7.3 Homeostasis

Plant sterols and plant stanols are found in a range of food, including vegetables, cereals, seeds and nuts. Plant sterols and stanols have a similar structure to cholesterol and reduce the absorption of cholesterol in the small intestine, so more cholesterol is lost in the faeces. This helps to lower the levels of cholesterol in the blood and in turn, reduces the risk of cardiovascular disease (CVD). 

A group of scientists wanted to investigate the effects of eating plant sterols and stanols on the risk of CVD. The scientists randomly divided healthy volunteers into two groups. Every day for eight weeks, one group was given plant sterols and stanols to eat. The other group acted as a control.

Each week, the scientists measured the diameter of the lumen of the main artery in the arm of the volunteers.

State how the control group should have been treated.

The results of the experiment in part (a) are shown in the table below.
 

A student reading the results concluded that there was sufficient evidence to assume a causal relationship between plant sterols and stanols, and a reduced risk of CVD.

Evaluate the student's conclusion.

An electrocardiogram (ECG) measures the electrical fluctuation within a cardiac muscle as a heart is beating. The diagram below shows an ECG trace for a normal, healthy person and an ECG trace for a person suffering from heart disease.

Describe and explain the path that electrical impulses follow to cause contraction in the ventricular muscle in a healthy heart.

Explain how the information from part (c) shows that the damage caused to the diseased heart is likely to have affected the sinoatrial node (SAN).

The graph below shows the change in the volume of the left ventricle during the cardiac cycle.

Identify the time(s) at which the semilunar valves opened.

(1)

Use the information in the graph to explain your answer at part (i).

(1)

The person that was used to draw the graph at part (a) has a cardiac output of 7.92 dm³ min^-1.

The cardiac output was calculated using the following formula:

Cardiac output = heart rate × stroke volume

Use the information provided in part (a) and (b) to calculate the heart rate of this person.

Cardiovascular disease (CVD) can cause damage to the cardiac muscle of the left ventricle which decreases the stroke volume of the heart.

Use the information in part (b) to explain the effect this may have on the heart rate of a person.

A mountaineer hikes up a mountain. 

With an increase in altitude there is a decrease in temperature.

Describe the homeostatic mechanism that will regulate the mountaineers' body temperature near the top of the mountain.

When hiking in extremely cold conditions, it is very important that mountaineers wear waterproof gloves that are properly insulated to avoid frostbite in their fingers and hands.

Frostbite (pictured above) is damage to the skin and tissue when exposed to very low temperatures. Severe frostbite can cause tissue death and in extreme cases, amputation of fingers and toes is the only course of action doctors can take. 

Deduce the reason why these gloves would prevent frostbite.

The tissue death caused by severe frostbite is known as gangrene. In such cases, the affected parts must be amputated.

Use your knowledge of skin structure and blood vessels to explain why gangrene is more likely in extremities, such as fingers and toes.

Prolonged exposure to extremely cold conditions leads to hypothermia. This occurs when a dramatic decrease in body temperature leads to a decrease in metabolism, which in turn results in less heat energy being released leading to a further drop in body temperature.

In this sense, hypothermia is an example of a positive feedback loop.

Explain why positive feedback loops cannot be considered a homeostatic mechanism.

The following image shows the electrocardiogram (ECG) of a healthy heart and a heart with an abnormal rhythm.

Which of the following provides the correct description of the ECG shape of a healthy heart?

Use the graph at part (a) to identify and explain the possible heart problem in the abnormal ECG.

Damage to the atrioventricular node (AVN) will severely affect the heart rate of an individual.

Explain the effect this would have on the heart rate.