Human Lungs – Important Points

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 Human Lungs  Lungs are the primary organ of the respiratory system, located in the thoracic cavity. Humans have two lungs: Right lung (3 lobes) and Left lung (2 lobes) — left is smaller to accommodate the heart. Covered by a double-layered membrane called Pleura (visceral & parietal), with pleural fluid reducing friction. Main function: Gas exchange – oxygen intake and carbon dioxide removal. Air pathway: Nose → Pharynx → Larynx → Trachea → Bronchi → Bronchioles → Alveoli. Alveoli are tiny air sacs (~300 million) where actual gas exchange occurs via diffusion. Alveolar walls are lined with a single layer of squamous epithelium for fast diffusion. Surfactant (produced by Type II alveolar cells) reduces surface tension, preventing alveolar collapse. Lungs are not muscular; breathing is driven by diaphragm and intercostal muscles. Diaphragm contraction → lung volume increases → inhalation (negative pressure). Diaphragm relaxation → lung volume decreases → exhalation. Right l...

Fertilisation in Human Beings

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• Fertilisation is the fusion of a male gamete (sperm) and a female gamete (ovum).

• It usually occurs in the ampulla region of the fallopian tube.

• The sperm carries 23 chromosomes.

• The ovum also carries 23 chromosomes.

• After fusion, a diploid zygote with 46 chromosomes is formed.

• Fertilisation restores the normal chromosome number of humans.

• Millions of sperms are released, but only one sperm fertilises the ovum.

• The acrosome of the sperm helps it penetrate the ovum.

• The ovum prevents entry of additional sperms after fertilisation.

• The nucleus of the sperm fuses with the nucleus of the ovum.

• The fertilised egg is called a zygote.

• The zygote is the first cell of a new individual.

• Cleavage (rapid cell division) begins soon after fertilisation.

• The zygote moves toward the uterus while dividing.

• A morula stage is formed after repeated cell divisions.

• The morula develops into a blastocyst.

• Implantation occurs when the blastocyst attaches to the uterine wall.

• Fertilisation determines the genetic makeup of the offspring.

• The sex of the child is determined by the sperm (X or Y chromosome).

• Fertilisation marks the beginning of embryonic development.


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