Gingival Micro Circulation: Nature of Vasculature

Introduction

Branching network of micro-vessels where transfer and trading of nutrients, waste products, hormones, heat, respiratory gases and water takes place between the blood and body tissues is termed as the microcirculation. These micro-vessels comprise of four components - arterioles, metarterioles, capillaries and venules.¹ Cardiovascular system ultimately ends in the microcirculation. Important function of microcirculation includes transport of oxygen from the capillary red blood cells (RBCs) to the parenchymal cells. Here, oxygen is delivered to meet the energy requirements of the tissue cells in collaboration with their functional activity. Apart from this, micro-vessels of the microcirculation also contribute to peripheral vascular resistance.²

Parameters that affect microcirculation³

• Patient’s body temperature

• Systemic blood pressure 

• Psychological stress

• Nutrition

• Intake of medicinal drugs

• Habit of consuming tobacco

• Mental activity

• Physical activity

• Age

• Diabetes

• Arterial insufficiency (microcirculatory reserve)

Structure of the Microcirculation and Capillary System

• Micro-circulatory vessels lined entirely by endothelial cells consists of pores and fenestration. They are tightly held together by numerous molecules like cadherins and gap junctions, allowing upstream electrical communication between the endothelial cells.²

• On the luminal aspect of the endothelium, an important sub-cellular structure is seen known as the Glycocalyx, which is a gel like structure and is around 0.2 - 0.5 µm in dimension. It majorly consists of the following three components: plasma protein, proteoglycans and glycosaminoglycans. Glycocalyx plays a vital role in various processes such as homeostasis, immunological function and solute transport.²

• Pericytes being the smooth muscle cells of the microvasculature participate directly or indirectly in maximum functions related to the microcirculation. They control diameter of the micro-vessels and are involved in maintaining microvascular hemodynamic resistance, pressure, and luminal fluid flow.⁴

• The microcirculation begins when a chief artery enters a tissue or organ which divides into first-order arterioles. (Figure 1)

• Second-order arterioles divergently divide from first-order arterioles, and this process continues for few more generations (dependent upon the type of the tissue), ending into terminal arterioles from which capillaries arise. 

• Second-order arterioles through terminal arterioles have a single smooth muscle cellular layer.

• Each terminal arteriole gives rise to 1 to 20 capillaries, which are endothelial cell tubes, 5 - 7 μm in diameter, 100 - 1000 μm long, surrounded by a basement membrane and lacks a smooth muscle cell layer.

• Two or more capillaries converge to form postcapillary venules, which further joins to form collecting venules. Post–capillary venules, in general, do not have a smooth muscle cell layer, but are covered with pericytes.

• Arterial side of microcirculation consists of vessels called as the arterioles. Arterioles are both innervated as well as surrounded by smooth muscle cells measuring 10 – 100 μm in diameter.

• Arterioles further carry the blood to the capillaries. Capillaries are neither innervated nor surrounded by smooth muscle cells. Capillaries are about 5 – 8 μm in diameter.

• Blood in the capillaries flows out and enter into the venules. Venules are surrounded by smooth muscle cells and are 10 – 200 μm in diameter.

• The blood eventually flows out from the venules into the veins.⁴

• Metarterioles are the connecting links between arterioles and capillaries.¹

• Thoroughfare channel is a tributary to the venules.¹

• Pre-capillary, capillary, and post-capillary network are the three major parts of the microcirculation.¹

• In the pre-capillary sector, arterioles, and pre-capillary sphincters participate.¹

Functions^1,2,5

• To supply oxygen as well as nutrients to the tissues and removal of carbon dioxide from the tissues.

• Regulating both tissue perfusion as well as blood flow, thus maintaining blood pressure and responses to inflammation in the form of oedema / swelling.

• Maintains exchange of solute between intra-vascular and interstitial tissue spaces.

• Delivery and transportation of all blood borne hormones and nutrients to various tissues.

• Mediates functional activity of immune system and is involved in maintaining homeostasis.

• Exchange and delivery of nutrients, heat, respiratory gases, water, hormone and waste products between the body tissues and blood.

• Leukocytes and lymphocytes present in the blood of microvasculature are carried towards the target organs. Here, trafficking of both immune as well as inflammatory cells takes place between blood and tissues.

• Microcirculation is also involved in maintaining peripheral vascular resistance, regulation of blood pressure and vascular capacitance.

• Regulates blood flow to and within the body tissues and organs.

Mechanism of capillary blood flow – Vasomotion⁶

• The blood flow in the capillaries is intermittent instead of continuous, that turns on and off alternatively.

• This intermittent blood flow is due to contraction of the pre-capillary sphincters and meta-arterioles. The reason for this intermittent phenomenon is termed as vasomotion.

Methods to estimate capillary pressure^6,7

Capillary exchange (Starling Equilibrium)^6,8

• E. H. Starling pointed that under normal conditions, a state of near equilibrium exists at the capillary membrane, whereby the amount of fluid filtering outward from some capillaries equals almost exactly the quantity of fluid that is returned to the circulation by absorption through other capillaries.

Vascular distribution of gingiva^9-12

• One of the highly vascularized structures of human gingiva is the lamina propria.

• Primary blood supply to maxillary gingiva is derived from the third part of maxillary artery (Pterygopalatine).

• Posterior superior alveolar artery supplies the buccal gingiva (from first premolar to third molar), infraorbital artery supplies the labial gingiva (from central incisor to canine) and the greater palatine artery along with the lesser palatine artery supplies the palatal gingiva (from central incisor to third molar) of maxilla.

• Primary blood supply to mandible is derived from the first part of maxillary artery (mandibular).

• Inferior dental artery supplies the buccal gingiva (from first premolar to third molar), mental and incisive branches of the inferior alveolar artery supply the labial gingiva (from central incisor to canine), and lingual artery supplies the lingual gingiva (from central incisor to third molar) of mandible.

• Branches of the above arteries reach the gingiva from the following sites: Supraperiosteal arterioles, vessels of the periodontal ligament, arterioles which emerges from the interdental septa and crest. (Figure 2, 3)

• The gingiva relies for its blood supply primarily on the supraperiosteal vessels.

• Venous vessels are assumed to follow the course taken by the arteries and arterioles.

• Each of the papillary projections / component of the gingival connective tissue carries a terminal capillary loop, with an ascending arterial limb and descending venous limb. (Figure 4)

• Vascular network located lateral to and beneath the smooth junctional epithelial collar is like a thin vascular basket, rich in anastomosis. This is termed as the gingival plexus / dento–gingival plexus. (Figure 5, 6)

• Dento–gingival plexus consists mainly of post capillary venules and extends from the coronal to apical termination of junctional epithelium. (Figure 5, 6)

• In healthy gingiva, there are no capillary loops in the gingival plexus.

• Number of gingival plexuses increase with age.

Maxillary and mandibular microcirculatory blood flow

Baab et al., (1986) investigated blood flow patterns in attached gingiva and free gingiva, interdental papillae, and alveolar mucosa using Laser Doppler Flowmetry (LDF).¹³

There was a significant difference in the blood flow rate in the maxillary anterior gingiva. In the mandibular anterior gingiva, there was significant difference in the blood flow rates of the alveolar mucosa as compared to the other sites. The gingival blood flow was the lowest in free gingiva and it was highest in the alveolar mucosa. There was significant difference in the blood flow rate in the maxillary anterior gingiva as compared to the blood flow rates in the mandibular anterior gingiva (the interdental gingiva, attached gingiva, and alveolar mucosa).¹⁴

Different types of tissue showed different blood flow patterns. Both heat and cold produced initial hyperaemia, which then returned rapidly towards the baseline. It has been proved that the average blood flow velocity through the larger vessels of the gingiva and periodontal ligament is faster than the blood flow rate of red blood cells in the dental pulp.14 Better healing efficiency is seen in maxillary anterior gingiva because of the better blood flow found in the maxillary anterior gingiva.14 Blood flow patterns in disease Baab et al., in their study concluded that, patients suffering from either juvenile periodontitis or from rapidly progressive periodontitis showed faster recovery of blood flow rates when compared to healthy control subjects when subjected to gingival cooling. Wounds or tissue injuries such as plaque-induced gingivitis need sufficient blood supply to heal them. Wounds or injuries with poor blood supply heal more slowly than those with adequate blood supply, e.g., facial wounds heal faster than those in less well vascularized areas, such as the feet. Slow wound healing is often associated with old age where delays are caused by impaired circulation.^13,14

Aging and microcirculatory dynamics in human gingiva

Systolic blood pressure is higher in older people as compared to young and middle-aged people. Diastolic blood pressure is higher in middle aged and older people compared to the young people. Volume of blood flow in the marginal gingiva, as indicated by laser doppler flowmetry, increased slightly with age, whereas laser doppler blood velocity decreased with age. Tissue blood flow assessed by laser doppler flowmetry was variable with age.¹³

Maxillary and mandibular gingival blood flow

When compared among maxilla and mandible, gingival blood flow in maxillary anterior gingiva was more than the mandibular anterior gingiva. The difference in blood flow values between the mandibular anterior gingiva and the maxillary anterior gingiva was due to higher bone density in the mandible as compared to that in the maxilla. The free gingiva showed lowest blood flow rates, while the alveolar mucosa showed the highest blood flow rates.¹⁴

Various conditions of changes in gingival capillary blood flow¹⁵

• Heating: There is vasodilation by stimulation of temperature receptors.

• Cooling: Initial vasoconstriction; if temperature goes less than 15°C, vasodilation with increased blood flow occurs.

• Occlusal forces: There is decreased blood flow from the gingival margin.

Effect of smoking on gingival microvasculature

• A study by Kumar V et al., (2011) evaluated the vascular and epithelial changes in gingiva of smokers and non-smokers with chronic periodontitis. Authors found that there was reduced vascular density and smaller lumen area in patients who smoked as well as increased epithelial thickness as compared to non-smokers. However, these changes were not statistically significant.¹⁶ Similar findings were noted in the previous studies. Authors demonstrated decreased bleeding on probing and reduced inflammatory response among smokers, which was due to alterations in gingival microvasculature as well as gingival epithelium.¹⁷

Conclusion

This review paper highlights the structure of microcirculation, its functions, vascular distribution of gingiva, differences in the maxillary and mandibular blood flow rates, aging and microcirculatory dynamics in human gingiva, effects of smoking and changes in gingival microcirculation from health through gingivitis to periodontitis.

Conflicts of Interest

Nil