Cardiovascular System
David Sturgeon in Introduction to Anatomy and Physiology for Healthcare Students, 2018
The final type of blood vessels are capillaries (from the Latin word capillaris for ‘hair-like’). These microscopic tubes consist of a single layer of endothelial cells through which plasma and other small molecules can pass. There is no tunica media or adventitia and the diameter of each capillary is only slightly larger than that of a single erythrocyte (remember how they can stack themselves like dinner plates to avoid blockage when travelling through these narrow vessels). Capillaries connect small arteries (arterioles) to small veins (venules) and the capillary network (or bed) between the two is the site of nutrient and waste exchange between the blood and interstitial fluid that surrounds the cells (Figure 7.3). The flow of blood through the capillary bed is known as microcirculation and takes place in two types of capillary. The first type of capillary is known as a metarteriole (or vascular shunt) and directly connects the arteriole to the opposing venule. True capillaries, on the other hand, branch from metarterioles and provide nutrient and waste product exchange between the plasma and interstitial fluid. The entrance to each true capillary is protected by a band of smooth muscle called a precapillary sphincter. This contracts or relaxes in order to control the flow of blood through the capillary bed. For example, when precapillary sphincters contract (close), blood flows directly through the metarteriole and bypasses the tissue. This occurs during vigorous exercise when blood is re-routed from the gastrointestinal tract to skeletal muscle in order to prioritise blood supply to the latter.
Hyperspectral image analysis for subcutaneous veins localization
Ahmad Fadzil Mohamad Hani, Dileep Kumar in Optical Imaging for Biomedical and Clinical Applications, 2017
These are the smallest, thin-walled blood vessels that are connected to the smallest arteries and the smallest veins. The capillaries are responsible for the exchange of nutrients, oxygen, waste products and electrolytes between tissues and the blood [4]. The oxygenated blood carried by arteries reaches the tissue's level through capillaries and the de-oxygenated blood is distributed to the veins, which are responsible to carry it towards the heart. The blood flow is slower in capillaries and this provides sufficient time diffusion for the exchange of materials between blood and tissues. Capillaries have dense interconnected network called capillary bed throughout the body. An estimated length of capillaries bed is about 60,000–100,000 miles long in an average human adult [5].
The Dermal Microvascular Unit: Relationship to Immunological Processes and Dermal Dendrocytes
Brian J. Nickoloff in Dermal Immune System, 2019
The microanatomical features and the 3-D organization of the microcirculation described above clarify how the microvasculature participates in a variety of physiologic and pathologic states. The resistance vessels of the skin involved in reactive vasoconstriction and vasodilation are represented by the arterioles of the lower plexus and the ascending segments with their immediate branches that give rise to the upper plexus. The precapillary sphincter is placed at the distal end of these branches, thereby controlling tissue perfusion through the dermal capillary beds and the capillary loops in the dermal papillae. The upper plexus, which is situated 2 mm below the skin surface, is the major site of thermoregulation. The resistance vessels and the precapillary sphincters regulate the volume of blood in the upper plexus from which heat is radiated. The smooth muscle sphincters of the terminal arterioles appear to be the sites of the pulsatile vasomotor activity detected by laser Doppler flowmetry.10,15 This vasomotion facilitates tissue perfusion through the capillary beds.
The Association of Acute Cerebrospinal Fluid Pressure Reduction with Choroidal Thickness
Published in Current Eye Research, 2021
Xiangxiang Liu, Mohamed M. Khodeiry, Danting Lin, Yunxiao Sun, Caixia Lin, Wei Feng, Jing Li, Yaxing Wang, Qing Zhang, Kai Cao, Jiawei Wang, Ningli Wang
To the best of our knowledge, no studies have examined subfoveal choroidal thickness after CSFP reduction. A previous population-based study found that SFCT was associated with CSFP.14 These results consist of the present data. Additionally, we extended previous findings suggesting a significant association between CSFP and the ratio of SMVL thickness to total choroidal thickness. Histologically, choroidal blood drains into the intracranial cavernous sinus through the vortex veins and the superior venous plexus, and these anatomical features indicate that CSFP may influence the choroidal thickness.1 The choriocapillaris is a specialized capillary bed with the greatest density of capillaries that arises from the arterioles in Sattler’s layer.1,15 In addition, the extravascular tissue in the medium vessel layer contains collagen and elastic fibres, etc.16 These structural features make the small to medium vessel layer more flexible than other regions in response to pressure changes, indicating that the SMVL is more sensitive to pressure alterations. Furthermore, the primary function of the choroid, especially the choriocapillaris, is supplying oxygen and nutrients to the retina, so attenuation of the SMVL may be correlated with decreased choroidal circulation and may produce a relatively ischemic environment, leading to retinal dysfunction.
Recognition and management of idiopathic systemic capillary leak syndrome: an evidence-based review
Published in Expert Review of Cardiovascular Therapy, 2018
Noor Ul-Ain Baloch, Marvi Bikak, Abdul Rehman, Omar Rahman
At a molecular level, capillaries are the relay between arterial and venous circulations. Flow into the capillary beds is controlled by periarteriolar muscular sphincters, which contract and relax serving as a valve to forward flow. Moreover, exchange of particles at the molecular level is governed by the differences in hydrostatic and oncotic pressures of the capillary and interstitium (Starling equation). A delicate balance of chemical mediators is responsible for maintaining normal flow to the capillary bed and ensuring normal exchange of substances at the molecular level without promoting capillary leak. Patients with SCLS have unexplained episodes of capillary leakage [15] that may or may not be preceded by a clear precipitant. In the cohort of Kapoor et al. (2010), 56% of patients reported a flu-like illness prior to the development of hypotension and shock [9]. In a few recent reports, influenza A virus infection was implicated as a possible precipitant of capillary leakage in patients with SCLS [16,17]. In the published literature, reports of capillary leakage following administration of general anesthesia [18], gemcitabine [19], trastuzumab [20], and filgrastim [21] have been described.
Antibiotic exposure at the site of infection: principles and assessment of tissue penetration
Published in Expert Review of Clinical Pharmacology, 2019
Nynke G. L. Jager, Reinier M. van Hest, Jeffrey Lipman, Jason A. Roberts, Menino O. Cotta
Pulmonary infection begins within the inner part of the lungs, i.e. the airspace, and thus antibiotic concentrations within the airspace are of direct importance. The inner part of the lungs is covered by a thin aqueous layer, which is called epithelial lining fluid (ELF). ELF is a natural barrier against pathogens, but also a medium in which the pathogens multiply before entering the lungs. It has been considered the likely site of extracellular respiratory infection [17]. Before an antibiotic agent enters the ELF, it has to pass the blood-alveolar barrier. This barrier, between the systemic circulation and the airspace, is composed of two membranes; the capillary wall and the alveolar wall separated by a compartment filled with interstitial fluid. The capillary wall contains fenestrations and allows free diffusion of small molecules from the bloodstream to the interstitial fluid. The alveolar wall, which separates the interstitial fluid from the inner part of the lungs, has no fenestrations to facilitate diffusion. Therefore, the antibiotic must pass through the alveolar epithelial cells themselves in order to reach the ELF [18]. Consequently, lipophilic agents, such as fluoroquinolones and macrolides, experience higher distribution into ELF, where hydrophilic agents such as aminoglycosides and glycopeptides, experience low distribution into ELF.