Structure of the nephrone
The nephrone is the basic is the structurak and functional unit of the kidney. A human contains approx. 1 million nephrones each approx. 3cm which offers a large surface area for exchange of material. The nephrone is made up of 6 parts.
- Renal capsule made up of Bowman’s capsule and Glomerulus (blood capillaries).
- Proximal/first convoluted tubule.
- Descending limb of loop of Henle
- Ascending limb of loop of henle
- Distal convoluted tubule
- Collecting duct.
- Excretional/removal of metabolic wastes like urea, uric acid, creatinne etc in form of urine.
- Carries out osmorgulation by regulating amount of water content in blood to produce either hypotonic or hypertonic urine.
- Excretes excess salts like sodium chloride and potassium ions thereby regulating the concentration of blood.
- Regulates sodium ion level in blood through the aldosterone hormone.
- Regulate blood pH.
Process that leads to urine formation
- Ultra filtration in the Bowman’s capsule.
- Selective reabsorption in convoluted tubules, loop of Henle and collecting duct.
- Active tubular secretion in the proximal and distal convoluted tubules.
This is filtration under high pressure. The following is required for ultra filtration.
- The barrier must be constructed so as to retain molecules above a certain size, while allowing smaller ones to pass through.
- There must be high pressure to force fluid through the filter.
Adaptations for achieving the first condition
The blood passes through barriers as it is filtered from the glomerular capillary into the Bowman’s capsule.
- The endothelium of the glomerular capillary.
- The basement membrane of glomerular capillary
The epithelial cells of the Bowman’s capsule called podocytes
Adaptations of endothelium
It has numerous pores ~ 0.14m in diameter large enough to allow through all components of plasma including plasma proteins except the cells.
Adaptations of basement membrane.
- It is thin to ease diffusion of substances.
- It has pones that act as dialyzing membrane which does no allow plasma proteins to go through.
Adaptations of the epithelial cells of the capsule (podocytes)
Each cell (podocyte) bears numerous major processes from which minor foot life processes (pedicels) extend to the basement membrane of the capillary.
Narrow species lie adjacent epithelial cells which attach them to the basement membrane. These spaces allow passage of blood plasma into capsular space without giving much resistance.
Adaptations of achieving high pressure
- The diameter of the lumen with the afferent arteriole that serves blood to the glomerulus is wider than the efferent arteride which drains blood from it.
- The afferent vessels being a branch of renal artery which in turn also branches directly from the aorta is under high pressure due to the pumping action of the heart.
- The coiling of the glomeruli offers resistance for movement of blood which results in high pressure building up.
The process with ultra-filtration is also facilitated by the capillary/glomeruli lying very close to the inner wall of the capsule. The high pressure developed forces the fluid component of blood together with dissolved substances into the capsular space. This hydrostatic pressure is opposed by the osmotic pressure of plasma proteins which tends to hold plasma back in the capillaries. However this is less than the hydrostatic pressure, so there will be a pressure gradient ensuring formation of glomerular filtrate.
The glomerular filtrate has all components of blood plasma like glucose, amino acids, vitamins, some hormones, uric acid, urea, creatinine, ions, H20 except WBCs, RBCs, large plasma proteins like albumins and globulins.
The table showing the mean composition of human plasma and glomerular filtrate.
An investigation carried out to determine the rates of flow and the composition of fluids in the human kidney. These were measured at positions A, B, C, and D shown in the diagram below which represents a nephron and associated blood vessels. The results are given in the table.
Explain how the following are brought about.
- The change in flow rate between B and D.
Reduction in flow rate is due to decrease in volume of fluid because of reabsorption and widening of the bore of tube.
- The change in protein concentration between A and D.
- The change in glucose concentration between A and D.
- The change in area concentration between B and E and C and D.
The smallest plasma proteins molecules have molecular weights of about 69000. Haemoglobin has molecular weight of about 65,000. If RBCs are damaged in the blood stream, hemoglobin may appear in the urine. Comment on the significance of this observation.
By the kidney
Tissue fluid must be in osmotic balance with tissue cells; this balance is affected by the amount of H20, salts and organic compounds in the fluids and the cells. Exactly how much H20 is lost in the urine varies under different conditions. A person may need to conserve H20 if much has been lost because of sweating, illness or low H20 intake acid, or taken in food with low salt content may need to lose much. The amount of H20 lost in urine is controlled by anticiurectic hormone (ADH) a.k.a vasopressin produced by neurosecretory cells of the hypothalamus and stored in the posterior lobe of the pituitary gland.
Response to their H20 content.
- Concecrcepters in the hypothalamus detect on increase in H20 concentration in blood plasma.
It then produces the neurocecretory hormone ADH
- ADH is secreted in the posterior lobe of the pituitary gland.
- ADH then passes into the blood and finally reaches its target, the kidneys.
- Here it increases the permeability of collecting ducts and distal convoluted tubules. This result in more H20 being reabsorbed back into blood and resulting urine is therefore low in H20 …………. hence between in volume and high concede with solutes.
Response to H20 content in blood (e.g. during cold)
- Conmoreceptors in the hypothalamus detect a decrease in solute concentration of blood plasma.
- Low impulses flow to the pituitary gland.
- Less ADH is produced from the pituitary gland and therefore little reaches the kidney.
- The collecting ducts and distal convolute tubule become less permeable to water hence little H20 is reabsorbed into the blood.
- Urine with high volumes of H20 is then produced.
This is a condition in which excessive amount of watery urine is produced. It is due to lack of enough production of ADH. The person is said to suffer from diabetes inspidus.
Regulation of pH of blood and tissue fluid in human kidney at about 7.9. In the cells of the distal convoluted tubule, CO2, RNS with H20 to form H2C03 which dissociates to give H+ and HCO–3.
H+ are pumped into the lumen where they are buffered by NaHPC4.
It takes up H+ to form NaH2PO4 when the acidity of the renal fluid is exceptionally higher; the cells of the distal convoluted tubule produce NH3 form an amino acid called glutamine. NH3 combines with the excess NH+4 which are in excreted.
Hormones regulate ions by:
- Controlling their uptake from the gut.
- Controlling their release from organs which contain them in high concentration.
- Controlling their elimination of the kidney