Discussion – effects of diurnal changes, postural changes and temperature on heart rate and blood pressure

In experiment 1, the results show that there is diurnal variation of heart rates with a significant rise in the heart rate in the evening; the hypothesis was proven to be true. This diurnal variation can be attributed to the hormones produced by the body as well as environmental factors during the various times of the day (Singh et al. 2003). The body secretes endocrine hormones which affect the autonomic nervous system that in turn affects the heart rate. Thyroid and stress hormones (cortisol) cause an increase in the heart rate (Goldberger 2001). In the morning, the levels of cortisol and other hormones are low, hence a lower heart rate; their secretion gradual increases during the day, thus, the higher heart rates in the evening. The highest levels of cortisol are recorded between ten in the morning and six in the evening (Goldberger 2001). Some foods, drinks and drugs such as caffeine, alcohol and nicotine as well as physiological effects of stress and burn out are also known to trigger increase in heart rates (Glavas & Weinberg, 2006). These can also account for higher heart rates in the evening.

In experiment 2, the hypothesis was also proven to be true; the results showed an immediate increase in the heart rate after change of posture from the supine position to upright position. In the supine position, the heart rate was lower because the return of the venous blood from the lower parts of the body to the heart is not much of a problem and pumping of the blood is easier to all parts of the body because there is less resistance (Noble 2004). In the upright posture, blood has to return from the lower part of the body against gravity; and the pumping has also to be done against gravity. This necessitates increase in the heart rate to overcome the effect of gravity (Noble 2004). After two minutes while still in the upright position, the heart rate starts to decline and eventually levels off by the fourth minute as the heart has maintained the stroke volume to overcome gravity.

In experiment 3, part 1; the aim of the experiment was to investigate the effect of heat on heart rate. The hypothesis that the heart rate will increase with exposure to heat was found to be true. In response to elevation of body temperature due to immersion in hot water, surface cooling is enhanced as blood is diverted to the skin, the baroreceptors in the blood vessels trigger vasodilatation in order to dissipate off the heat and maintain the body temperature in the normal range. This will result in lowering of the blood pressure. In order to maintain the normal blood pressure, the heart has to increase the rate of pumping blood thus, the increase in the heart rate (Guyton & Hall 2006).

In experiment 3, part 2; the aim of the experiment was to investigate the effect of cold temperature on the heart rate. The hypothesis was also found to be true since the results indicate that there is a gradual decline in the heart rate from 78bpm before immersion to 66bpm after 30 minutes of immersion. According to Charkoudian (2003), the exposure to cold temperature causes the baroreceptors in the blood vessels to trigger vasoconstriction of the vessels in order to prevent heat loss. The narrowing of the vessels results in an increase of blood pressure and thus the heart rate is reduced in order to reduce the blood pressure. The blood flow to the limbs and legs is also reduced but the blood flow to the core and vital organs such as the liver, kidneys and brain is not affected. However, it is documented that the heart rate would rise if the cold temperatures are lowered to the extremes as the body tries to keep the body temperature in the normal range (Charkoudian 2003).

Pulse rate and blood pressure are directly correlated. Pulse rate is the number of times an individual’s heart beats per minute; blood pressure is the force that the pumped blood exerts on the walls of the arteries (Guyton & Hall 2006). When the pulse rate increases, the blood pressure rises and when the pulse rate reduces, so does the blood pressure. The body controls blood pressure and the pulse rate through a negative feedback mechanism-baroreflex or baroreceptors reflex (Sleight et al. 1995). The walls of the arteries, especially the aortic arch and carotid sinuses, possess stretch receptors (baroreceptors) that are sensitive to stretch produced by increased blood pressure (Ehrman et al. 2009). When the blood pressure is raised, the baroreflex leads the decrease of heart rate and, hence, blood pressure is lowered; in a similar way, lowering of blood pressure stimulates the baroreflex, leading to increase of heart rate and, therefore, the blood pressure elevates. The baroreceptors are linked to the glossopharyngeal nerve which relays the information to the autonomic nervous system that mediates the heart rates and blood pressures (Sleight et al. 1995).

According to Ehrman (2009), there are many other factors that affect the heart rate and blood pressure. Physical activities, nutrition, gender; age, emotions such as anger, worry, and anxiety, temperatures- heat and cold, stress, drugs such as alcohol, infections and diseases are all know to influence an individual’s pulse and blood pressure. The effect of these factors is exerted on the pulse and blood pressure through the negative feedback mechanism.

Conclusion

The aims of the experiments were to investigate how diurnal, postural changes and temperature affect the heart rate. The results of the experiments confirmed that these external stimuli affect the heart rate. Experiment 1 confirms that the heart rate has a diurnal variation with the rates being higher in the evening than in the morning. Experiment 2 confirms that there is a variation of the heart rate depending on the posture of the individual with rates being lower in the supine position and higher in the upright position. In the experiment 3, the findings validate the hypothesis that temperature affects heart rate. In this respect, causes an increase in the heart rate while exposure to cold temperature lowers the heart rate.

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