(a) How does the placenta support the life and development of the embryo? (4 marks). b). Describe the hazards to which the embryo in the uterus may be subjected. (4 marks) (c)How is the embryo maintained until parturition? (6 marks) (d). Explain by reference to the placenta how materials in the blood pass between mother and foetus. What additional mechanisms increase the efficiency of oxygen transfer? (6 marks)

(a) How does the placenta support the life and development of the embryo? (8 marks) 

The placenta produces hormones oestrogen and progesterone; which are essential in maintaining the pregnancy; it provides a large surface area for the exchange of antibodies; glucose; amino acids and oxygen from the mother to the foetus; and nitrogenous wastes like urea; carbon (IV) oxide from the foetus to the mother; it prevents passage of harmful substances such as bacteria; viruses; the mother's blood cells etc;

(b). Describe the hazards to which the embryo in the uterus may be subjected. (4 marks)

-Alcohol; drugs such as heroin; and viruses such as rubella may all cross the placenta and harm the embryo (also poisons from cigarette smoke, AIDS virus, medicines Rhesus antibodies etc)

(c)How is the embryo maintained until parturition? (6 marks)

High levels of progesterone and oestrogen have a negative effect on the secretions of the FSH and LH; 

LH must remain high to sustain the corpus luteum; thus the outer layer of the developing cell mass forms a hormone called human chorionic gonadotropic hormone (HCG) which maintains the corpus luteum for the first three months of pregnancy; the functioning corpus luteum continues producing progesterone and oestrogen, which in turn maintain the endometrium; the endometrium and embryo thus remains in the uterus; cells of the embryo and endometrium combine to form the placenta, through which materials are exchanged between the mother and the developing embryo; approximately the fourth month of pregnancy, the placenta begins to produce oestrogen and progesterone; high levels of oestrogen and progesterone prevent further ovulation; the pregnancy is thus maintained until parturition;

(d). Explain by reference to the placenta how materials in the blood pass between mother and foetus. What additional mechanisms increase the efficiency of oxygen transfer? (6 marks)

- The concentration of glucose and amino acids etc, is higher in the maternal circulation, so dissolved

foods diffuse down the concentration gradient from the maternal to foetal circulation;

- The concentration of carbon (IV) oxide and urea is higher in the foetal circulation, so these diffuse from the foetal to the maternal circulation;

- The maternal blood entering the uterine lining contains a higher concentration of oxygen than the foetal blood entering the placenta As a result, oxyhaemoglobin in the maternal red blood cells breaks down to haemoglobin and oxygen. The oxygen diffuses across the placenta and unites with the haemoglobin in the foetal red blood cells;

- Antibodies also diffuse from the mother to the foetus, so providing some protection against disease in the early months of life;