NEONATAL BIOLOGY – AN OVERVIEW
FACT FILE 2D
For health professionals caring for neonates, it is important to understand the unique biological features of this special group in order to accurately assess normality and any potential deviation from the ‘norm’ which may require referral and further management. This Fact file provides an overview of key specific anatomical and physiological features of the term neonate as compared to the older child and / or adult. Any further differences in the preterm neonate will also be highlighted but more detail for this group of neonates can be seen in unit 3E.
1) Respiratory system
Overall, the neonate’s respiratory system is smaller, shorter and less efficient in terms of the reserve and ability to cope with compromise. An increased basal metabolic rate means that oxygen consumption is significantly higher than that of an adult. A neonate will become tachypnoeic in response to compromise (e.g. hypoxia) rather than increasing vital capacity of the lungs as an adult would do. They have a relatively high respiratory rate and short inspiratory time and are obligatory nose breathers up to 3 months of age. Structurally, the epiglottis is floppy and situated at a higher position than a child of over 8 years of age. The narrowest part of the neonate airway is the subglottic region with a more funnel shape overall whereas, over 8 years, the airway shape is more cylindrical.
The diaphragm is the main respiratory muscle. Neonates have short necks, small mouths, narrow nares and large tongues compared with adults. Therefore, there is less airway protection and more resistance. The thorax is softer with a greater proportion of cartilage which means there is lower elastic recoil during breathing.
In the preterm neonate, there is an underdeveloped respiratory centre leading to a predisposition to apnoea of prematurity. There is also immature pulmonary function due to less alveoli growth meaning the surface area for gaseous exchange is reduced and the lung functional residual capacity is lower. This is further exacerbated by potential surfactant deficiency particularly at very early gestations meaning the alveoli are more fragile with a high surface tension. Therefore the lungs are less compliant and so more easily damaged by the shearing forces of mechanical ventilation.
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2) Cardiovascular and haematological systems
As with the respiratory system, the cardiovascular system is less efficient in terms of the reserves to deal with compromise and illness. The specific features are as follows;
The neonatal heart is less contractile than the adult heart with lower myocardial reserve. Cardiac output is dependent on rate rather than stroke volume. There is also a relatively high pulmonary vascular resistance in the first few week s of life until this lowers to reach neonatal values after the first month of life. Heart rate variability is high between resting and active values. Being born preterm means the reduction in cardiac contractility is even greater and there is less sensitivity to the constricting effects of oxygen at birth to close the ductus arteriosus in the heart; therefore this may remain open.
A neonate’s total blood volume is low (85 mls / kg). There is a higher percentage of haematocrit (45-65%) with high haemoglobin levels in the newborn period. These fall during the first 8-10 weeks of life to reach childhood values at approximately 6 months of age. In the newborn, the majority of haemoglobin is fetal (HbF)– this has an increased affinity to oxygen according to the oxygen dissociation curve with less being given up to the tissues (left shift). This HbF is rapidly broken down by haemolysis leading to a high risk of developing physiological jaundice. Red blood cells have a shorter life span in neonatal life (60-70 days approximately whereas it is up to 120 days in adults). This further increases the chance of physiological jaundice.
There is no vitamin K production until the neonate is fully fed – this, along with liver immaturity can lead to prolonged clotting times and is the rationale for administering vitamin K at birth.
In the preterm neonate, the total blood volume is further reduced, the red blood cells have an even shorter life span (approximately 30-40 days) and there is a more rapid decline in HB. Clotting times are also more prolonged.
3) Immune system
Neonates have reduced or immature specific and non-specific immune systems. For example...
Specific immunity is via the action of B and T lymphocytes which form immunoglobulins against certain diseases or antigens.
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Immunoglobulins such as IgM and IgG start to be produced in utero in mid trimester but the full quantity required for c omplete immuno-protection is not reached until at least 1 year of age. IgG can cross the placenta but then the levels will fall in the first year of life rendering the neonate prone to infection. Protection against certain diseases and general defences tak e at least the first year to build up as immunoglobulins are produced and specific antibodies are formed against certain antigens. Immunisations are essential to further assist with the build of defences against specific conditions.
Prematurity means tha t neonates will fail to receive the transfer of IgG across the placenta during the last trimester and will be further immune -compromised.
Non- specific immunity refers to the action of granulocytes which engulf bacteria and the release of Opsonins, chemi cal agents necessary for phagocytosis. Neonates have a less effective phagocytic action as well as impaired opsonic activity with relatively reduced complement levels.
4) Digestive System
The digestive tract is structurally complete at term but functional ly immature due to reduced levels of certain digestive enzymes in the first 9 -12 months of life
The first feed triggers postnatal changes in the function of the gastro -intestinal tract so that hormones are then stimulated and the gut matures. Production o f natural skin flora also commences at this stage, essential to protect the gut from infection and a source of natural vitamin K.
Gastric acid production is lowered with relatively prolonged rate of gastric emptying The cardiac sphincter of the stomach is often weak predisposing the neonate to gastro-oesophageal reflux.
Until feeding is established, the healthy newborn is able to cope with the sudden cessation of glucose via the placenta by relying on alterative substrates such as ketones for brain energy and metabolism. Following this metabolic adaptation and once feeding starts, glucose is obtained from an exogenous source and levels normalise.
5) Hepatic System
The term neonate has immature liver and hepatic enzyme systems. This results in a slower rate of metabolism of many endogenous and exogenous substances such as clearance of drugs, breakdown of haem in bilirubin metabolism and reduced formation of clotting factors. Liver metabolism is further slowed, the lower the gestation.
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Physiological jaundice is common in the term neonate due to liver enzyme immaturity (e.g. gluconeryl transferase) and even more so in prematurity.
A delay in establishing feeding can lead to slow digestive transit time leading to increased reabsorption of bilirubin back into the hepatic pathway and then circulation.
6) Renal and urinary-genital Systems
Kidney development is complete by week thirty-six of pregnancy but the nephrons continue to mature for longer. At birth, there is relatively low glomerular filtration rate (GFR) compared to the older child / adult which does not mature until approximately one year of age.
There is therefore, a poorer response to excessive fluid load with the kidneys being unable to tolerate large volumes with a lower ability to concentrate urine, excrete excess acids or solutes or drugs and toxic waste. Kidney immaturity is even greater in the preterm neonate. Increased renal vascular resistance is also present initially.
At birth, there is a contraction of the extracellular fluid volume in the immediate postnatal period, another aspect of adaptation to extra-uterine life. 75% of the birth weight is total body water with a higher extracellular to intracellular fluid ratio compared to adults. Neonates lose 10% of their body water is lost in the first week of life demonstrated by postnatal weight loss.
External genitals should be fully developed and sex differentiation should be clearly visible at birth. In the term male, the testes should have descended into the scrotum and are palpable with the scrotal skin showing deep rugae. In the female, the labia majora should cover the labia minora and clitoris fully.
In the preterm neonate, differences may be seen in the genitalia due to immaturity – for example, the testes may not have descended and the labia majora may not cover the minora, depending on the gestational age.
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7) Thermoregulatory System
The neonate has an immature thermoregulatory system and a greater predisposition to heat loss for the following reasons...
The hypothalamus, the central control centre for thermoregulation, is immature. This has a central role in non-shivering thermogenesis so this process may be hindered leading to an inability to generate heat physiologically.
Neonates have a high surface area to volume ratio with relatively large heads, a site of potentially high heat loss via exposure to cooler environmental air temperature or convective air currents, from radiation towards cooler surfaces and through direct contact with cold surfaces from conduction.
In addition, newborns due to their wet skin can easily lose significant and rapid heat through evaporation.
The most effective way to prevent heat loss by these means in the healthy, term newborn is to dry, cover the head and place on to the mother’s bare chest for skin- to-skin contact which has proven physiological benefits.
The preterm neonate particularly those at early gestations (less than 30 weeks) lack adipose tissue (brown fat) which is also a necessary component for heat generation via non-shivering thermogenesis. Therefore, they are more at risk of poor thermoregulation. They also lack keratin, the water proofing layer of the skin at early gestations meaning the risk of evaporative heat loss is even greater. Preterm and small for gestation neonates will also be lacking in subcutaneous tissue, insulation under the skin which helps prevent heat loss. As with glucose homeostasis, there is reduced ability to adapt to the extra-uterine environment after birth due to these factors and so hypothermia is a risk if not addressed properly.
8) Metabolism
At birth, a healthy term newborn will undergo metabolic adaption to extra-uterine life with regard to glucose, thermoregulatory and fluid homeostasis.
Neonates have a higher basal metabolic rate (BMR) as compared to the older child / adult necessitating a much higher glucose and oxygen need. This high energy demand is also required for the high rate of growth in the neonatal period.
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There are more limited glycogen stores particularly in the preterm and / or small for gestational age neonate who may also be lacking the nutri tional reserves (fat, iron, and vitamins) laid down normally in the third trimester of pregnancy. In addition, glycogen stores are more easily exhausted in response to compromise. The preterm neonate is less able to achieve normal metabolic adaptation to extra-uterine life.
9) Neurological System
The brain comprises a higher percentage of total body weight compared to the adult with a much higher glucose requirement for growth. The central (CNS) and peripheral nervous system is not fully complete at birth although the autonomic nervous system is. Myelination still has to complete within the CNS and development of more refined coordination, motor and developmental milestones along with higher cognitiv e functions that continue to mature over the childhood years.
Neonates in the early weeks are reflexive in nature with the presence of reflexes indicating normality in the CNS – For example; Moro, ventral suspension, sucking, rooting, grasp (palmar), plan tar, tonic neck reflex.. Eventually, the higher centres within the cortex will take precedence over the reflexive primitive behaviours and the neonate will learn eventually to engage in more purposeful movements and actions with reflexes diminishing.
The presence of an anterior and posterior fontanelle on the surface of the head indicates that the skull sutures are still soft and have not fused – this allows for early brain growth.
The preterm neonate is more vulnerable to neurological injury due to an i mmature germinal matrix within the ventricles before 34 weeks gestation and the immature vascular cerebral blood supply. Therefore, they are prone to bleeding and ischaemic damage to the brain. The preterm neonate also has a poorer ability for autoregulati on in response to compromise (the ability to maintain adequate cerebral perfusion in the event of low systemic blood pressure ). The preterm neonate has an underdeveloped brain stem along with an increased sensitivity of the vagus nerve. This means they are more prone to apnoea , bradycardia and their chemoceptor responses to hypoxia and increased CO2 are immature compared to term neonates.
10) Sensory System
A neonate’s senses are fully intact and will mature over the first few months of life. A neonate fro m birth is able to see, hear, taste, smell, respond to touch and feel pain.
Page | 6 J Petty They are able to fix and follow on an object of interest or face from birth if the object is placed at the appropriate distance from the eyes (no greater than 20 centimetres) with full visual acuity at around 6 months. The red reflex should be elicited on a formal examination which shows reflection from the retina. Pupils should constrict to light rapidly and equally on both sides. Their heads turn to sounds and they will startle. They are responsive to positive touch, kinaesthetic stimulation, (e.g. rocking), skin to skin contact, all of which can soothe and console them. They respond to pain stimuli by exhibiting key pain cues – crying, bulging of the brow, grimace of face and eyes, agitation, tensing and increased heart rate. They can differentiate different tastes (sweet / sour) and can smell breast milk and mother’s skin.
The Behaviour and appearance of the newborn along with the musculoskeletal and integumentary (skin) systems and normal parameters will be discussed in the following Unit 2E.
NB: All images are taken from http://commons.wikimedia.org/ where permission is granted for use
KEY READING
GENERAL ANATOMY AND PHYSIOLOGY WEBSITES ... GRAYS ANATOMY ONLINE http://www.bartleby.com/107/ ONLINE LEARNING CENTRE FOR A and P - Anatomy and Physiology, 6/e Rod Seeley, http://highered.mcgraw-hill.com/sites/0070272468/student_view0/ http://highered.mcgraw-hill.com/sites/0072351136/student_view0/getbodysmart.html Interactive Body Guide by Merck Source http://www.mercksource.com/ppdocs/us/cns/content/adam/visualbody/frameParent.html
Get Body Smart – online Textbook on Human A and P – visual learning http://www.getbodysmart.com/ http://www.innerbody.com/htm/body.html http://en.wikipedia.org/wiki/Human_anatomy
ANATOMY AND PHSYIOLOGY OF THE NEONATE
Astuto M, Paratore AL and Gullo A Anatomy and Physiology in Neonates and Children Chapter 1 in Astuto M (ed) Anaesthesia, Intensive Care and Pain in Neonates and Children (2009) Springer ; Milan
Aylott, M. (2006a) The Neonatal energy triangle part 1; Metabolic adaptation. Paediatric Nursing . 18(6):38-42.
Aylott, M. (2006b) The Neonatal energy triangle part 2; Thermoregulatory and respiratory adaptation Paediatric Nursing . 18(7):38-43
Baston H and Durward H (2010) Examination of the Newborn – A Practical Guide (2 nd Edition) Routledge, London
Davies L and McDonald S (2008) Examination of the Newborn and Neonatal Health: A Multidimensional Approach: A Multidimensional Approach ; Churchill Livingstone, Oxford
English PJ and Wilding JPH (2006) Applied Physiology; The control of weight Current Paediatrics 16, 439-446
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Gennery AR, Cant AJ (2006) Applied Physiology; Immune competence Current Paediatrics 16, 447-452
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Jacob S (2008) Human Anatomy; A clinically orientated approach Churchill Livingstone; Oxford
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Lote CJ (2006) Applied Physiology; The renal tubule Current Paediatrics 16, 453-457
MacGregor J (2008) Introduction to the Anatomy and Physiology of Children: A Guide for Students of Nursing, Child Care and Health Routledge; London http://www.amazon.co.uk/Introduction-Anatomy-Physiology-Children- Students/dp/0415446244/ref=sr_1_2?ie=UTF8&s=books&qid=1265294936&sr=1- 2#reader_0415446244
Milner AD and Greenough A (2006) Applied Respiratory Physiology Current Paediatrics 16, 406-41
Netter FH (2010) Atlas of Human Anatomy; Professional edition (5th edition) Saunders, Philadelphia
Petty.J. (2010) Normal Postnatal Adaptation to extra-uterine life Part b- Thermoregulation and glucose homeostasis Journal of Neonatal Nursing 16, 5, 198-199
Petty.J. (2011) Fact sheet: Neonatal Biology – An Overview parts 1, 2 and 3 – Series in Journal of Neonatal Nursing From 2011
Schoenwolf GC, Bleyl SB, Brauer PR and Francis West PH (2008) Larsens Human Embryology (4 th edition) Churchill Livingstone; Oxford
Skinner S (2000) Understanding clinical investigations- a quick reference guide Bailliere Tindall, London
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Tortora GJ and Grabowski S (2003) Principles of Anatomy and Physiology (10 th Edition ) John Wiley and sons: Chichester
Tortora GJ, and Derrikson BH (2009) Principles of anatomy and physiology. (8 th Edition) John Wiley and sons: Chichester
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