Phenotype, skin, and musculoskeletal system
A healthy full-term infant has pink skin. The connection between the epidermis and dermis is weak, and the capillary network is dense. Lanugo hair (lanugo), which lacks a core, is absent on the face but permitted in small amounts on the trunk. Nail plates are developed and completely cover the nail bed. The auricles are formed due to well-developed cartilage.
The reproductive system is differentiated: in boys, the testes are descended into the scrotum; in girls, the labia minora are covered by the labia majora. Head size is 1/4 of the total trunk length.
Cartilage tissue predominates in the skeleton (in the epiphyses, bones of the hand and foot). The lateral fontanelles are closed by birth. The posterior fontanelle is closed in 75% of infants (in the rest, it closes by the 8th week), while the anterior fontanelle is open—its physiological closure occurs between 12 and 18 months of age.
Respiration and circulation
The nasal passages are anatomically narrow and short, so even mild rhinitis quickly obstructs breathing. The lumen of the bronchioles is narrowed, which increases the risk of bronchoobstruction and atelectasis during mucosal edema. Due to the horizontal position of the ribs and weak respiratory muscles, the infant is unable to significantly increase tidal volume under exertion.
The heart is spherical in shape and positioned high and horizontally. Blood pressure on the first day is low (about 75/20 mmHg), but then physiologically increases. The contractility of cardiomyocytes is reduced, and the conduction system of the myocardium is still developing, which causes a predisposition to arrhythmias.
Digestion and urinary excretion
The oral cavity and stomach have a small volume. Due to an imbalance of the gastric sphincters (the cardiac part is weakly developed, while the pyloric part is well developed), there is a high risk of regurgitation with overfeeding or swallowing air. The secretion of most enzymes is reduced. An exception is lactase—its activity is 10 times higher than in adults, which is an adaptation to breastfeeding. The main pathway for nutrient assimilation is intracellular digestion via pinocytosis.
The kidneys are relatively large, retain fetal lobulation, and their cortical layer is thinned. The ureters are wide and tortuous, with an underdevelopment framework, which predisposes to vesicoureteral reflux. The kidneys have poor urine-concentrating ability and excrete sodium poorly, so fluid excess easily provokes edema syndrome. It is important to note the close lymphatic connection between the intestine and kidneys, which increases the risk of infection translocation.
Nervous system and thermoregulation
The cerebral cortex is formed, but the relief of sulci and gyri is smoothed out. The spinal cord, in contrast, is the most functionally mature part of the central nervous system. Physiological sympathicotonia is characteristic due to insufficient myelination of the fibers of the autonomic nervous system.
Newborns have a high pain threshold, but tactile sensitivity is excellently developed. Vision is characterized by physiological farsightedness (hyperopia), and the gaze does not fixate. Movements are chaotic, and there is physiological hypertonus of flexor muscles and the presence of primitive reflexes.
To maintain body temperature, the infant uses non-shivering thermogenesis through the breakdown of brown adipose tissue. Due to the predominance of cold receptors, the child is more vulnerable to hypothermia than to hyperthermia.
Hematopoiesis and immunity
In the newborn's blood, fetal hemoglobin (HbF) accounts for up to 85%. In the first hours of life, physiological polycythemia is pronounced: erythrocytes reach 5.9×10¹²/L, hemoglobin up to 193 g/L. Pronounced reticulocytosis is observed, which rapidly decreases from the 2nd day of life. Under stress, hematopoiesis can return to the embryonic type (extramedullary hematopoiesis).
The hemostasis system exhibits physiological hypocoagulation—the activity of vitamin K-dependent clotting factors is reduced. This is the main risk factor for hemorrhagic disease.
The immune system is in a state of transient immunodeficiency. The bactericidal activity of leukocytes is reduced, there is a shortage of T-helper cells and interleukins, and secretory immunoglobulin A (sIgA) is completely absent on the mucosa.
The rule of the 5th day: on the 5th day of life, the "first crossover" of the leukocyte formula occurs, when the number of neutrophils and lymphocytes equalizes.
Class and exam questions
What weight and height parameters are considered normal for a full-term infant?
Normally, body weight exceeds 2500 g, and body length exceeds 46 cm. However, in 10% of healthy full-term infants, these indicators may be below or above the specified limits.
Why are newborns prone to frequent regurgitation?
They exhibit an imbalance of the gastric sphincters: the input (cardiac) section is weakly developed, while the output (pyloric) section is well developed. This provokes the return of food, especially when swallowing air (aerophagia).
What causes the physiological predisposition of infants to edema?
The concentration ability of the kidneys in newborns is reduced, and they cope poorly with the excretion of excess sodium salts. Any water or salt load easily leads to edema syndrome.
What else to study on this topic
- Peculiarities of the sebaceous and sweat glands in the newborn
- Mechanisms of non-shivering thermogenesis and brown adipose tissue
- Timing and order of fontanelle closure
- Dynamics of the leukocyte formula in the neonatal period
- Risk of developing hemorrhagic disease of the newborn
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Other topics in «Chapter 2. Neonatology»
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