1.3, 1.4 The Heart and Circulation

Question 1

What is the structure and function of an artery?

Answer 1

• arteries carry blood away from the heart to the rest of the body at high pressures
• the outer connective tissue contains collagen (a tough fibrous protein) for strength and durability
• arteries have thick walls containing elastic tissue and smooth muscle
• the thick walls allow the artery to withstand the high pressure generated by the blood being forced against the walls
• the elastic tissue allows for stretch and recoil. the elastic walls stretch to accomodate the blood being forced through the arteries. the elastic walls recoil behind the blood, pushing the blood forward and smoothing blood flow. this stretch and recoil is how blood moves through the arteries, and can be felt by the pulse.
• the smooth muscle allows for vasoconstriction and vasodilation. this allows the arteries to regulate blood pressure and blood flow.
• the narrow lumen maintains the high blood pressure
• the folded endothelium (inner lining) allows the artery to expand under high pressure
• there are no valves

Question 2

What is the structure and function of a vein?

Answer 2

• veins take blood back to the heart at low pressures
• the outer connective tissue contains collagen (a tough fibrous protein) for strength and durability
• the thin walls contain less smooth muscle and elastic tissue, as the blood moves through at lower pressure
• the wide lumen allows for a larger volume of blood to flow with less friction/resistance
• the smooth endothelium lining reduces friction and restistance, allowing blood to flow faster
• semilunar valves prevents backflow of blood
• blood flow is assisted by the contraction of skeletal muscles during movement of limbs and breathing

Question 3

What is the structure and function of a capillary?

Answer 3

• capillaries are where metabolic exchange occurs (substances exchanged between cells and capillaries)
• the network of capillaries is called a capillary bed, which increases surface area for diffusion
• endothelium is one cell thick to reduce the diffusion distance, which speeds up the diffusion of subtances
• the very narrow lumen increases the friction, reducing the speed of blood flow into a steady flow, allowing for exchange of substances

Question 4

What is the structure and function of the heart?

Answer 4

• the heart pumps blood around the body
• the right side pumps deoxygenated blood to the lungs
• the right side consists of the superior and inferior vena cava, the right atrium, the right ventricle, and the pulmonary artery
• the superior vena cava takes blood from the head and arms, and the inferior vena cava takes blood from the lower body
• the left side pumps oxygenated blood to the body
• the left side consists of the pulmonary vein, the left atrium, the left ventricle, and the aorta
• the left ventricle is the largest, as it has thick muscular walls which contract powerfully to pump blood around the entire body
• the ventricles have thicker walls than the atria, as they need to push blood out of the heart, whereas the atria only need to push blood into the ventricles
• atrioventricular valves prevent backflow of blood from the ventricles to the atria when the ventricles contract
• semilunar valves prevent the backflow of blood from the arteries back into the heart when the ventricles relax
• coronary arteries supply the heart with oxygenated blood

Question 5

How can the heart be investigated practically?

Answer 5

• external examination
• internal examination (cutting it open)

Question 6

How do valves work?

Answer 6

• valves only open one way
• when there is high pressure behind the valve, the valve is forced open and blood is forced through
• there is now higher pressure in front of the valve, and so it is forced closed, and and so blood cannot flow backwards

Question 7

What is the cardiac cycle?

Answer 7

• the continuous contraction (systole) and relaxation (diastole) of the atria and ventricles
• there are three phases: atrial systole (and ventricular diastole), ventricular systole (and atrial diastole), and cardiac diastole

Question 8

Describe atrial systole:

Answer 8

• the ventricles are relaxed
• both the atria contract
• this decreases the volume and pressure of the atria
• blood is forced through the AV valve into the ventricles
• as the ventricles fill with blood, there is a slight increase in ventricular pressure and volume

Question 9

Describe ventricular systole:

Answer 9

• the atria relax
• the ventricles contract
• this decreases the volume but increases the pressure of the ventricles
• the pressure is now higher in the ventricles than in the atria, so the AV valve is forced shut to prevent backflow
• the pressure is also higher in the ventricles than in the aorta and pulmonary artery, so the SL valves are forced open
• blood is forced through the arteries

Question 10

Describe cardiac diastole:

Answer 10

• both the atria and ventricles are relaxing
• there is higher pressure in the aorta and pulmonary artery than in the ventricles, so the SL valves are forced shut (gravity also influences this)
• elastic recoil of the atrial walls generates low pressure in the atria
• there is higher pressure in the vena cava and pulmonary vein than in the atria, so the atria begin to fill with blood
• this increases the pressure in the atria, pushing open the AV valves, allowing blood to passively flow into the relaxing ventricles