11.3 The Kidney
1. Define excretion.
- the removal from the body of the waste products of metabolic pathways
2. Draw and label a diagram of the kidney.
- including cortex, medulla, pelvis, ureter and renal blood vessels
3. Annotate a diagram of a glomerulus and associated nephron to show the function of each part.
4. Explain the process of ultrafiltration including blood pressure, fenestrated blood capillaries, and basement membrane.
- high blood pressure in the glomerulus
- forces 1300 ml of blood filtrate into kidney nephridia per minute (2000 L/day)
- as filtrate moves from glomerulus to nephron, it undergoes ultrafiltration
- ultrafiltration
- three layers of filtration:
- fenestrated (porous) endothelial wall of glomerulus
- basement membrane (extracellular protein) between glomerulus and Bowman’s capsule
- slits of interdigitating extensions of podocytes making up Bowman’s capsule squamous epithelium
- any substance small enough to be forced through the capillary wall and between the podocytes by blood pressure (blood serum and smaller dissolved particles) enters the lumen of the nephron tubule
- larger particles (RBCs, WBCs, platelets, proteins) are excluded from entering nephron, keeping them within the blood vessels
- ultrafiltration is nonselective, allowing water, urea, glucose, amino acids, salts, vitamins, hormones, minerals, and any other small particles to enter the nephridia
5. Define osmoregulation.
- the control of water potential and solute potential of the blood, tissue or cytoplasm of a living organism
6. Explain the reabsorption of glucose, water and salts in the proximal convoluted tubule , including the roles of microvilli, osmosis and active transport.
- glomerular filtrate moves through nephron lumen into PCT
- microvilli:
- of PCT endothelial lining greatly increase surface area for reabsorption
- include membrane proteins for facilitated diffusion of glucose, sodium ions, amino acids
- active transport:
- Na+/K+ pump drives Na+ from PCT cells into peritubular capillaries,
- decreasing N+ concentration in PCT cells
- Na+ moves down concentration gradient from PCT lumen into PCT cells
- glucose and amino acids cotransport with Na+ as it moves from lumen into PCT cells
- facilitated diffusion moves glucose and amino acids from PCT cells to peritubular capillaries
- Cl- follows Na+ from PCT lumen to PCT cells to peritubular capillaries by electrical charge
- osmosis:
- H2O follows Na+ from PCT lumen to PCT cells to peritubular capillaries by osmosis
- counter transport moves H+ from PCT cells to PCT lumen, helping control blood pH
- ultimate effect of selective reabsorption is the return of essential substances to the blood that had left the blood during ultrafiltration
7. Explain the roles of the loop of Henle, medulla, collecting duct and ADH (vasopressin) in maintaining water balance of the blood.
- loop of Henle:
- descending limb:
- water leaves by osmosis (and enters vasa recta = vessels along loop of Henle)
- due to increasing concentration of salt
- as filtrate moves from cortex to medulla;
- thus, urine becomes concentrated, or hypertonic
- ascending limb:
- salt is pumped out by active transport,
- but water is kept within lumen as walls are impermeable to water;
- thus, urine is dilute, or hypotonic
- medulla:
- creates a salt gradient, with increasingly higher osmolarity (= more concentrated = hypertonic)
- with greater depth into the medulla
- collecting duct:
- entering urine is hypotonic
- CD travels from cortex through medulla to renal pelvis
- CD endothelium is impermeable to H2O in absence of ADH, thus releasing hypotonic urine
- CD endothelium is permeable to H20 in presence of ADH, thus releasing hypertonic urine
- as CD passes through medulla it encounters increasingly hypertonic environment
- H20 reabsorbed by osmosis
- ADH:
- anti-diuretic hormone secreted by pituitary
- target cells: collecting duct endothelium
- ADH makes collecting duct endothelium permeable to H2O,
- allowing for reabsorption of H2O
- as collecting duct travels through hypertonic environment of medulla,
- thus producing a hypertonic urine
- absence of ADH
- makes collecting duct endothelium impermeable to H2O,
- allowing hypotonic urine from ascending limb of loop of Henle
- to pass through collecting duct without being affected by the hypertonic environment of the medulla;
- thus, a hypotonic urine is produced
- water potential gradients:
- water moves from higher to lower concentration, i.e., from hypotonic to hypertonic environment
- by creating the hypertonic environment of the medulla,
- the flow of hypotonic urine through the collecting duct can either be maintained,
- or altered to hypertonic, by controlling the permeability of the collecting duct to H2O as it passes through the medulla
8. Explain the differences in the concentration of proteins, glucose and urea between blood plasma, glomerular filtrate and urine.
concentration (mg/100 ml)
blood plasma glomerular filtrate urine
protein 740 0 0
glucose 90 90 0
urea 30 30 1200
- proteins:
- large molecules
- do not pass from glomerulus into nephron during ultrafiltration
- glucose:
- small moleucles
- pass from glomerulus into nephron during ultrafiltration
- reabsorped from filtrate back to blood plasma at proximal convoluted tubule
- by co-transport with sodium ions
- active transport of sodium ions
- drives glucose reabsorption by facilitated diffusion
- urea:
- small moleucles
- pass from glomerulus into nephron during ultrafiltration
- concentrated in medulla
- as water is reabsorbed
- but urea is not reabsorbed
9. Explain the presence of glucose in the urine of untreated diabetic patients.
- untreated diabetic patients have elevated blood glucose levels
- after ultrafiltration, the glomerular filtrate therefore also has elevated glucose levels
- reabsorption in the convoluted tubules cannot move enough glucose back to the blood plasma
- therefore, the urine contains glucose
