acquire some muscle, circular and longitudinal, in the media and adventitia. In general, elastic is not needed for recoil, for variations in pressure between diastole and systole are insignificant, but the vena cava has significant numbers of elastic fibres.

C HEART

1 Thick-walled, hollow, muscular pumping, and endocrine, organ.

2 Heart structures

       |                                                |
Systemic Veins                                    Pulmonary Veins
       |        Coronary sinus                          |
       |       /                                        |
       |      /                                         |
 RIGHT ATRIUM/                                     LEFT ATRIUM
       |                 . . . . . . . .  .             |
Tricuspid Valve . . . .  . Annuli fibrosi . . . . . Mitral Valve
       |                 .                .             |
       |                 . Trigona fibrosa.             |
       |                 .                .             |
       |                 .                .             |
RIGHT VENTRICLE          .    Septum      .        LEFT VENTRICLE
       |                 .  membranaceum  .             |
       |                 .                .             |
Pulmonary Valve . . . .  . Annuli fibrosi . . . .  Aortic Valve
       |                 . . . . . . . .  .             |
       |                         .                      |
Pulmonary artery                 .                    Aorta
       |                         .                      |
                         [Cardiac Skeleton]

Fig. 7 Heart structures.

3 Heart wall`s three layers

  1. Endocardium (innermost)
  2. Myocardium
  3. Epicardium (visceral pericardium) and subepicardium
  4. Pericardium (parietal)
    CT membrane of fibres supporting a mesothelium. This faces the epicardium across the pericardial cavity containing a small amount of lubricating fluid.
4 Cardiac skeleton of dense fibrous CT, with a tendency to turn into fibrocartilage. Elements are listed above in Fig. 7.

5 Heart valves
1 Atrio-ventricular valves
... (a) Leaflets are covered with endothelium on a
... (b) core of dense CT fused to the supporting annulus.
... (c) Cords of CT (chordae tendineae) connect the valve to
... (d) the papillary muscles in the ventricular wall.
2 Semi-lunar valves
... (a) Deploy three leaflets.
... (b) Thinner than the atrio-ventricular valves.
... (c) Lack chordae tendineae.
... (d) Fibrous core enlarges to the nodule of Arantius at the free margin.

6 Impulse-conducting system (coordinates myocardial contractions)
l Sino-atrial node of thin, modified, cardiac muscle fibres, influenced by parasympathetic (ganglionic neurons are found in the heart) and sympathetic autonomic nerve fibres, initiates contraction (pacemaker).
2 Contraction spreads through the atrial myocardium to the
3 atrio-ventricular node (Tawara's) consisting of a tangled plexus of modified cardiac fibres in the medial wall of the right atrium.
4 These fibres enlarge into Purkinje fibres and continue through the septal CT as the bundle of His, which then branches.
5 Purkinje fibres are rich in sarcoplasm and glycogen, but poor in myofilaments. They lack T-tubules, and are connected by intermediary transitional cells with ordinary myocardial fibres, whose contraction they can thus evoke in many regions of the ventricles.
7 In ungulates, Purkinje fibres are very large, pale and easily recognized: in man, the system is less obvious.

7 Endocrine role of heart
Atrial myocytes synthesize atrial natriuretic factor (ANF), which relaxes blood vessels and increases the excretion of sodium and water by the kidney. ANF is thus a partial counterweight to the renin-angiotensin system.

D LYMPHATIC VESSELS

l Lymphatic capillaries
l Network of blindly ending or anastomosing tubes, 5-50 µm wide.
2 The wall is made of an endothelial tube, with a discontinuous basal lamina and fine anchoring fibrils.
3 The wall permits the capillary to collect water, solutes and macromolecules from the tissue spaces.
4 Capillaries (i.e., a lymphatic drainage) are absent from the CNS, bone marrow, eye, and parts of the spleen.

2 Collecting vessels
l Lymph passes from capillaries into larger lymphatic vessels with very thin walls of endothelium, basal lamina and collagen, and numerous valves.
2 Lymph is led to small protective ovoid bodies - lymph nodes - through whose tissues it must filter before going further.
3 Lymph collects in the thoracic duct before entering the circulating blood at the left innominate vein; the right lymphatic duct also collects lymph for return to the bloodstream.

4 Thoracic duct

3 Lymph
l Adds to the blood proteins leaked from blood capillaries, new and recirculated lymphocytes, and antibodies, fat droplets (chylomicrons), etc.
2 Fat is collected from the gut in blind lymphatic capillaries lying centrally in intestinal villi. The fat-whitened lymph (chyle) gives these vessels a milky colour, hence their name lacteal.

4 Oedema and its causes
Oedema is an excessive accumulation of tissue fluid, involving mainly the extracellular space (except in CNS), and making the tissue swollen and puffy. It is caused by:

  1. Venous obstruction, e.g., from cardiac incompetence, which
    .. (a) raises intracapillary hydrostatic pressure, thus forcing more fluid into the tissues;
    .. (b) reduces the volume of blood collected from the capillaries.
  2. Injured capillary walls, e.g., from heat, become permeable permitting greater egress of fluid, solutes and colloids.
  3. Resulting reduction in intracapillary colloid at the venous end of the blood capillary lessens the osmotic attraction for tissue fluid to come back into the capillary.
  4. Lowering of systemic plasma colloids (proteins), from
    .. (a) proteinuria (excretion of protein in the urine),
    .. (b) protein starvation, or
    .. (c) exudation from burnt skin surfaces,
    will likewise reduce the osmotic return of extracellular fluid to capillary blood.
  5. Obstruction of lymphatic vessels receiving the lymph drained from tissue fluid by lymph capillaries, for instance, blockage by metastatic cancer cells. The tropical filaria parasites often block the lymphatics of their host causing gross swelling (elephantiasis) of affected extremities.

E VESSEL DEVELOPMENT

l Blood and lymphatic vessels (except sinusoids) form initially as simple endothelial tubes developed from mesenchymal cells - angioblasts.
2 Larger vessels and systems start independently of one another.
3 Their tunics with muscle and CT are added from mesenchymal condensations around the endothelium.
4 Capillaries of the adult can multiply or regenerate by extending cords of endothelial cells, which arrange themselves into a tubule. Cords can fuse with one another to build an anastomosing network.
5 Various cytokines promote or inhibit angiogenesis, e.g., vascular endothelial growth factor (VEGF).
William A Beresford, Anatomy Department, School of Medicine, West Virginia University, Morgantown, WV 26506-9128, USA - - e-mail: -- wberesfo@wvu.edu -- wberesfo@hotmail.com -- beresfo@wvnvm.wvnet.edu -- fax: 304-293-8159