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Body Cavities, Mesenteries, and Diaphragm

Early in the fourth week of development, the intraembryonic coelom —the primordium of the body cavities—appears as a horseshoe-shaped cavity ( Fig. 9.1 A ). The bend in this cavity at the cranial end of the embryo represents the future pericardial cavity, and its limbs indicate the future pleural and peritoneal cavities. The distal part of each limb of the intraembryonic coelom is continuous with the…

Placenta and Fetal Membranes

The fetal part of the placenta and fetal membranes separate the embryo or fetus from the endometrium —the inner layer of the uterine wall. The chorion, amnion, umbilical vesicle, and allantois constitute the fetal membranes. An interchange of substances (e.g., nutrients and oxygen) occurs between the maternal and fetal blood through the placenta. The vessels in the umbilical cord connect the placental circulation with the fetal…

Fetal Period: The Ninth Week to Birth

Development during the fetal period is concerned primarily with body growth and differentiation of tissues, organs, and systems. Rudimentary organ systems were formed during the embryonic period. The rate of body growth during the fetal period is rapid, and fetal weight gain is phenomenal during the terminal weeks ( Table 7.1 ). Ultrasonographic measurements of the crown – rump length (CRL) can be used to determine…

Fourth to Eighth Weeks of Human Development

All major external and internal structures are established during the fourth to eighth weeks. By the end of this period, the main organ systems have begun to develop. Exposure of embryos to teratogens (e.g., drugs and viruses) during this period may cause major birth defects (see Chapter 19 ). As the tissues and organs form, the shape of the embryo changes so that, by the eighth…

Third Week of Human Development

Rapid development of the embryo from the trilaminar embryonic disc during the third week is characterized by: Appearance of the primitive streak Development of the notochord Differentiation of three germ layers The third week of development occurs during the week of the first missed menstrual period, that is, 5 weeks after the first day of the last normal menstrual period. Cessation of menstruation is often the…

Second Week of Human Development

Implantation of the blastocyst is completed during the second week of development. As this process takes place, changes occur, producing a bilaminar embryonic disc composed of two layers, the epiblast and hypoblast ( Fig. 4.1 A ). The embryonic disc gives rise to germ layers that form all the tissues and organs of the embryo. Extraembryonic structures forming during the second week include the amniotic cavity,…

First Week of Human Development

Development begins at fertilization when a sperm penetrates an oocyte to form a zygote. A zygote is a highly specialized, totipotent cell, which has the ability to differentiate into any type of cell. It contains chromosomes and genes derived from the mother and father. The zygote divides many times and is progressively transformed into a multicellular human being through cell division, migration, growth, and differentiation (see…

Human Reproduction

Puberty begins when secondary sex characteristics appear, usually between the ages of 10 to 13 years in females and 12 to 14 years in males. Menarche (first menstrual period) may occur as early as 8 years. Puberty in females is largely completed by age 16. Puberty in males is also largely completed by age 16; it ends when the first mature sperms are formed. Reproductive Organs…

Thrombocytopenia

Thrombocytopenia is a low platelet count below 150 × 10 9 The causes of a low platelet count can be grouped according to reduced production, decreased survival or sequestration in the spleen. Causes Reduced production ● Aplastic anaemia ● Drugs, e.g. tolbutamide, alcohol, cytotoxic agents ● Viral infections, e.g. EBV, CMV ● Myelodysplasia ● Bone marrow infiltration, e.g. carcinoma, leukaemia, myeloma, myelofibrosis ● Megaloblastic anaemia…

Polycythaemia

Polycythaemia is an increase in red cell concentration above the normal limit, usually accompanied by a corresponding increase in haematocrit and haemoglobin concentration. Polycythaemia can be ‘real’ due to a true increase in red cell concentration or ‘apparent’ due to low plasma volume (e.g. dehydration). Primary polycythaemia (polycythaemia vera) is due to a neoplasm of the myeloid cells whilst secondary polycythaemia is due to raised erythropoietin…

Leucopenia

Leucopenia is a reduction in circulating white blood cells. The normal range is 4–11 × 10 9 In practice, the commonest form is neutropenia – a deficiency of neutrophil granulocytes. Neutropenia may be selective or part of a pancytopenia. Causes Neutropenia You’re Reading a Preview Become a Clinical Tree membership for Full access and enjoy Unlimited articles Become membership If you are a member. Log…

Leucocytosis

Leucocytosis is an increase in the absolute count of circulating white blood cells. The normal range is 4–11 × 10 9 It may involve any of the types of white cells, but a polymorphonuclear leucocytosis, i.e. neutrophilia, is the most common. Causes Neutrophilia Physiological ● Neonates ● Exercise ● Pregnancy ● Childbirth ● Lactation ● Stress Bacterial infection, e.g. acute appendicitis, pyelonephritis Acute non-infective inflammation…

Clotting disorders

Disorders of clotting can present with an increased tendency to clotting (thrombophilia) or a decreased ability to form clots and therefore an increased risk of bleeding. Causes Increased clotting Congenital ● Factor V Leiden deficiency ● Protein C deficiency ● Protein S deficiency ● Antithrombin III deficiency Acquired ● Malignancy ● Pregnancy ● Infection ● Prolonged immobility ● Autoimmune: anti-phospholipid syndrome (can be related to SLE)…

Anaemia

Anaemia is defined as a haemoglobin concentration of <13.5 g/dL in men and <11.5 g/dL in women. Classification of anaemia according to red cell indices is useful because specific investigations can be tailored accordingly. Causes Microcytic (MCV <80 fL) ● Iron deficiency ● Anaemia of chronic disease ● Thalassaemia (more common in certain groups) Normocytic (MCV 80–95 fl) ● Acute blood loss ● Haemolytic anaemia ●…

Uraemia

Blood urea is a useful test of renal function. However, other factors such as liver failure, dehydration and protein breakdown may alter levels of blood urea. The normal level of blood urea is 2.5–7.8 mmol However, values may be lower in pregnancy and the newborn, and in the elderly, values may be slightly increased due to lack of renal concentration. Causes You’re Reading a Preview Become…

Respiratory alkalosis

Respiratory alkalosis is a common disorder in critically ill patients. It occurs when carbon dioxide is lost via excessive pulmonary ventilation (↑ pH, ↓ p CO 2 ). Compensatory mechanisms are: ■ ↓ HCO 3 − by bicarbonate buffer system ■ ↑ H + by kidneys (slow process – may take several days). Causes (any cause of hyperventilation) Central nervous system ● Pyrexia ● Pain ●…

Respiratory acidosis

Respiratory acidosis is caused by CO 2 retention due to inadequate pulmonary ventilation (↓ pH, ↑ pCO 2 ). Compensatory mechanisms are: ■ ↑ HCO 3 − by bicarbonate buffer system ■ ↓ H + by kidneys (can take several days). Causes (any cause of hypoventilation) (all causes of respiratory acidosis require urgent assessment) Impaired gaseous exchange ● Chronic obstructive pulmonary disease ● Alveolar disease, e.g.…