Hair germs begin from an aggregation of keratinocytes in the stratum basale of the epidermis. The initiating factor is the underlying dermal fibroblast cells. The keratinocytes elongate, divide and relocate to the dermis. Dermal fibroblasts then form a dermal papilla beneath the hair germ. This causes stimulation of the basal stem cells to up-regulate their cycle, producing cells that will keratinise and form the hair shaft. Two swellings form on the shaft, one containing stem cells for follicle regeneration, the other becomes a sebaceous gland which will secrete sebum onto the hair shaft. The follicles develop from an ectodermal bud which invades the mesenchyme during embryonic development. The mesoderm also condenses during the development creating an outer mesodermal component to the embedded part of the hair.

Hair germs begin from an aggregation of keratinocytes in the stratum basale of the epidermis. The initiating factor is the underlying dermal fibroblast cells. The keratinocytes elongate, divide and relocate to the dermis. Dermal fibroblasts then form a dermal papilla beneath the hair germ. This causes stimulation of the basal stem cells to up-regulate their cycle, producing cells that will keratinise and form the hair shaft.

Learn the two ways that oxygen moves from the lungs to the tissues, and the three ways that carbon dioxide returns from the tissues to the lungs. Rishi is a pediatric infectious disease physician and works at Khan Academy.

Learn how oxygen content (CaO2) is related to Hemoglobin concentration (Hb), oxygen saturation (SaO2), and the partial pressure of oxygen (PaO2). Rishi is a pediatric infectious disease physician and works at Khan Academy.

Learn how there are two distinct Bohr effects happening at the same time in the placenta. Rishi is a pediatric infectious disease physician and works at Khan Academy.

Although mom controls the oxygen source, the fetus has a couple of clever tricks to get the most oxygen possible! Rishi is a pediatric infectious disease physician and works at Khan Academy.

Learn how blood flows through the heart, and understand the difference between systemic and pulmonary blood flow. Rishi is a pediatric infectious disease physician and works at Khan Academy.

Watch how the fetal heart allows blood to simply bypass the lungs altogether using the Foramen Ovale and the Ductus Arteriosus! Rishi is a pediatric infectious disease physician and works at Khan Academy.

Watch as fetal blood goes through umbilical vessels and takes a shortcut through the Ductus Venosus to make it back to the heart. Rishi is a pediatric infectious disease physician and works at Khan Academy.

Want to avoid getting sick with the flu? If so, get a flu vaccine, it's 60-70% effective! You have a couple of options: TIV (dead virus, injection) or LAIV (weak virus, spray). They can cause some side effects like a sore arm (TIV) or a runny nose (LAIV), but isn't that better then lying in bed with a cough and fever for a week? Don't worry though, if you do get really sick with the flu, we have some medications that can help.

Getting the flu is awful! You get respiratory symptoms (Example: stuffy nose, sore throat, or cough) and constitutional symptoms (Example: fevers, chills, or body aches), and youre usually in bed for 3-7 days. The flu virus spreads from person to person through tiny little droplets and is really common during the winter. Learn how were getting smart about tracking the flu, and how you can avoid getting sickĚä

Find out how the your body uses special cells that are peripheral to the brain (outside the brain) to sense levels of O2, CO2, and pH. Rishi is a pediatric infectious disease physician and works at Khan Academy. These videos do not provide medical advice

Find out how to calculate exactly how much oxygen is deep down inside your lungs! Rishi is a pediatric infectious disease physician and works at Khan Academy. These videos do not provide medical advice and are for informational purposes only. The videos are not intended to be a substitute for professional medical advice, diagnosis or treatment. Always seek the advice of a qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read or seen in any Khan Academy video.

Students learn about the anatomy of the ear and how the ears work as a sound sensor. Ear anatomy parts and structures are explained in detail, as well as how sound is transmitted mechanically and then electrically through them to the brain. Students use LEGO® robots with sound sensors to measure sound intensities, learning how the NXT brick (computer) converts the intensity of sound measured by the sensor input into a number that transmits to a screen. They build on their experiences from the previous activities and establish a rich understanding of the sound sensor and its relationship to the TaskBot's computer.

The formation of the mammalian heart is a fairly complex process. It begins when angiogenic mesodermal cells in the cardiogenic plate coalesce to form the endocardial tubes. The endocardial tubes then fuse to form a single duct, the cardiac tube. This undergoes a process of distension, folding and septation and a four chambered, dual circuit pump is formed . The simple heart seen in fish or amphibians forms via the same path but development ceases at an earlier stage.

The heart is located in the thoracic cavity in between the lungs, 60% of it lying to the left of the median plane. The heart’s lateral projection extends from rib 3 to 6. Most of the heart’s surface is covered by the lungs and in juveniles it is bordered cranially by the thymus. Caudally the heart extends as far as the diaphragm. Variations in position and size exist among individuals depending on species, breed, age, fitness and pathology. Roughly speaking, the heart is responsible for about 0.75% of the bodyweight.

This lesson describes how the circulatory system works, including the heart, blood vessels and blood. Students learn about the chambers and valves of the heart, the difference between veins and arteries, and the different components of blood. This lesson also covers the technology engineers have developed to repair the heart if it is damaged. Students also understand how the circulatory system is affected during spaceflight (e.g., astronauts lose muscle in their heart during space travel).

Hepatic stellate cells (HSC) can also be referred to as vitamin A-storing cells, lipocytes, interstitial cells, fat-storing cells and Ito cells. HSC exist in the space between parenchymal cells and sinusoidal endothelial cells of the hepatic lobule and store 80% of retinoids in the whole body as retinyl palmitate in lipid droplets in the cytoplasm. In physiological conditions, these cells play pivotal roles in the regulation of retinoid homeostasis; they express specific receptors for retinol-binding protein (RBP), a binding protein specific for retinol, on their cell surface, and take up the complex of retinol and RBP by receptor-mediated endocytosis.

In this lesson on the brain's neural networks, students investigate the structure and function of the neuron. They discover ways in which engineers apply this knowledge to the development of devices that can activate neurons. After a review of the nervous system specifically its organs, tissue, and specialized cells, called neurons students learn about the parts of the neuron. They explore the cell body, dendrites, axon and axon terminal, and learn how these structures enable neurons to send messages. They learn about the connections between engineering and other fields of study, and the importance of research, as they complete the lesson tasks.

The hind brain is also called the rhombencephalon and is the brain stem that provides the connection between the spinal cord and the rest of the brain. The hind brain contains many vital structures including the Medulla Oblongata, the Pons (the link between the cerebellum, forebrain and mid-brain) and the majority of the cranial nerves, III to XII. In general the brain stem governs essential functions that are carried out sub-consciously via reflexes.