Transport, B3.2

• What is the circulatory system? The circulatory system consists of blood vessels, and is a network of tubes that carry blood around the body.

  Arteries transport high pressure blood away from the heart and branch into smaller vessels, forming arterioles.

  Arterioles distribute blood to capillaries, which are the sites of exchange of material between blood and the internal/external environment.

  Capillary vessels combine into venules that carry blood into a vein, which is a larger blood vessel that returns low pressure blood to the heart.
• What are capillaries? Capillaries are blood vessels that are responsible for the exchange of substances between the bloodstream and the surrounding tissue. They do this almost everywhere in the body- apart from the lens and the cornea of the eye.
• How are capillaries adapted for their function of exchange? - branched

  - narrow lumen diameter

  - thin vessel wall of flattened endothelial cells

  - basement membrane

  - fenestrations

  - pericytes wrapped around them
• How does branching aid capillaries in their function? Capillary vessels branch from arterioles to form capillary networks.

  This branching increases the surface area, so there is more area available for exchange to occur.

  This facilitates a faster rate of exchange.
• How does the narrow lumen diameter of capillaries aid their function? Capillaries have the narrowest lumen of all vessels- maximum 10 micrometres.

  This narrow diameter has two benefits:

  Increased surface area (in relation to its volume), allowing for more efficient exchange

  Slows down blood flow, giving more time for diffusion to occur
• How does the thin vessel wall of flattened endothelial cells aid capillaries in their function? The capillary walls are only one cell thick. It is comprised of flattened endothelial cells.

  This allows for efficient exchange between the blood and surrounding tissues, as it minimises the distance that the molecules need to travel to cross from the bloodstream into tissue cells and vice versa.

  The fact that the endothelial cells are flattened means that they are thin and flat. This provides more membrane surface area, which facilitates more efficient exchange.
• How does the basement membrane aid capillaries in their function? The basement membrane- a coating of extracellular fibrous proteins- is secreted by the endothelial cells.

  The basement membrane anchors the capillary wall to the surrounding tissue, and also regulates the passage of substances between blood and tissues.

  It allows small particles like water and ions to pass through, but prevents large proteins and cells from being exchanged.
• How do fenestrations aid capillaries in their function? Some capillaries are fenestrated. This means that they contain small pores that allow larger molecules to pass through. The pores allow part of the blood plasma to leak out, but are too small for cells the move through.

  The number and size of fenestrations will depend on the location of the capillary.

  Most fenestrated capillaries are found in tissues where large volumes of tissue flid and plasma are exchanged (like the small intestine or the pancreas)
• What is the heart? The heart is a muscular organ that pumps pressurised blood through the body.

  It pumps high pressure (oxygen-poor) blood out int the arteries and receives low pressure blood back in via the veins
• What are arteries? Arteries are blood vessels that are responsible for carrying blood way from the heart under high pressure.
• How are arteries adapted for their function? Thick vessel wall

  Narrow lumen

  Elastic fibres

  Collagen

  No valves
• How is the artery wall adapted to its function? The artery wall consists of three layers:

  The tunica interna

  This is the innermost layer.

  It is comprised of endothelium cells.

  It lines the lumen of the blood vessel, and provides a smooth surface for blood to flow through with low resistance.

  The tunica media

  This is the middle layer.

  It is the thickest layer, comprised mostly of elastic fibres and smooth muscle.

  This provides support for the vessel and changes vessel diameter in order to regulate blood flow and blood pressure.

  The tunica externa

  This is the outermost layer.

  It is comprised of connective tissue with varying amounts of elastic and collagenous fibres that attach the vessel to the surrounding tissue.
• What is the tunica interna? This is the innermost layer.

  It is comprised of endothelium cells.

  It lines the lumen of the blood vessel, and provides a smooth surface for blood to flow through with low resistance.
• What is the tunica media? This is the middle layer.

  It is the thickest layer, comprised mostly of elastic fibres and smooth muscle.

  This provides support for the vessel and changes vessel diameter in order to regulate blood flow and blood pressure.
• What is the tunica externa? This is the outermost layer.

  It is comprised of connective tissue with varying amounts of elastic and collagenous fibres that attach the vessel to the surrounding tissue.
• How is the small lumen of arteries adapted to its function? A lumen is a hollow passageway through which blood flows.

  The smaller lumen of arteries helps them to maintain high blood pressure and high velocity of blood flow.

  This is because the blood puts pressure on the artery walls as it squeezes through the narrow lumen. As fluids move from areas of high pressure to areas of low pressure, the blood is consequently propelled forward to capillaries where the pressure is lower.
• How do elastic fibres aid arteries in their function? Within the tunica media there are elastic fibres.

  Peak pressure in an artery (systolic pressure) causes the wall of an artery to be pushed outwards, widening the lumen and stretching the wall, When this occurs, elastic fibres store potential energy. At the end of each heartbeat, the pressure within the arteries falls and the stretched elastic fibres return the energy by recoiling and squeezing the blood in the lumen. Therefore, the elastic fibres help to reduce the amount of energy expended in transporting blood in the body.

  They also prevent minimum pressure (diastolic pressure) within the arteries from becoming too low, evening out blood flow.
• How does smooth muscle aid arteries in their function? Within the tunica media there are rings of smooth muscle. Smooth muscle cells are of especially high density in the arterioles. These muscle cells are circular, so when they contract, the lumen's diameter is narrowed. This is called vasoconstriction. The muscle cells can respond to hormonal and neural signals, enabling the adjustment of blood flow to tissues depending on availability and need. Vasodilation widens arteries, increasing blood flow to a tissue.

  Vasoconstriction narrows arteries, decreasing blood flow to a tissue.
• How does collagen aid arteries in their function? Collagen is a fibrous protein within the extracellular matrix of the tunica externa of artery walls.

  Fibrous proteins consist of repeating amino acid sequences that form long, helical structures. These structures allow them to stack and intertwine, creating strong and insoluble fibres. This provides support, protection and elasticity to the body's tissues.

  Collagen has different functions:

  Provides high tensile strength (the ability to withstand pressure before breaking when stretched or pulled)- allowing the artery to withstand high pressures without bulging (aneurism) or rupturing.

  Maintaining artery vessel shape (this is why they are more rounded than veins)

  Anchoring arteries to nearby tissues

  Providing elasticity for the recoil of the artery wall.
• What is an aneurysm? An aneurysm is a weakened and bulging section of an artery wall. A ruptured aneurysm leads to a stroke, heart attack or internal bleeding.
• Why do arteries have no valves, and how does this aid them in their function? A valve is a structure that allows blood to flow in only one direction.

  It prevents backflow, ensuring that blood moves forward through the vessels or heart chambers. Arteries do not need valves because blood in arteries is pumped directly from the heart, so it moves under high pressure. This prevents backflow naturally.
• What are veins? Veins are blood vessels responsible for carrying blood towards the heart under low pressure.
• How are veins adapted to their function? Thin vessel wall

  Wide lumen

  Valves

  Skeletal muscle pump
• How is the thin vessel wall of veins adapted to its function? The walls of veins have the same three layers as arteries (tunica interna, tunica media, tunica externa), but contain less smooth muscle, elastic fibres and connective tissue.

  Therefore, their walls are thinner.

  This is optimal as the blood in the veins has less pressure compared to arteries.

  There is no need for elastic fibres to extend or recoil, as blood flows through veins without a pulse.

  There is no need for smooth muscle for vasoconstriction or vasodilation, as veins are not used to adjust blood flow to areas of the body. As they contain less smooth muscle cells, veins typically have a distorted, irregular shape.
• Why do veins have wide lumens, and how does this aid them in their function? A significantly larger amount of blood is found in the veins compared to capillaries and arteries. Therefore, the wide lumen diameter of veins allows a large volume of blood to flow through with minimal resistance.
• Why do veins have valves, and how does this aid them in their function? Veins have valves, which are structures that prevent the backflow of blood. If blood begins to flow backwards, it gets caught in what is known as a pocket valve (three cup-shaped flaps of tissue), which, when filled with blood, closes the valve and prevents further backflow.
• What is the skeletal muscle pump? The skeletal muscle pump is a physiological mechanism that aids in the circulation of blood. The thin wall of the veins allows them to be squeezed by the surrounding skeletal muscles.

  When the skeletal muscles contract, this increase the pressure within the veins, forcing blood upwards and towards the heart.

  The skeletal muscle pump is most prominent in the legs, due to the need to return blood from the lower body to the heart, against gravity.
• How can you identify veins and arteries on a micrograph? To identify veins or arteries on a micrograph, look for the thickness of the walls and the shape and size of the lumen.

  Arteries have a thicker wall and a smaller, more rounded lumen.

  Veins have a thinner wall and a larger, more variable shaped lumen.
• What is the cardiac cycle? The cardiac cycle is a series of actions that generate one pump of the heart. The duration of the cardiac cycle varies depending on the heart rate. Ata a normal heart rate of 70 beats per minute, each cycle lasts approximately 0.8 seconds.
• What is heart rate? Heart Rate: Refers to the number of times the heart beats per minute (BPM) and is a direct measure of heart activity.
• What is pulse rate? Pulse Rate: Refers to the number of times the arteries expand and contract with each heartbeat, which can be felt at various pulse points on the body.

  Each time the heart beats, it propels blood through the network of arteries of the body. The blood applies pressure to the vessel walls, which becomes stretched and then recoils. This can be felt as a pulse.

  Between contractions, the heart relaxes, which brings the pressure back down again. There is one pulse per beat of the heart.
• How can you measure pulse rate? The carotid (neck) and radial (wrist) arteries are commonly used to check a pulse because those arteries come close to the surface of the skin. In order to measure pulse rate:

  -Find the pulse (using two fingers, not the thumb as it also has a pulse)

  -Count the beats for thirty seconds

  -Double it

  Or digital tools may be used, that rely on optical sensors. Pulse oximeters are often used, clipped to a fingertip. The device emits red and infrared light into the tissue. A detector on the device then detects the amount of light that passes back through the tissue. The volume of blood in the tissue changes with each heartbeat, which alters the amount of light absorbed.
• What factors influence pulse rate? -Age (decreases)

  -Exercise (increases)

  -Stress (increases)

  -Medications (decreases or increases)

  -Temperature (increases)

  -Health conditions (abnormalities e.g. arrhythmias)
• What is an occlusion? An occlusion is a blockage of a blood vessel.

  What a blood vessel is occluded, the flow of blood to the tissue downstream is reduced or completely stopped, preventing oxygen and nutrients from reaching the cells. Cells may die if this blockage is not resolved. This is especially dangerous in vital organs like the heart and brain where oxygen demand is high (as the oxygenated blood cannot flow there).
• What is Ischemia? Ischemia is a condition where there is reduced blood flow to a specific area of the body.
• What is atherosclerosis? Atherosclerosis is the accumulation of plaque (atheroma) in an artery. Plaque is a substance made of cholesterol, fat, calcium deposits, cellular debris and the clotting protein fibrin. Calcium salts make the artery harden and it's inner surface rough.
• What is thrombosis? A thrombosis is a clot of platelets, red blood cells, and the clotting protein fibrin. Thrombi form when a blood vessel is damaged and the clotting mechanism is activated. Hypertension increases the risk of this.
• What is embolism? An embolism is a travelling blood clot. Embolisms occur when a thrombosis breaks off the vessel wall and is carried through the bloodstream. The embolus can get stuck in smaller vessels and block blood flow.
• What is a coronary artery? The blood within the heart chambers is not used by the heart muscle itself- the heart receives its own blood supply through the coronary arteries.

  Coronary arteries branch from the aorta and deliver oxygen-rich blood directly to the heart muscle.
• What is a coronary occlusion, and what are its consequences? A coronary occlusion is the partial or complete obstruction of blood flow in a coronary artery. Coronary occlusions restrict the flow of blood to the heart wall, depriving the heart of oxygen and other essential nutrients.

  Without enough oxygen and glucose, the heart muscle cells are unable to synthesise ATP.

  Without a supply of ATP, the cardiac muscle cells cannot contract in synchrony.

  The heart will therefore spasm and may eventually stop beating altogether. This is known as a heart attack, or myocardial infarction (MI).