Heart rate is moderated through sympathetic and parasympathetic (vagus) nerve

From the medulla oblongata of brain to heart, nerve impulses are sent through sympathetic nerves

Parasympathetic nerves send information about oxygen levels and pH of blood to medulla

Medulla sends nerve impulses to the heart to increase/decrease heart rate

Medulla releases epinephrine via sympathetic nerve to increase heart rate or sends acetylcholine via parasympathetic nerve to decrease heart rate

Air enters respiratory system through nose or mouth and passes through pharynx to trachea

Air travels down trachea until it divides into two bronchi which lead to lungs

Inside each lung, bronchi divide into many smaller airways called bronchioles, greatly increasing surface area

Each bronchiole contains a cluster of air sacs called alveoli, where gas exchange with bloodstream occurs

• Alveoli have specialized structural features to facilitate gas exchange with blood:

  • Have very thin epithelial layer (one cell thick) to minimize diffusion distance of gasses

  • Spherical in shape to maximize surface area

  • Surrounded by a rich capillary network

  • Internal surface covered with a layer of fluid (surfactant) as dissolved gases are better able to diffuse into bloodstream

Inspiration (inhaling) and expiration (exhaling) are controlled by two sets of antagonistic (working oppositely) muscle groups

• During inspiration:

  • Diaphragm muscles contract, causing the diaphragm to flatten and increasing the volume of the thoracic cavity

  • External intercostals contract, pulling ribs upwards and outwards (expanding chest)

  • When volume of lungs increase, pressure in lungs decreases

  • Gases will move from areas of high pressure to low pressure, hence air will move into the lungs

• During expiration:

  • Diaphragm muscles relax, reducing volume of the thoracic cavity

  • Internal intercostal muscles contract, pulling ribs inwards and downwards (reducing breadth of chest)

  • When volume of lungs decreases, pressure in lungs increases

  • Gases will move from areas of high pressure to low pressure, hence air will move out of the lungs

Spermatogenesis describes the production of sperm in the seminiferous tubules of the testes

• Process begins at puberty when the germ-line epithelium of the seminiferous tubules divides by mitosis to form spermatogonia

• These cells then undergo a period of cell growth to become primary spermatocytes

• Primary spermatocytes undergo a first meiotic division to form two secondary spermatocytes

• Secondary spermatocytes undergo a second meiotic division to form four haploid daughter cells called spermatids

• Spermatids then undertake a process of differentiation in order to become functional sperm cells

Spermatogenesis is a lifelong, uninterrupted process that happens continuously