6.2 The blood system

Question 1

What did William Harvey do?

Answer 1

Discovered that the heart was the pump, and that blood was unidirectional.

Question 2

Structure of the arteries?

Answer 2

The arteries convey blood at high pressure from the ventricles to the tissues of the body.
Artery walls contain elastin fibres, which store the energy that stretches them at the peak of each pumping cycle. Their recoil helps to propel the blood on down the artery.
Contraction of the smooth muscle in the artery controls the diameter of the lumen and to some extent the rigidity of the arteries, thus controlling the overall flow through them.
The elastic and muscular tissues contribute to the toughness of the arteries which have to be strong to withstand the constantly changing and intermittently high blood pressure without bulging outwards.

The artery walls have muscle and elastic fibres in their walls,

• tunica externa - a tough outer layer of connective tissue
• tunica media - a thick layer containing smooth muscle and elastic fibres made of the protein elastin
• tunica intima - a smooth endothelium forming the lining of the artery.

Question 3

How is arterial blood pressure controlled?

Answer 3

The muscle and elastic fibres assist in maintaining blood pressure between pump cycles.
The blood entering the artery is at very high pressure, the maximum pressure is called the systolic pressure. It pushes the wall of the lumen out, stretching the elastic muscle and thus storing potential energy.
At the end of each heart beat the pressure in the arteries will fall, the stretched muscle fibres will then use their potential energy to squeeze the blood in the lumen preventing the diastolic pressure from becoming too low.
The circular muscles in the wall of the artery can contract or relax adjusting the diameter of the lumen, is it contracts this is called vasoconstriction and if it dilates it is called vasodilation.

Question 4

What is the systolic pressure?

Answer 4

The peak pressure reached in an artery. It pushes the wall of the artery outwards widening the lumen and stretching elastic fibres in the wall, thus storing potential energy.

Question 5

What is diastolic pressure?

Answer 5

The minimum pressure reached in the artery.

Question 6

Structure of the capillaries?

Answer 6

They are the smallest blood vessels that branch and join repeatedly to form a capillary network with a huge total length.
The capillary wall consists of one layer of very thin endothelium cells, coated by a filter like protein gel, with pores between the cells. The wall is thus permeable and allows some plasma to leak and form the tissue fluid. This fluid then flows between the cells in a tissue allowing the cells to absorb useful substances and excrete waste. It then re-enters the capillary network.
The permeability of capillary walls differ between tissues, enabling particular proteins and other large particles to reach certain tissues but not others. Permeabilities can also change over time and capillaries repair and remodel themselves continually in response to the needs of tissues that they perfuse.

Question 7

Structure of veins?

Answer 7

Veins collect blood at low pressures from the tissues of the body and return it to the atria of the heart. The blood is at a much lower pressure and therefore veins do not need to have as thick wall as the arteries and that wall contains far fewer muscle and elastic fibres. They can therefore dilate to become much wider and thus hold more blood than arteries. Blood flow is assisted by gravity and by other tissues. Contraction makes a muscle shorter and wider and therefore it squeezes on adjacent veins like a pump. Walking, sitting or even just fidgeting greatly improves venous flow.

VEINS HAVE VALVES

• They protect circulation by preventing back flow.
• If blood starts to flow backwards it gets caught in the flaps of the value which fill with blood blocking the lumen.
• When blood is flowing forward it pushes on the flaps opening them.

Question 8

How many layers in each of the vessels?

Answer 8

3 layers in the arteries AND the veins
tunica externa
tunica media
tunica intima

JUST ONE LAYER IN CAPILLARIES.

Question 9

What are the two double circulation systems?

Answer 9

The pulmonary circulation - to lungs
The systemic circulation - to body

FISH only have one circulation system, they pump deoxygenated blood at high pressures to their gills to be oxygenated and it flows back slowly enough to oxygenate the organs, however we are a bit bigger and more complicated.

Question 10

Structure of the heart?

Answer 10

Blood comes in to the vena cava, into the right atria and pushes on the atrioventricular value and then goes into the right ventricle. It then is pushed out and past the semilunar value into the pulmonary artery.

Question 11

What is atherosclerosis?

Answer 11

This is the development of fatty tissue called atheroma in the artery wall adjacent to the endothelium. Low density lipoproteins containing fats and cholesterol accumulate, and phagocytes are then attracted by signals, they engulf the LDL’s and then grow very large. Smooth muscle cells migrate to form a tough cap over it, and so the artery wall bulges into the lumen narrowing it and thus impeding blood flow.
Sometimes this cap will rupture and therefore a scab will be formed which can block the artery completely.

Question 12

What causes atherosclerosis?

Answer 12

• Lots of LDL’s in blood
• High glucose concentration, due to overeating, obesity or diabetes
• Chronic high blood pressure
• Trans fats which damage the endothelium of the artery
• Infection

Question 13

What controls the heartbeat?

Answer 13

The heartbeat is initiated by a group of specialised muscle cells in the right atrium called the sinoatrial node.
The heart is unique in that it can be stimulated to contract without stimulation from motor neurons, it is therefore myogenic, because it can generate its own contraction.

The membrane of the heart cell depolarises, when the cell contracts depolarising the other cells around it. The heart therefore contracts almost simultaneously at the rate of the fastest.
The fastest is a special group of muscle cells in the wall of the right atrium called the sinoatrial node. This region therefore initiates each heartbeat, because the membranes of its cells are the first to depolarise in each cardiac cycle.

Question 14

What is myogenic contraction?

Answer 14

What the heart does, being able to contract itself without stimulation from motor neurones.

Question 15

Where is the sinoatrial node?

Answer 15

Right atrium (by the vena cava)

Question 16

What is the pacemaker?

Answer 16

The sinoatrial node because it contracts first and therefore causes everything else to contract. It therefore sets the pace for the beating of the heart and is often called the pacemaker. It sends out a signal in the form of depolarisation that spreads throughout the walls of the atria. This can happen because there are interconnections between adjacent fibres across which the electrical signal can be propagated. Also the fibres are branched so each fibre passes the signal on to several others. This causes the atria to contract, then less than a few seconds later the electrical signal is conveyed to the ventricles. This time delay allows time for the atria to pump the blood that they are holding into the ventricles.

Question 17

How does the heart beat?

Answer 17

The sinoatrical node, in the right atrium of the heart depolarises, this causes the cells around it to depolarise too. This signal spreads through the walls of the atria, this can happen because there are interconnections between adjacent fibres, and also they are branched which allows the signals to be propagated (breed). The blood is pushed through the atrioventricular valve into the ventricles, these are then stimulated 0.1 seconds later, the delay time allows the atria to pump blood into the ventricles, and then blood is pushed out into the arteries through the semi lunar value.

Question 18

Describe the cardiac cycle?

Answer 18

0. 0-0.1 seconds
   - The atria contract
   - The atrioventricular values open
1. 1-0.15
   - The ventricles contract, the pressure increases and the atrioventricular values close.
   - The pressure in the ventricles increases
2. 15-0.40
   - The pressure in the ventricles rises above the pressure in the arteries and so the semi lunar values open and blood flows in.
3. 40-0.45
   - The pressure in the ventricles drops below the pressure in the artieries causing the semilunar value to close.
4. 45-0.8
   - The pressure in the ventricles drops below pressure in the atrium and so the atrioventricular values close.

Question 19

How is the heart rate controlled?

Answer 19

The heart rate can be increased or decreases by impulses brought to the heart through two nerves from the cardiovascular centre in the medulla of the brain.

The cardiovascular centre sends impulses down the two nerves to the sinoatrial node to make it either beat faster or slower depending upon factors.

Low blood pressure, low pH, low oxygen concentration all indicate that the heart rate needs to speed up.

High blood pressure, high pH or high oxygen concentration all indicate that the heart needs to slow down.

Question 20

What is Epinephrine?

Answer 20

This is a hormone, it goes from the adrenal glands (on the kidney) THIS HORMONE IS ALSO CALLED ADRENALINE!
Secretion is controlled by the brain and rises when physical activity may be needed because of a threat.