Introduction

A significant function of the cardiovascular system is to deliver oxygen and nutrients to tissues via arteries and their branches. Subsequently, the venous system receives carbon dioxide and other waste products expelled by the tissues. A vital transition point exists between the arterioles and the venules known as the capillary, which is ultimately where the exchange occurs. In short, capillaries are thin-walled vessels that allow for the transportation of nutrients and metabolites from the vasculature and into the interstitium to be taken up by cells. Capillaries are ubiquitously organized throughout the human body and exert their function in every tissue. This article strives to illustrate the anatomical characteristics of the capillary and its function in meeting the metabolic demands of tissues and demonstrate features that are evident upon histological staining.[1]

Structure

Before exploring the anatomy of the capillary, it is first reasonable to better understand the anatomy of the arteriole and venule that directly surround it. Arterioles vary from 8 to 60 micrometers in diameter and may even further subdivide into meta-arterioles.[2] All arteries possess a basic structure consisting of three layers: a tunica intima, tunica media, and tunica adventitia.[3] In addition to carrying metabolites to the capillaries, arterioles also play a significant role in maintaining peripheral vascular resistance mediated by input from the autonomic nervous system.[2] On the other side of the capillary, venules are thin-walled vessels mainly composed of endothelial cells. They function in receiving capillary blood containing tissue waste products and destining them to be processed by other organs. Also, post-capillary venules facilitate leukocyte diapedesis and plasma protein leakage during times of inflammation. While it has been shown that venules may also partake in fluid and nutrient exchange, the capillary primarily assumes the task.[4]

Upon arriving at a tissue, arterioles branch into capillary beds that provide the means of gas and nutrient exchange. Capillaries are thin-walled vessels composed of a single layer of simple squamous epithelium, a basement membrane known as tunica intima, and scattered connective tissue cells called pericytes. The bloodflow into the capillaries is primarily controlled by precapillary sphincters (smooth muscle bands that wrap around metarterioles).[5]

There are a few different structural types of capillaries that research has identified in the human body. One subtype is known as continuous non-fenestrated capillaries and is present in the skin, lungs, and blood-brain barrier. They are connected via cellular junctions, contain a basement membrane, and lack fenestrations (pores) in the plasma membrane. Continuous, fenestrated capillaries, found in intestinal villi and endocrine glands, have a similar structure however contain diaphragmed fenestra in the membrane. Discontinuous capillaries, found in the liver, have large gaps and an incomplete basement membrane.[6] The structure of the capillary in a given organ ultimately determines its permeability to solutes and the extent of exchange that can occur. Furthermore, arterioles can also dictate exchange in the capillary through the regulation of bloodflow. Arteriolar contraction decreases flow to the capillary, whereas arteriolar dilation will increase flow. Arterioles can also shunt blood via metarterioles to post-capillary venules, completely bypassing the capillary unit altogether.[4] For example, in cold weather situations, vasoconstriction and physiologic shunting occur in skin arterioles to minimize heat loss and supply blood to vital organs such as the brain, heart, and lungs.[7] In summary, the capillary has a unique anatomical relationship with arterioles and venules that ultimately governs its vascular function.

Function

As previously stated, the primary goal of the capillary is to allow for the exchange of fluid and metabolites between the capillary and tissue interstitium. The capillary will simultaneously provide nutrients from the arteriole and carry away waste products of cellular metabolism expelled by the tissues. However, anatomical considerations and intrinsic forces of the capillary govern the mechanism by which exchange occurs. In general, most organs in the human body, such as the heart, lungs, skeletal muscle, and GI tract, contain continuous, non-fenestrated capillaries. These allow water and solutes smaller than 3 nm to pass freely via simple diffusion. Conversely, molecules over 3 nm pass through selectively, often with the help of a transporter.[4]

There are intra-capillary forces that also dictate molecular movement, known as Starling forces. The difference in hydrostatic and oncotic pressures between the capillary and the tissue interstitium is the primary determinant of these forces.[8] Capillary hydrostatic pressure is the pressure exerted by the fluid on the capillary endothelium. The oncotic pressure the osmotic pressure that proteins and other colloids exert on fluid. The mechanism by which fluids and nutrients diffuse across the capillary membrane involves a combination of hydrostatic and oncotic pressures that vary along the length of the capillary.[9]

As arteriolar blood first enters the capillary, the hydrostatic pressure is higher than that of the interstitium, thus favoring flux of fluid into the interstitial space. Capillary blood has a high concentration of albumin, which cannot diffuse through the capillary membrane; this exerts a strong oncotic pressure favoring nutrient flux back into the capillary. However, the hydrostatic pressure is greater than the oncotic pressure, which causes fluid and nutrients to diffuse into the interstitial space. As blood moves along the capillary bed, capillary hydrostatic pressure starts to decrease since the fluid is leaving the vasculature, and oncotic pressure decreases slightly as proteins that are small enough may filter through the capillary. Ultimately, hydrostatic pressure drops more significantly, and the net oncotic pressure prevails, causing fluid and waste products to diffuse from the interstitium back into the capillary to be carried away by venules.[4] Excess fluid in the interstitium may be absorbed by lymphatics to be returned later to the venous system. Note that specific patient characteristics such as blood pressure, venous or lymphatic insufficiency, and body protein content can impact the capillary forces, thus affecting exchange.

Tissue Preparation

Tissue preparation for the capillaries requires several steps owing to the size of the capillaries. It requires several filtration steps needed to separate the capillaries, red blood cells, and cell debris. Starting with a 300 um mesh, which would filter the suspension. Capillaries pass through the mesh, while larger blood vessels can not. The mesh is then carefully washed with 50 mL of 1% BSA (Bovine serum albumin). Mesh should be discarded. Then a 30 um cell strain filter is used to separate the capillaries from the red blood cells. The filtrate from the last step should be distributed over five 30 um cell strain filters. This time the capillaries do not pass, whereas the red blood cells pass through the filter. The filters are washed with 25 mL 1% BSA. After this, pour all the filtrates over another filter. Wash each one with 50 mL of 1% BSA. The cell strain filters containing the capillaries should be kept. Finally, the filters are turned upside down and washed with 50 mL 1% BSA for each filter into 50 mL tubes. Pressure is applied gently with the pipet tip of a 5 ml pipettor and moved across the filter to wash off the capillaries. After the capillaries are collected, they are centrifuged and then seen under a microscope (100X magnification).[10]

Microscopy, Light

The capillary anatomy described above can be better appreciated with the assistance of histological staining and microscopic analysis. In review, the basic structure of a capillary a single layer of simple squamous epithelium, a basement membrane, and few pericytes. One method used to appreciate the features is with Hematoxylin and Eosin (H&E) tissue staining and a light microscope. The Hematoxylin component binds nucleic acids found in the cell nucleus and reflects a basophilic color. On the other hand, the eosin component binds to proteins primarily present in the cytoplasm and extracellular matrix and reflects an eosinophilic color.[11] An exemplary H&E stain would portray a thin endothelial cell layer with basophilic nuclei and eosinophilic cytoplasm and sometimes a red blood cell filling the capillary lumen.

Microscopy, Electron

Another mode of molecular analysis that takes a closer look at the capillary structure is known as transmission electron micrography (TEM). This functions by staining a particular tissue and passing a short wavelength electronic beam through the sample, which becomes absorbed and produces a contrast image appreciated with an electron microscope. In addition to the structures highlighted in the H&E stain, TEM can even detect small transcytosis vesicles in the capillary lumen that aid in transporting large molecules across the capillary endothelium.[12] While other histological techniques are usable, H&E and TEM are two methods commonly employed to evaluate the molecular structure and function of capillaries.

Pathophysiology

The capillary unit possesses a vital role in allowing the exchange of metabolites between the vascular system and tissues. Capillaries have a unique morphology, consisting of only a single layer of endothelium, enabling them to carry out their function in delivering needed nutrients to organs and tissues. Furthermore, despite the thin diameter of capillaries, they are the most numerous of the vessel types and thus possess the highest total surface area enhancing overall exchange.[9] This fact gives an excellent example of the common theme in biology that structure meets function. However, there can also be associated with dysfunction, as capillaries can be involved in many different disease processes. For instance, during septic shock, the immune system responds to bacterial toxins by releasing cytokines and inflammatory mediators such as histamine and nitric oxide. This reaction ultimately leads to vasodilatation and increased capillary permeability, causing leakage of large proteins and fluid into the tissue interstitium. Patients may develop hypotension refractory to fluid resuscitation due to the profound decrease in peripheral vascular resistance and tissue third-spacing caused by the inflammation.[13] All in all, the capillary unit is an imperative structure for maintaining the viability of living tissues. Furthermore, the assistance of histological technologies has aided in a deeper understanding of the anatomy and function of capillaries.

Clinical Significance

• Some diseases are related directly to the capillaries and can present with specific signs and symptoms. They can be either hereditary or nonhereditary.
• The hereditary diseases include (HHT) which also known as Osler–Weber–Rendu syndrome. It is an autosomal dominant disorder with abnormal blood vessel formation in the mucous membranes, skin, and some organs, including the liver, lungs, and brain.[14]
• Benign nevus or spider angioma are solitary or multiple benign vascular lesions present in 10 to 15% of healthy adults and children.[15]
• Systemic capillary leak syndrome is a disease that causes leakage of fluid and proteins out of tiny blood vessels into surrounding tissues. It presents as hypotension, hypoalbuminemia, and hemoconcentration. Elevated white cell count may also be present.
• A capillary hemangioma is also known as a strawberry birthmark. It is a benign tumor showing abnormal overgrowth of tiny blood vessels. Capillary hemangiomas sometimes appear within the first six months of life and may not be present at the birth.[16]
• A capillary angiosarcoma is a rare malignant tumor involving the capillaries. It is a cancer of the endothelial cells that line the walls of blood vessels. Only a few reports exist in the literature regarding angiosarcomas occurring on capillary malformations.[17]