Vascular System

VASCULAR SYSTEM CIRCULATORY SYSTEM The circulatory system is composed of a closed network of vessels that move materials around the body. These materials are moved within a fluid called blood and the movement of blood is driven by the ac,on of a muscular heart. The blood is transported at high pressure from the heart via arteries, which become split to form smaller arterioles. These arterioles then branch into capillaries that func,on as the site of material exchange with the body ,ssues. The capillaries then pool into venules, which converge to form veins. The veins then act to return the blood back to the heart at low pressure. ARTERY VEIN CAPILLARIES BLOOD Blood is the fluid medium in which materials are transported around the body. The blood contains three types of cells that each perform a specific func,on – red blood cells (erythrocytes) transport oxygen to the ,ssues, white blood cells (leukocytes) fight off pathogenic infec,ons and platelets (thrombocytes) are used in cloOng. The fluid component of blood is called plasma and is involved in transpor,ng many materials: • Hormones (chemical messengers) • Plasma proteins (e.g. albumin) • Urea (nitrogenous waste product) • Immunoglobulins (an,bodies) • Gases (oxygen and carbon dioxide) • Glucose (and other nutrients) • Electrolytes (minerals and ions) • Solvent (water as a medium) VESSELS Blood is transported in three types of vessels – all of which have dis,nc,ve morphologies according to their main role: • Arteries have thick walls and comparably narrow lumen (this is because the blood is moving at a high pressure) • Veins have thinner walls and an extremely wide lumen (as the blood is returning to the heart at low pressure) • Capillaries are very small (site of material exchange) VEIN ARTERY ARTERIES Arteries func,on to transport blood from the heart at high pressure and have specialised structures to accomplish this. They have a thick wall composed of an outer layer of collagen, which acts to prevent the artery from rupturing under the high pressures. The inner layer of the arterial wall consists of muscle and elas,c fibres, which can stretch and recoil with each pulse of blood to help maintain a stable blood pressure between pump cycles. The lumen is compara,vely narrow compared to a vein, which func,ons to keep pressure high. VEINS Veins act to transport the blood towards the heart at low pressure and have specialised structures to accomplish this. The wall is much thinner because blood pressure is lower (less chance of rupturing), however, the lumen is much wider (op,mising the return of blood). The muscle and elas,c fibres in the vessel wall provide flexibility, allowing veins to be compressed by skeletal muscle contrac,ons (which helps to propel blood back towards the heart). Because the pressure is low, veins have valves to prevent the backflow of blood. CAPILLARIES Capillaries func,on as the site of material exchange with ,ssues. The pressure in capillaries is low, due to the repeated branching of arteries. The lumen is extremely narrow, with only one erythrocyte able to pass through a capillary at a ,me – op,mising the exchange of oxygen with the ,ssues. The wall of a capillary is extremely thin (composed of a single layer of endothelial cells), which minimises the diffusion distance between the blood and the inters,,al fluid. Capillaries may be fenestrated (contain pores) to improve transfer. Not to scale (capillaries are small) BLOOD VESSEL COMPARISON The differences between the three types of blood vessels can be summarised by the following table: Arteries Veins Capillaries FuncIon Transports blood from the heart Transports blood towards the heart The site of material exchange Blood Pressure High (80 – 120 mmHg) Very low (>10 mmHg) Low (~15 mmHg) Lumen Diameter Narrow Wide Extremely narrow Wall Thickness Thick (three ,ssue layers) Thin (three ,ssue layers) Extremely thin (single cell layer) ElasIc Fibres Large amounts Small amounts None Valves None Present None HEART The human heart is a four chambered organ, consis,ng of two atria and two ventricles. Atria act as reservoirs, by which blood returning to the heart (via veins) is collected and passed on to the ventricles. The ventricles then act as pumps, expelling the blood (via arteries) to the various body ,ssues. The heart can be func,onally divided into two sides. The right side of the heart collects the deoxygenated blood from the body ,ssues and pumps it to the lungs (pulmonary circula5on). The le\ side of the heart then collects the oxygenated blood from the lungs and pumps it to all the body ,ssues (systemic circula5on). The le\ side of the heart has a thicker muscular lining (myocardium) because it needs to pump blood greater distances. Hearts are drawn as they would appear when looking at a chest – so, the le\ side of a drawing represents the right side of the heart. HEART RATE Blood circulates around the body as a consequence of the rhythmic contrac,on of the heart. Each of these contrac,ons (or heart beats) forces a wave of blood through the arteries, which can be detected as a pulse. The heart rate is monitored by the medulla oblongata (brainstem) and can be regulated by either nervous impulses or hormonal signals (specifically adrenaline). Heart rate can be affected by a range of condi,ons, including physical ac,vity (exercise), age, specific diseases, body temperature and emo,onal states (stress). CORONARY HEART DISEASE The blood that is being pumped through the heart cannot be used to supply the heart ,ssue with oxygen or glucose. This func,on is achieved by a network of vessels surrounding the heart – called coronary arteries. If a coronary artery becomes occluded, then the heart ,ssue that is nourished by the blocked artery begins to die, resul,ng in the eventual development of coronary heart disease (CHD) and a poten,al heart a_ack. Atherosclerosis is the hardening and narrowing of arteries due to the deposi,on of cholesterol. When fa_y deposits (atheromas) develop in the arteries, the lumen diameter becomes greatly reduced. The restricted blood flow increases pressure in the artery, leading to damage to the arterial wall and the development of fibrous lesions called atherosclero,c plaques. If a plaque ruptures, blood cloOng is triggered, forming a thrombus which restricts blood flow. The forma,on of a thrombus within a coronary artery leads to CHD. A variety of factors can increase the risks of developing occlusions in the coronary arteries, including: • • • • GeneIcs – hypertension may increase risks Obesity – being overweight strains the heart Diseases – diabetes can increase the risks Diet – salty and fat-rich diets pose a threat • • • • Exercise – sedentary lifestyles are dangerous Sex – males are at greater risk (less estrogen) Smoking – nico,ne causes vasoconstric,on Age – the risks generally increase with age