Renal system

• The urinary system: kidneys, ureter, urethra, bladder
• Glomerular Filteration: plasma passes through capillary wall, through the basement membrane and through the podocytes of bowmans capsule
• The two bean-shaped kidneys are supplied with blood directly from the descending aorta, via the renal arteries.
• each kidney filters the blood, removing waste products which leave the body in the form of urine and maintains the correct balance of salts and fluids within the body. 
•  As blood passes through the kidneys, a proportion of the plasma is filtered out for further processing. The rest of the blood passes through the capillaries of the kidneys and then leaves via the renal veins to the inferior vena cava. 
• Plasma passes through the nephrons where selective secretion and reabsorption processes can adjust the amount of water, salt and other substances to be excreted. The nephrons channel urine into the collecting ducts which converge into one funnel-like structure called the renal pelvis. 
• The renal pelvis connects to the ureter, which is a long tube that connects the kidney to the bladder.The bladder is a hollow, bag-like organ which is lined with smooth muscle. It stores urine until it is released (usually voluntarily), via the urethra.
• On the medial (inner) side of the kidney, there is a a dip known as the hilus. this is the point where the renal artery and vein ,renal pelvis ,renal nerve and renal lymph system connect to the kidney from the rest of the body.
• the hilus contains the renal artery (supplying oxygenated blood to the kidney) and the renal vein (draining deoxygenated blood away).
• The nephron can be thought of as a tube that is closed at one end (Bowman's capsule) and open at the other (the collecting duct).
• The glomurulus provides a supply of blood to the nephron, Each glomerulus is supplied with an afferent (into) and efferent (out of) arteriole.
• the afferent arteriole subdivides into many smaller capillaries to form the glomerulus which then re-join to form a single exit point in the form of the efferent arteriole. 
• As the blood passes through the glomerulus, around 20% of the plasma (and some of the substances dissolved in it) will be filtered into the nephron via Bowman's capsule, this means 80% passes through the glomerulus unchanged. This is important because the removal of too much fluid at this stage would increase the viscosity of the blood and cause blockages. 
• Bowman's capsule leads into the proximal tubule. The epithelial cells which form the tubule have a 'brush border', with millions of microvilli present on the inside of the tubule. This increases the available surface area, allowing efficient reabsorption of any substances.
• The proximal tubule has a high concentration of enzymes such as peptidases which break down proteins into amino acids. The amino acids are then actively transported back into the bloodstream by carrier molecules.
• The loop of Henle consists of two parts; the descending limb and ascending limb.
• The distal convoluted tubule continues the process of selective reabsorption of water and solutes from the filtrate. It also plays a role in regulating the pH of the body fluids.
• Glucose is also actively transported from the filtrate back into the bloodstream using a sodium/potassium pump.
• Amino acids and glucose are both examples of small organic molecules being actively transported back into the bloodstream.
• Water is passively absorbed across the wall of the proximal tubule due to osmosis.
• Cortical nephrons are associated with glomeruli that are closer to the cortex (surface) of the kidney and have loops of Henle that dip only a little into the medulla, while juxtamedullary nephrons have glomeruli that are deeper in the cortex and have long loops of Henle that go deep into the medulla. In humans, around 20% of the nephrons are juxtamedullary, but in species that need to conserve water, such as desert-dwelling mammals, there is a much higher proportion of juxtamedullary nephrons.
• The later part of the distal tubule eventually connects with the collecting ducts. Each collecting duct joins with approximately 6 distal tubules in the cortex and as the collecting ducts descend down into the medulla, they join together to eventually join into a renal calyx. 
• A study in 1998 painstakingly mapped all the vessels in a glomerulus and they found that the total length of the capillaries in a single glomerulus is about 0.95 cm, making a total of 19 km for all 2-million glomeruli.
• The molecular weight cut off from the glomerulus to the bowmans capsule is 70kDaltons, so anything larger than that will not be able to pass through, but smaller things can. This means that water and very small molecules such as amino acids, glucose and ions can pass through, but larger molecules such as proteins or red blood cells cannot.
• Filtrate passing through 3 layers

  1. Layer of epithelial cells in the glomerular capillaries

  2. Basement membrane made of collagen and glycoproteins

  3. Epithelial cells of Bowman's capsule, known as podocytes
• Layer of epithelial cells in the glomerular capillaries

  • Gaps allow filtering by size
  • Filters molecules less than 70kDa
• Basement membrane


  • Made of collagen and glycoproteins
  • Glycoproteins have a slight negative charge which repels negatively charged plasma proteins
  • Allows selection for size and charge
• Podocytes


  • Epithelial cells of Bowman's capsule
  • 'Podo' means feet and refers to the many foot processes that spread out from the podocytes and surround the capillaries of the glomerulus
• glomerular capillary blood pressure= High due to afferent and efferent arterioles size difference= 55mmHg

  plasma colloid osmotic pressure= opposes filtration

  bowmans capsule hydrostatic pressure= opposes filtration
• The kidneys need to filter 125ml/minute in men or 115ml/min in women, in order to make sure any toxic waste products are removed
• Extrinsic control of the glomerular filtration rate. A drop in bp will be detected by baroreceptors in the aorta and carotid artery. Causes less neuronal signalling to the brain. Activates sympathetic nervous system. Increased heart rate, increased bp, increased glomerular filtration rate.
• Myogenic mechanism

  1. Blood pressure increases

  2. Walls of arteriole are stretched

  3. Opposing contraction of smooth muscle in vessel walls
• Tubuloglomerular feedback

  1. Loop of Henle dips down

  2. Loop of Henle resurfaces into cortex as distal tubule

  3. Distal tubule passes close by glomerulus and between afferent and efferent arterioles

  4. Macula densa cells in distal tubule detect changes

  5. Granule cells surrounding afferent arteriole detect changes

  6. Cells adjust constriction of arteriole to influence filtration rate
• Tubuloglomerular feedback

  • Specialised cells within the juxtaglomerular apparatus
  • Macula densa cells in distal tubule
  • Granule cells surrounding afferent arteriole
• none of the filtered creatinine, which is a waste by-product of muscle activity, will be reabsorbed, because the body does not want to keep this molecule.
• The goal of the useful molecules is to get from inside the tubule, through the luminal membrane, through the cytoplasm of the proximal cell, through the basolateral membrane, diffuse across the interstitial fluid between the proximal cell and nearby blood vessels and then finally through the epithelial cell layer of the capillary wall, so that it can re-enter the blood stream.
• In the proximal tubule, the active reabsorption of sodium ions also leads to the passive reabsorption of water via osmosis. It is able to pass through the cells via specialised channels known as aquaporins. The proximal tubule has AQP1 channels, which are always open and the water will simply follow the sodium ions.
• The descending limb is highly permeable to water because of the presence of numerous AQP-1 aquaporin channels and unlike the rest of the nephron.
• the function of the long loops of Henle is to establish what is known as a vertical osmotic gradient
• Vasa Recta: These are blood vessels that run parallel to the Loop of Henle. They help preserve the osmotic gradient by carrying away the reabsorbed water and solutes without washing away the gradient