• Nephron Structure

    • Long tubules along with bundles of capillaries where the blood is filtered are through the nephrons
    • one million nephrons per kidney
    • The top section of the kidney lies in the renal cortex whereas the lower section is in the medulla

    Ultrafiltration

    “Ultrafiltration is the movement of blood being forced at high pressure against the basement membrane, which optimises filtration”

    Structure:

    • Ultrafiltration occurs in the glomerulus and the glomerular filtrate (GF) before passing into the Bowman’s capsule.
    • Renal artery branches into one million arterioles where each one enters into a renal (Browman’s) capsule of a nephron.
    • The afferent arteriole divide to give the glomerulus which later forms the efferent arteriole.
    • The arteriole leading into the glomerulus is wider than the one leading out, therefore the resulting blood pressure is high in the capillaries of the glomerulus
    • The Capillary wall and the capsule walls are formed from a epithelial cells, with gaps between them. Therefore, the molecules with a molecular mass of < 68,000 are filtered out of the blood to form a filtrate in the renal capsule.
    • The inner lays of the renal capsule is lined with podocyte cells with regular intervals between them. This is optimised to allow filtrate to pass between the gaps. Filtrate passes through the gaps: not through them.
    • Endothelium of the glomerular capillaries too have spaces between them (approx. 100nm wide between cells)

    Process:

    • The renal artery splits into arterioles which run between different nephrons, called the afferent arteriole.
    • One of the branches of renal capillaries enters into the Renal’s (Bowman’s) capsule
    • The Renal capillaries form a ‘knot’ inside the capsule, the
    • The result of the high pressure , which causes resistance by:
      • Capillary epithelial cells
      • Connective tissue and epithelial cells of the capillary
      • Epithelial cells of the renal capsule a
      • Hydrostatic pressure of the fluid in the renal capsule space
      • Low water potential of the blood when in the glomerulus
    • The total resistance will prevent the filtrate from leaving the glomerular capillaries.
    • The high pressure results in the plasma moving out of the blood
    • What remains in the capillaries is only blood cells and large proteins (e.g. antibodies)
    • The kidneys produce about 180 litres of glomerular filtrate per day.
    Before Ultrafiltration After Ultrafiltration
    Red blood Cells Red blood Cells
    White blood cells White blood cells
    Platelets Platelets
    Other Large Proteins Large Proteins
    Glucose
    Water
    Urea
    Salts
    Blood plasma
    • Useful products such as glucose, salts, water etc. are reabsorbed back into the blood through the next step, Selective Reabsorption

    Selective Reabsorption

    “The absorption of some of the components of the glomerular filtrate back into the blood as the filtrate flows through the nephrons of the kidney.”

    • Selective reabsorption occurs as the glomerular filtrate flows along the proximal convoluted tubules (PCT) through the loop of Henle and along the distal convoluted tubule (DCT)
    • Useful substances leave the tubules of the nephrons and enter the capillary network
    • Epithelium of the wall of the PCT contains microvilli which proves a large surface area for the reabsorption of useful materials from the glomerular filters (in the tubules) into the blood
    • Useful solutes (i.e. glucose) are reabsorbed along the PCT via active transport and facilitated diffusion
    • Water enters the blood by osmosis
      • Water potential of blood is lower than the filter
      • Water is absorbed from the PCT, Loop of Henle, DCT and collecting duct
    • Filtrate which remains is urine which passes through the ureter to the bladder

    Urine

    • Urine comprises mostly of water and dissolves salts, urea and other substances (notably hormones and excess vitamins)
    • Urine should not normally contain proteins or blood cells
      • Due to their large size they should not be able to pass out normally
    • Glucose is actively reabsorbed back into the blood so it is not usually found in blood either