1) a, 2) d, 3) a, 4) b, 5) d, 6) d, 7) a, 8) b, 9) d, 10) c
Further reading
Boore J et al. (2016) Chapter 2: The human cell, in Essentials of Anatomy and Physiology for Nursing Practice. London: SAGE Publications Ltd.
A textbook to develop your knowledge of human anatomy and physiology that is aimed specifically at nurses.
Knight J and Andrade M (2018) Genes and chromosomes 1: Basic principles of genetics. Nursing Times, 114(7), 42–5.
A gentle overview of how DNA is organised and the nature of chromosomes.
Tortora G and Derrickson B (2017) Chapter 3: The cellular level of organization, in Tortora’s Principles of Anatomy and Physiology (15th edition). New York: John Wiley & Sons.
In-depth coverage of human anatomy and physiology.
Useful websites
www.medicalnewstoday.com/articles/320878.php
A simple overview of cell structure and function.
www.kenhub.com/en/library/anatomy/introduction-to-histology
An overview of histology including real tissue sections as viewed under a microscope.
Chapter 2 Homeostasis
Chapter aims
After reading this chapter, you will be able to:
define homeostasis and describe the key elements of negative feedback;
explain what is meant by the terms ‘set point’ and ‘normal range’;
explain why nurses need to know the normal physiological ranges of major variables such as body temperature, electrolytes and blood glucose;
describe how positive feedback differs from negative feedback.
Introduction
Case study: Ian – type II diabetes
Recently Ian has been waking up several times during the night to pass urine; as a result, he has a permanently dry mouth and a raging thirst which he has been able to relieve by drinking from the large bottle of water which he now keeps on his bedside cabinet. These disturbances to his sleep have left Ian permanently exhausted and of late it has been a real struggle to even get out of bed in the morning. While at work Ian has been tired and crotchety and has found it increasingly difficult to concentrate on even the simplest tasks.
During an appointment with his GP a random blood glucose reading of 24.5 mmol/l was recorded and a urine sample revealed the presence of large amounts of glucose. Ian was asked to return the following morning after skipping breakfast to have his fasting blood glucose level recorded, which at 11.2 mmol/l confirmed his GP’s suspicion of diabetes mellitus and Ian was promptly referred to his local diabetes clinic. Further tests confirmed that Ian has type II diabetes mellitus which he is currently managing via a combination of diet, exercise and drugs.
To ensure optimal health, the cells of the human body need to be maintained within a stable environment with minimal fluctuations in temperature, pH and chemical composition, otherwise pathological states may arise. This chapter will explore the mechanisms by which key internal variables of the body are maintained and balanced. We shall begin by defining homeostasis and exploring the nature of the negative feedback mechanisms that allow variables to be held within their normal ranges. We shall then examine the role of the endocrine and nervous system in maintaining homeostasis and highlight problems that may occur when these mechanisms do not function optimally. Finally, we shall introduce the concept of positive feedback and highlight how positive feedback mechanisms differ from those of negative feedback. Throughout the chapter we shall link the often abstract concepts of homeostasis to common clinical scenarios encountered by nurses.
Homeostasis
Homeostasis can be defined as the ability to maintain a relatively constant internal environment. There are multiple physical and chemical variables within the body that are subject to fluctuation; these include: temperature, pH, blood pressure and all of the dissolved components of plasma such as glucose, sodium, potassium, calcium and bicarbonate.
Negative feedback mechanisms
For each physiological variable there is an optimal value termed the ‘set point’.
For example, the set point for blood glucose is around 5 mmol/l. At this optimal concentration cells throughout the body will have a steady supply of glucose, allowing them to undertake efficient cellular metabolism. The body attempts to maintain each variable as close to its set point as possible by a process called negative feedback. During negative feedback deviations from the set point are minimised and resisted to constrain the variable within its ‘normal range’.
Requirements for negative feedback
All homeostatic processes that rely on negative feedback must have the following elements.
A stimulus: this is the trigger that causes the variable to deviate from its set point, e.g. moving to a cold environment will lower the core temperature.
A sensor: to detect deviations from the set point, e.g. the core and skin thermoreceptors that measure temperature changes.
A control centre: to decide how to bring the variable back towards its set point, i.e. the thermoregulatory centre of the hypothalamus.
Effector organs and tissues: to effect the physiological changes necessary to bring the variable back towards its set point, e.g. the sweat glands and blood vessels in the dermis of the skin that help control heat loss and retention.
Homeostatic control of blood glucose
The normal range for blood glucose is roughly between 4 and 6 mmol/l (Figure 2.1). When blood glucose rises, for example after eating a slice of sweet cake, this is detected by the pancreas and the hormone insulin is released. Insulin stimulates cells throughout the body to take up glucose from the blood; gradually blood glucose levels return back towards the set point. Conversely, if carbohydrates are not consumed for a few hours, e.g. you have just gone to bed and fallen asleep, blood sugar will fall and the hormone glucagon is released from the pancreas, stimulating the liver to release glucose increasing concentrations back towards the set point.
Figure 2.1 Homeostatic control of blood glucose via negative feedback
The
