Copyright EMAP Publishing 2018 This article is not for distribution Clinical Practice Keywords Arteries/Veins/Capillaries/ Blood flow/Fluid movement Systems of life This article has been Vascular system double-blind peer reviewed In this article... ● Anatomy of the vascular system and structure of blood vessels ● The process of gas exchange and nutrient transfer between capillaries and tissues ● Physiological processes used by the body to regulate blood pressure Vascular system 1: anatomy and physiology Key points Author Selina Jarvis is a research nurse and former Mary Seacole development The vasculature scholar at Kingston and St George’s University of London and King’s Health Partners, works with the Guy’s and St Thomas’ Foundation Trust. heart to supply the body with oxygen Abstract The vasculature is a network of blood vessels connecting the heart with and nutrients and all other organs and tissues in the body. Arteries and arterioles bring oxygen-rich to remove waste blood and nutrients from the heart to the organs and tissues, while venules and products veins carry deoxygenated blood back to the heart. The exchange of gases and transfer of nutrients between blood and tissues take place in the capillaries. A solid There are five understanding of how the vasculature works is key to understanding what can go classes of blood wrong with it. This first article in a three-part series covers anatomy and physiology; vessels: arteries, parts 2 and 3 will discuss the pathophysiology of the vascular system. arterioles, veins, venules and Citation Jarvis S (2018) Vascular system 1: anatomy and physiology. Nursing Times capillaries [online]; 114: 4, 40-44. Capillaries allow the diffusion of gases he body requires oxygen and and arterioles. Veins are described as ‘con- and transfer of nutrients and needs to eliminate verging’ or ‘joining’ vessels, as venules and nutrients and waste waste products to maintain meta- veins join to return blood to the heart products between Tbolic stability. The vascular through the largest veins (such as the supe- blood and tissues system has a crucial role in bringing oxygen rior and inferior venae cavae) (Marieb and and nutrients to every organ and tissue, and Hoehn, 2015). Capillaries are in intimate con- Blood flow and removing waste products, via a series of tact with the tissues, providing nutrients blood pressure blood vessels. In conjunction with the heart, and removing waste products through their are regulated by which acts as a pump, it forms the cardio- thin walls at a cellular level. Table 1 details nervous, chemical vascular system (Jarvis and Saman, 2018). the functions of the five blood vessel types. and hormonal Arteries leaving the heart with oxygenated mechanisms blood provide oxygen, nutrients, hormones Structure of blood vessels and other substances throughout the body. Blood vessels, except the smallest ones, Some organs and Veins leaving the organs and tissues return are made of three layers: the tunica tissues can to the heart carrying metabolic waste. interna, tunica media and tunica externa automatically adjust (or adventitia). their own blood flow Five classes of blood vessels There are five classes of blood vessels: Tunica interna arteries and arterioles (the arterial system), The tunica interna (innermost layer) is a veins and venules (the venous system), and single layer of squamous (flat) epithelial capillaries (the smallest bloods vessels, cells called the endothelium; this smooth linking arterioles and venules through net- lining in direct contact with the blood works within organs and tissues) (Fig 1). offers little resistance to blood flow (Marieb Arteries are described as ‘branching’ or and Hoehn, 2015). The endothelial cells can ‘bifurcating’ vessels, as great arteries (such easily be damaged by hypertension, toxins as the aorta) branch off into smaller arteries such as cigarette smoke, or hyperglycaemia; Nursing Times [online] April 2018 / Vol 114 Issue 4 40 www.nursingtimes.net Copyright EMAP Publishing 2018 This article is not for distribution Clinical Practice Systems of life this damage can result in atherosclerosis. Fig 1. These delicate cells rest on a thin layer of The five types of blood vessels connective tissue made of elastin and col- lagen (elastic and structural support fibres) that anchors the tunica interna to the tunica media. The endothelium regulates blood flow and prevents clotting; it produces chemicals such as nitric oxide that help reg- Veins Arteries ulate blood flow by relaxing the smooth muscle within blood vessels. Tunica media The tunica media (middle layer) takes up most of the arterial vessel wall and is com- posed of smooth muscle fibres and elastin. This is where an activated sympathetic Capillaries nervous system can stimulate the smooth muscle fibres to contract, provoking blood vessel narrowing (vasoconstriction) and decreasing blood flow (Marieb and Hoehn, Venules Arterioles 2015). When the sympathetic nerves are inhib- ited, the muscle fibres of the tunica media relax, the blood vessels increase in diameter (vasodilation) and blood flow increases. Tunica externa regions, the ascending aorta, aortic arch, determined by the diameter of the arterioles The tunica externa (outer layer) consists thoracic aorta and abdominal aorta. Table 2 and can be increased through vasodilation. mainly of connective tissue fibres that pro- lists the major branches off the aorta. tect the blood vessels and attach them to Arteries can be divided into elastic Venous system any surrounding tissues. In larger blood arteries, muscular arteries and arterioles. The veins are thin, elastic vessels that act vessels, additional small vessels – vasa The elastic arteries are the largest (1-2.5cm as a reservoir of blood. They do not need vasorum – supply blood and nutrients to in diameter) and comprise large amounts large amounts of elastin and smooth the tunica externa and tunica media. of elastin as well as smooth muscle. They muscle, since they transport low-pressure have a large lumen with low resistance to blood back to the heart. They have a large Anatomy of the vasculature blood flow, and can expand and recoil to lumen, as well as valves that ensure a one- Arterial system accommodate changes in blood volume. way flow of blood to the heart. Arteries supply the body with oxygenated Muscular arteries regulate local blood Venules measure 8-100µ in diameter blood – with the exception of the pulmo- flow and deliver blood to individual and the largest ones possess a thin tunica nary arteries from the heart; these carry organs. They measure 0.3mm-1cm in deoxygenated blood to the lungs, and the diameter and possess more smooth muscle Table 2. Major branches off umbilical artery, which carries deoxygen- but less elastin than elastic arteries. the aorta ated blood from the foetus to the placenta. The arterioles are the smallest arteries Blood travels from the arteries to the arte- (0.01-0.3mm in diameter). In certain areas, Area of the aorta Arterial branches rioles and on to the capillaries, where gas- they have all three vascular layers (tunica Aortic arch Brachiocephalic eous exchange takes place. intima, media and externa). When they are Left common carotid The largest artery is the aorta, which close to the capillaries they comprise a single Left subclavian extends from the left ventricle down the left smooth muscle layer overlying endothelial Thoracic aorta Bronchial side of the body. It divides into four major cells. Blood flow into the capillaries is Pericardial Oesophageal Table 1. Five blood vessel types Mediastinal Posterior intercostal Vessel type Function Abdominal aorta Coeliac Arteries Transport high-pressure blood from the heart to smaller Phrenic arteries and arterioles Superior and inferior Arterioles Connect arteries and capillaries mesenteric Veins Act as reservoir of blood and transport low-pressure blood Suprarenal from venules to heart Renal Gonadal Venules Connect capillaries and veins Lumbar Capillaries Allow gas exchange, nutrient transfer and waste removal Middle sacral between blood and tissue fluid Common iliac PETER LAMB Nursing Times [online] April 2018 / Vol 114 Issue 4 41 www.nursingtimes.net Copyright EMAP Publishing 2018 This article is not for distribution Clinical Practice Systems of life Fig 2. hydrostatic pressure and oncotic pressure Fluid movement between capillary and tissue (Bit.ly/NottinghamUniStarling). Like any fluid pushed through a con- fined space, blood in a capillary exerts pres- sure on the wall of the vessel because of the Interstitial tissue fluid pressure exerted upstream by the blood coming from the arteriole. The blood pres- sure (BP) generates hydrostatic pressure, Arterial end Venous end which expels fluid from the pores of the capillary into the interstitial compartment. The size of the pores in the capillary dic- Net pressure Net pressure = fluid out = fluid in tates whether particular nutrients are delivered to particular tissues. Hydrostatic pressure is highest at the arterial end, and Direction of blood flow through the capillary lowest at the venous end, of the capillary. The other influencing force is oncotic pressure, which is underpinned by the principle of osmosis; this is the passive movement of water through a semiperme- able membrane from a region of low solute Hydrostatic pressure concentration to one of high solute con- centration, with the aim of achieving equi- Oncotic pressure librium. In blood, plasma proteins – which cannot easily pass through the capillary walls – exert an osmotic pressure that tends to pull fluid from the surrounding externa and a tunica media comprising Capillaries act as a semipermeable tissue (which has a higher water concen- two or three layers of smooth muscle cells. membrane allowing the diffusion of gases tration) into the capillary (which has a The venules join to form veins, in which and transfer of nutrients and waste prod- lower water concentration). This is the tunica externa, consisting of thick col- ucts. The single layer of flattened endothe- referred to as oncotic pressure. lagenous bundles, is the largest layer.