sunlight and nutrient availability
In humans, the yolk sac is the
site of origin for blood cells and primordial germ cells. The human yolk sac is
a membrane located outside the embryo and it is connected by a tube through the
umbilical opening to the embryo's midgut. The yolk sac serves as an early site
for the formation of blood and in time, is incorporated into the primitive gut
of the embryo.
Answer:
The answer is <em><u>D.
</u></em>
Explanation:
Viruses can evade the processing and presentation of antigens, for example by interfering with the expression of MHC class I proteins, although this increases their susceptibility to be detected by natural killer cells (NK). However, some members of the herpesvirus, papillomavirus, retrovirus, poxvirus and flavivirus families have also developed strategies to escape the attack of NK cells and promote their survival, inhibiting cell apoptosis. Finally, some pathogens often change surface antigens frequently, as in the case of influenza viruses (orthomyxovirus).
The human immunodeficiency virus (HIV) affects CD4 + T cells and degrades the host's ability to counterattack with a strong cell-mediated immune response. There are so many tactics of immune evasion used by HIV, which have so far hindered the development of an effective vaccine.
In human blood, there is a compound inside the RBCs called haemoglobin which ensures that the muscle will receive enough oxygen during exercise.
<h3><u>Explanation:</u></h3>
In human blood, the red blood corpuscles contain the haemoglobin. Haemoglobin is a iron chelated compound containing porphyrin ring and a globin tail which can establish co-ordinate covalent bond with both oxygen and carbon dioxide. The bonding element depends on the concentration of these two gases. In lungs, where the oxygen concentration is more than carbon dioxide, the haemoglobin bonds with oxygen and brings it to the tissues where carbon dioxide concentration is more. This makes the haemoglobin to release oxygen and bond with carbon dioxide which is brought back to lungs. This is the process by which each and every tissue including the muscles recieve oxygen.
In muscles there is Myoglobin which is another iron-porphyrin compound which has several times more affinity for oxygen than haemoglobin. This helps to extract more oxygen from haemoglobin in muscles.
