Blood Vessels PDF

9 Transport of substancesjnchumans O) Learning objective Blood vessels Comparethe structure of differenttypes of blood vessels Blood travels around the body in a series of tubes called blood Relatethe structu...

9 Transport of substancesjnchumans O) Learning objective Blood vessels Comparethe structure of differenttypes of blood vessels Blood travels around the body in a series of tubes called blood Relatethe structure of blood vesselsto traryport vessels. There are three main types of blood vessels: carry blood away from the heart..Y?.i.Q9. carry blood towards the heart. Gpill.qrie>, the smallest blood vessels, connect the arteries and the veins. The arrangement of blood vessels Blood vessels are arranged in branching networks. The aor!?, which is the largest artery in the body, comes out of the heart. It branches into other smaller arteries. These arteries will divide into many smaller branches called arteriq!99.(Figure 9.4 0). Each arteriole divides repeatedly to form a network of capillaries, which lie close to body cells The capillaries then join up, forming larger vessels called vppg!99 (O). The venules eventuallyjoin into veins (O). The venae cavae (singular:.vpna f.?.y.q)are two large veins that return blood from the body to the heart (see Section 9.4). to heart from heart vein artery o venule arteriole Key; —>direction Of blood flow body tissues capillary network in vessels and the direction of blood flow between different types of blood Fioure9.4 The relationship venøcgva aorta 9 Transport Jinhumans A. Arteries from the heart to body The function of arteries is to transport blood is high because of the tissues. The pressure of the blood in arteries pumping action of the heart. for the pulmonary The blood in the arteries is oxygenated, except baby). Most arteries artery (and the umbilical artery of an unborn are located deep in the body. in transport: Arteries are structurally adapted to their function 0). Remember this Arteries have very thick walls. This is to withstand the high Blood pressure is the force exerted : pressure of the blood. by blood against the inner wails of the blood vessels. it is created by The walls of arteries contain large numbers of elastic fibres. the rhythmic beating of the heart which drives the flow of blood This allows the walls to stretch when the heart contracts to through arteries and arterioles to pump blood into arteries and recoil when the heart relaxes. This capillaries. It can be referred to as helps maintain blood pressure and ensures a continuous blood hydrostatic pressure as it is pressure within a liquid. The walls of arteries have a thick muscle layer. These muscles can contract and relax to change the diameter of the vessels. This is to regulate the blood flow to certain organs or tissues. Cross section Longitudinal section small lumen lumen fibrous tissue fibrous tissue thick layer of muscles and muscle layer elastic fibres with elastic inner fibres inner lining lining (one layer of cells) Figure 9.5 Structureof an artery B. Veins The function of veins is to transport blood back to the heart. The blood pressure in veins is much lower than that in arteries because Whyis blood drawn from a vein the blood has passed through the arterioles, capillaries and venules but not from an artery during blooddonation? The blood in most veins is deoxygenated„ (Refertop.A4foeanswersJ except for the pulmonary vein (and the umbilical vein). Veins are located closer to the body surface when compared to arteries, umbilical'artery umbilical vein  In of substances 9 Transport function in transport: Veins are structurally adapted to their contain less muscular and The walls of veins are thinner, arteries. fewer elastic fibres than those of lumen arteries. The large Veins have a larger lumen than can continue to flow in the reduces resistance so that blood veins at low pressure. prevent the backflow of blood. This helps are present to only, i.e. the blood ensure that blood flows in one direction flows only towards the heart. Longitudinal section Cross section lumen large lumen fibrous tissue fibrous tissue thin layer of valve muscles and muscle layer elastic fibres with elastic inner lining inner fibres (one layer of cells) Figure 9.6 Structure of a vein The blood flow in veins is aided by the contraction of skeletal muscles adjacent to the veins. When these skeletal muscles contract, the veins are squeezed and the blood inside is forced towards the heart (Figure 9.7 0). valve opens blood is forced towards the skeletal heart muscles valve closes to prevent the valve opens backflowof blood O Muscles relax Muscles contract Figure 9.7 How the contraction of skeletal musclesand the action of valves help blood flow in veins valve 9-11 9 ,iTran'poit of.ubstanceöin' humane C. Capillaries vessels in the body. They Capillaries are the smallest of all blood networks called capillary receive blood from arterioles and form of the body. They are the beds, which are found in almost all tissues the blood and body cells. site for the exchange of materials between function: Capillaries are structurally adapted to their permeable. Some The walls of capillaries are differentially substances (e.g. oxygen, glucose, carbon dioxide and other metabolic wastes) are small enough to pass through them. very small Red blood cells and plasma proteins are too large to pass lumen through. capillary wall (one-cell thick) Their walls are one-cell thick (Figure 9.8). This provides a short diffusion distance for the exchange of materials. The highly branched networks of capillaries provide a large surface area for the rapid exchange of materials. In most tissues, o the capillary networkis so dense that each cell of the tissue is in close contact with a capillary. This also reduces the diffusion nucleus distance. Figure 9.8 The structure of a The lumen is so small that red blood cells must squeeze through capillary (Figure 9.9). This brings them into close contact with the capillary walls, reducing the distance for oxygen to diffuse from red blood cells to the body tissues. Blood flows slowly in capillary beds because they have a very Liquids in motion large cross-sectional area. This allows more time for the When @liquid completely fills exchange of materials. and flowsthrough a tube, the velocity (speed) of the liquid (v) is inversely proportional to the cross-sectional area ofthe $ tube capillary wall Q.iS the volume of the liquid V.that passes_ through any 3 section of the tube per unit red blood cell time. It is a constant. Hence, the larger the cross- sectional area of the tube, the x1000 slower the liquid flows Figure 9.9 Light micrograph of capillaries showing red blood cells passing through the capillaries one by one