HISTOLOGY FULL-TEXT

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.     

B SYSTEMIC BLOOD VESSELS

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)

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

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

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.)

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:
   • (a) diffusion or carrier-mediated transport through and between endothelial cells;
   • (b) active transcytotic transport through endothelial cells; then passage through the basal lamina.
   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

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]

3 Heart wall`s three layers

1. Endocardium (innermost)
   • (a) Lined by endothelium on a basal lamina.
   • (b) Subendothelial layer of collagenous and elastic fibres, fibroblasts and some smooth muscle cells.
   • (c) Subendocardial layer of CT with blood and lymphatic vessels, nerve fibres, and Purkinje fibres of the heart's conducting system. A layer worth calling a subendocardium is not everywhere present.
2. Myocardium
   • (a) Cardiac muscle fibres, bundled and wound in spiralling sheets, thickest in the left ventricle, thinnest in the atria.
   • (b) Blood vessels and lymphatics and fine CT.
3. Epicardium (visceral pericardium) and subepicardium
   • (a) Outer mesothelial sheet and BL on
   • (b) loose subepicardial CT of fat cells and collagen fibres with
   • (c) blood vessels (coronary), lymphatics and nerves to the heart nodes.
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.

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

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

• (a) Intima of endothelium, BL, CT, some longitudinal smooth muscle and an elastic lamina.
• (b) Thick media of mixed longitudinal and circular smooth muscle.
• (c) Thin adventitia of collagen and a little longitudinal smooth muscle, vasa vasorum and nerve fibres.
• (d) A valve is at the venous exit.

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