BioDigital Systems: Availble for free (individual) or for a fee (groups/businesses), this interactive system will first require you to sign in via your Facebook or Google account to gain access. Once inside the system, you can zoom and rotate your virtual skeleton. Eleven systems in total are able to view and examine. Note: this site allows you to repeatedly quiz yourself on all eleven systems that are covered within the site.
Anatomy Drill & Practice: This site covers the human body, the chemical, cellular, and tissue levels of organization, the integumentary system, skeletal system, muscular system, nervous system, cardiovascular system, respiratory system, digestive system, excretory system, and reproductive system.
This site not only includes images of and information on the above listed systems, but it also includes interactive drills and practice questions for students. NOTE: Flash required for the quizzes/practice questions.
The AK Lectures are a series of lectures from a (external) educational platform designed to "promote collaboration between our users and help spread knowledge to every part of the world."
These lectures vary in length, and will open in a new window when you click on the provided link.
Structure of the Nephron: The nephron is the basic unit of structure of the kidney. Each kidney contains over a million of these tiny nephrons. Oxygenated and nutrient-filled blood enters the nephron through the afferent arteriole. This blood also contains the many waste products that must be excreted by the kidneys. The blood empties into the glomerulus, which is a network of capillaries where filtration begins. About 20% of the blood plasma is filtered into a space called the Bowman's capsule while the remaining 80% of the blood enters the efferent arteriole and is carried to the second capillary network called the vasa recta. The blood plasma that enters the Bowman's capsule is now called the glomerular filtrate or simple the filtrate. This filtrate then travels through the proximal convoluted tubule, proximal straight tubule, the descending loop of Henle, thin ascending loop of Henle, thick ascending loop of Henle, the distal convoluted tubule and finally collecting duct before it enters the ureter. Within these different segments of the nephron filtration, absorption and secretion takes place.
Structure of the Kidney: The kidney is an important organ in the body that functions to (1) excrete waste products from the body, thereby filtering the blood (2) regulate the electrolyte concentration in the blood and (3) regulate the pH of the blood. Each person contains two kidneys that are symmetrical and identical. The outermost portion of the kidney is called the renal capsule and it is a thin, transparent and fibrous membrane that protects the kidney and gives it its shape. The uppermost portion directly below this membrane is the renal cortex. It contains the Bowman's capsule, the glomerulus, the proximal and the distal convoluted tubules of the nephron. The nephron is the basic unit of structure of the kidneys. The inner layer below the cortex is called the renal medulla. It contains the vasa recta, loop of Henle and the collecting duct. The innermost portion of the kidney is a funnel-shaped cavity called the renal pelvis. It contains the renal artery and renal vein as well as the ureter.
Filtration in Renal Corpuscle (Glomerulus and Bowman's Capsule): The renal corpuscle consists of two individual structures - the glomerulus and Bowman's capsule. The glomerulus is a network of capillaries that receives oxygenated blood filled with nutrients and waste products from the afferent arteriole. It filters about 20% of that blood into the cup-shaped structure called the Bowman's capsule and sends the remaining 80% of the blood to the efferent arteriole. The glomerulus consists of two important cell types - endothelial cells, which contain small holes that allow the process of filtration and mesangial cells, which are modified smooth muscles whose contraction creates hydrostratic pressure that allows the movement of blood within the capillaries of the glomerulus. The Bowman's capsule contains the Bowman's space and a visceral (inner) as well as parietal layer (outer layer). The visceral layer that faces the glomerulus contains specialized cells called podocytes. These podocytes contain tiny slits that allow the movement of small particles and molecules into the Bowman's space. Together, the endothelial cells of the glomerulus, the basement membrane and the podocytes of the glomerulus create the three-layer membrane that allows the passage of small and positively-charged particles such as sodium and potassium ions, glucose, amino acids, small proteins, etc. Large proteins, red blood cells and platelets cannot pass through due to the limited size of the pores of the endothelial cells and slits of the podocytes. This type of filtration is known as ultrafiltration and the normal glomerular filtration rate is 125 mL/min.
Proximal Convoluted Tubule: The proximal convoluted tubule is the tubular segment of the nephron that connects the renal corpuscle to the proximal straight tubule and ultimately to the loop of Henle. It is located in the renal cortex of the medulla and functions in both reabsorption and secretion. In fact, this is where the majority of the reabsorption of electrolytes and water takes place. Approximately two-thirds of the sodium ions and water and one-hundred percent of the glucose and amino acids is reabsorbed in the proximal convoluted tubule. The reason that so much absorption takes place within this section of the nephron is due to the presence of the brush border (microvili) on the epithelial cells. The brush border greatly increases the surface area and allows the membrane proteins to absorb and secrete the different types of molecules very efficiently. The proximal convoluted tubule also secretes things such as hydrogen ions and bicarbonate molecules. This allows the nephron to regulate the pH of the blood plasma.
Contercurrent Multiplier System and Loop of Henle: The U-shaped tubular structure of the nephron that is found within the renal medulla of the kidney is called the loop of Henle. It is divided into three segments - the descending loop of Henle, the thin ascending loop of Henle and the thick ascending loop of Henle. Each one of these structures has its own unique function. The loop of Henle utilizes the countercurrent multiplier system to increase the concentration of solute and ions within the interstitium of the medulla. This ultimately allows the nephron to reabsorb more water and concentrate the urine while at the same time using as little energy as possible. The thick ascending loop of Henle is impermeable to water. It uses energy (ATP molecules) to establish an electrochemical gradient and increases the amount of ions and solutes in the interstitium of the medulla. This makes the surrounding tissue in the medulla hypertonic and increases the osmotic pressure. As a result, the descending loop of Henle, which is permeable to water but impermeable to ions, uses the gradient established by the thick ascending loop of Henle to passively move water out of the tubule and into the surrounding tissue. As water continually moves out of the descending loop of Henle, this concentrates the filtrate and makes it hypertonic towards the bottom of the Henle. Once the filtrate makes its way into the thin ascending loop of Henle, which is impermeable to water but permeable to ions, the sodium and chloride ions move out of the tubule and into the surrounding area, down their gradient.
Distal Convoluted Tubule: The distal convoluted tubule is found in the renal cortex of the kidneys and connects the thick ascending loop of Henle to the collecting duct. It serves a function in both absorption as well as secretion. However, since most of the absorption took place in the proximal convoluted tubule, only a small percentage of ions are actually absorbed here. The cuboidal epithelial cells of the distal convoluted tubule do not have microvilli like the proximal convoluted tubule cells do. Approximately 5% of the sodium and chloride is absorbed here as well as a bit of calcium. The parathyroid hormone can act on this segment of the nephron and stimulate it to absorb more calcium. Aldosterone can also act on the distal convoluted tubule and increase the amount of sodium that is reabsorbed. This also secretes hydrogen ions and potassium ions into the lumen of the tubule. The antidiuretic hormone (ADH) can act on the final portion of the distal convoluted tubule, known as the collecting tubule, and increase the amount of water that is reabsorbed by the tubule.
Collecting Duct: The final tubular segment of the nephron that is capable of reabsorption is the collecting duct. It is found in both the renal cortex and the renal medulla. Under normal conditions, the epithelial cells of the collecting duct are impermeable to water but can absorb about 5% of the total amount of sodium found in the filtrate. However, in the presence of aldosterone and ADH, the collecting duct is made permeable to water. ADH causes the storage vesicles in the epithelial cells to release aquaporin proteins onto the membrane of the cells and these aquaporin proteins allow the passage of water molecules across the cell membrane. Since the loop of Henle used the countercurrent multiplier system to establish a hypertonic interstitium, water will be reabsorbed by the cells. Aldosterone on the other hand acts to create many more sodium channels on the membrane. As more sodium is reabsorbed by the cell, even more water travels into the interstitium. At times of extreme dehydration, the collecting duct can absorb about 20% of the total amount of water found in the filtrate from the collecting duct.
Renal Clearance, Renal Plasma Flow and Glomerular Filtration Rate: Renal clearance is a measurement that is used to analyze and study the function of the kidneys. It varies from one substance to another and it tells us the volume of blood plasma that is completely cleared of a given substance over a time period by the kidney. The units of renal clearance is given in mL/min. For instance, the renal clearance of urea is 65 mL/min. This value means that 65 mL of blood plasma is completed cleared of urea every single minute. The renal plasma flow is how much blood volume actually reaches the glomerulus of the kidney every single minute while the glomerular filtration rate is the volume of blood plasma that is filtered through the glomerulus and into the Bowman's capsule. For normal kidneys, the renal plasma flow is 625 mL/min while the glomerular filtration rate is 125 mL/min. Since the tubular portion of the nephron can secrete and reabsorb substances, there are only two ways by which a given substance can be cleared by the nephron (end up in the urine). It must either be (1) filtered through the glomerulus and not reabsorbed or (2) not filtered by the glomerulus but secreted in the tubular portion of the kidney. There is a polysaccharide called inulin, which is not normally found in humans, that is usually used to determine what the glomerular filtration rate is of the kidney. This is because it is freely filtered through the glomerulus but neither secreted nor reabsorbed by the tubular portion of the nephron. Therefore the renal clearance of inulin is equal to the glomerular filtration rate of 125 mL/min. Another interesting substance is PAH (para-amino hippuric acid), which is freely filtered, fully secreted and not reabsorbed. This means that the renal clearance of PAH is equal to the renal plasma flow of 625 mL/min. Nutrients such as amino acids and glucose are freely filtered, not secreted and completely reabsorbed. This means that the renal clearance of these nutrients is 0 mL/min.