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REPRODUCTION IN PLANTS AND ANIMALS
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Biology Form 3
About Lesson

The Role of the Placenta

Placenta is a temporary organ found in placental mammals.

It is the only organ in animals composed of cells derived from two different organisms, the foetus and the mother.

It is a site for exchange of materials between the mother and foetus.

It is a point of close association between the maternal and foetal circulation and facilitates the transfer of nutrients, and metabolic waste products between the foetus and the mother.

 The placenta is a discrete disc-shaped structure localized in one region of the uterus wall and as it develops, it takes over from the trophoblastic villi as the principle site of exchange of materials after 12 weeks.

The placenta selectively allows some substances to pass through and not others.

Functions of placenta

  • Exchange of substances between the mother and foetus
  • Produce hormone progesterone

 

What is allowed to pass through the placenta

What is allowed to pass through the placenta

(a)       From the mother to the foetus

(i) Oxygen

(ii) Digested food substances like; glucose

(iii) Vitamins

(iv) Mineral salts

(v) Mineral hormones

(vi) Water

(vii) Antibodies and antigens.

(b)      From the foetus to mother

(i) Carbon (IV) oxide

(ii) Nitrogenous waste

(i) All the blood cells

(ii) Plasma proteins

(iii) Most bacteria

 

Drugs, alcohol and some chemicals from cigarette smoke pass through the placenta hence pregnant mothers are advised not to take alcohol or smoke excessively.

If she has to take any drug like aspirin, it has to be under the instructions of a doctor.

The foetal blood does not mix with maternal blood in the placenta. The exchange of materials occur across the sinus in the uterine wall and capillary systems of foetus across intercellular space by diffusion.

This is because there is no direct connection between the foetal blood system and that of the mother. If the two systems were directly connected, the delicate blood vessels of the foetus would burst due to the higher blood pressure in the maternal circulatory system.

During pregnancy, the placenta takes over the role of producing hormones, oestrogen and progesterone.

 

Adaptations of the Placenta to its function

  • Numerous villi to increase surface area for exchange of substances;
  • Thin epithelium to shorten diffusion distance;
  • Highly vascularised to maintain a steep concentration gradient for efficient exchange;
  • Semi-permeable lining to selectively allow exchange of substances between the mother and foetus.

 

Study Question

Apart from the destruction of the tender blood vessels of the foetus if the two blood systems were to mix directly, suggest any other effects that are likely to occur.

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