Homeostasis and the kidney

• Homeostasis is the process of maintaining the body in a state of dynamic equilibrium, with a set point for each condition determined by a control centre within the body
• Deviation from the set point in homeostasis is corrected by negative feedback, where detectors/receptors monitor the condition and provide input to the control centre/coordinator, which then provides output to an effector to restore the set point
• Examples of negative feedback systems in homeostasis include the role of insulin in controlling blood glucose and the role of antidiuretic hormone (ADH) in controlling blood solute potential
• Positive feedback, on the other hand, enhances the size of the stimulus, like oxytocin's action on the uterus muscles during childbirth
• The urinary system's main functions are osmoregulation, regulating the balance of water and dissolved solutes in the plasma, and removal of nitrogenous metabolic waste (urea) from the body through excretion
• In the urinary system, oxygenated blood with a high level of urea flows from the heart to the kidneys via the aorta and renal arteries, while deoxygenated blood with a lower level of urea returns to the heart via the renal veins and vena cava
• Urine, containing a high concentration of urea, is passed via the ureter to the urinary bladder for storage, until it is eliminated from the body through the urethra
• The mammalian kidney has three main regions:

  • Cortex: the outer region where renal arteries divide into tiny arterioles serving kidney tubules called nephrons; ultrafiltration and selective reabsorption occur here
  • Medulla: the central region responsible for osmoregulation
  • Renal pelvis: the origin of the ureter, collecting urine formed and passing it to the ureter for transport to the bladder
• Each kidney is composed of about 1,000,000 nephrons
• It is crucial to know the position of the different parts of the nephron within the kidney
• Different parts of the nephron have distinct roles in osmoregulation and excretion
• In the cortex:

  • Ultrafiltration occurs in the glomerulus
  • Selective reabsorption happens in the proximal convoluted tubule
  • The excretion of urea in the urine occurs
• In the medulla:

  • Urine is formed
  • Osmoregulation occurs to control the volume of water and the amount of ions lost from the blood
  • These functions are carried out by the loop of Henle and the collecting duct
• The renal pelvis collects the urine formed and passes it to the ureter for transport to the bladder
• The position of a nephron is shown in the diagram.

  
• Nephrons are arranged in columns, with their loops of Henle running parallel to one another.
• The image shows the detailed structure of the cortex.

  The key features that help you to recognise that this is the cortex are:

  • The capillaries of the glomerulus stain as a darker group of cells / tissues.
  • The Bowman’s capsule appears as a clear zone surrounding each glomerulus.
  • The cells of the proximal convoluted tubules are cuboidal (square in cross section) with distinct nuclei.

  The image shows the detailed structure of the cortex.

  
• The image shows the detailed structure of the medulla.

  The key features that help you to recognise that this is the medulla are:

  • There are cross-sections of different tubular structures of different diameters and thickness of walls.
  • Collecting ducts appear larger than the other structures in traverse section.
  • The thin and thick parts of the descending and ascending limbs of the loop of Henle are quite different.
  • Blood in the capillaries stains as a solid mass.

  
• The image shows the detailed structure of the renal pelvis.

  The key features that help you to recognise that this is the renal pelvis are:

  • There are large numbers of tubules cut in longitudinal section – along the tubule.
  • Depending on the section, you may observe the tubules emptying into a space which is the origin of the ureter.

  
• Urea is removed from the blood by the kidney nephrons through a process called ultrafiltration
• The glomerulus is surrounded by the Bowman's capsule and, together with the blood supply, filters the blood under high pressure
• The blood supply to the glomerulus enters through the afferent arteriole and exits through the efferent arteriole
• The efferent arteriole, narrower than the afferent arteriole, carries blood to two other capillary networks:

  • Capillaries surrounding the proximal and distal convoluted tubules
  • Capillaries surrounding the loop of Henle known as the vasa recta
• The afferent arteriole is wider than the efferent arteriole, increasing the hydrostatic pressure of the blood plasma
• In the glomerulus, the endothelium of the capillaries has pores between the cells to speed up the process of filtration, also known as fenestrations or filtration slits
• The cells of the inner wall of the Bowman’s capsule, called podocytes, have many processes wrapped around the capillaries of the glomerulus
• Podocytes have large gaps between their processes, called filtration slits, which allow free passage of the filtrate from the blood into the lumen of the renal capsule
• Proteins and cells are mostly too large to pass through the filtration slits in the podocytes
• Basement membranes of capillaries and podocytes act as selective barriers, allowing water and small molecules to pass from the blood into the nephron
• Pores in the basement membrane are the main filtration site, smaller than the fenestrae and filtration slits
• Electron micrographs show the close association between podocytes and capillaries
• Efficiency of filtration is increased by:

  • The ‘feet’ of the podocytes which increase surface area for filtration
  • The short distance between podocytes and the capillary, decreasing the distance substances have to travel between the blood and the filtrate in the Bowman’s capsule
  • Channels between the ‘feet’ of the podocytes increase the concentration gradient between the tissue fluid
• The difference in diameters of the afferent and efferent arterioles increases the hydrostatic pressure of the blood in the glomerulus
• Consequences of increased hydrostatic pressure in the glomerulus:

  • Small, soluble molecules are forced out of the plasma
  • These molecules enter the channels between the ‘feet’ of the podocytes and the capillary walls
  • The concentration of these molecules increases in the channels
  • They enter the Bowman's capsule and form the glomerular filtrate
• The basement membrane of the capillary acts as a molecular sieve, allowing small-sized molecules to pass through while plasma proteins and cells remain in the blood due to their larger size
• Water and small molecules pass out of the plasma due to:

  • High hydrostatic pressure of the plasma from the narrower diameter of the efferent arteriole
  • Lower osmotic pressure of the plasma compared to the filtrate due to plasma proteins, causing water from the filtrate to move back into the plasma
  • Fluid pressure in the Bowman’s capsule increases as the volume increases
• The net filtration pressure is when the hydrostatic pressure is greater than the combined effect of the osmotic pressure of the plasma and the fluid pressure of the filtrate
• Water, ions, glucose, and amino acids were originally absorbed into the blood in the intestines
• In ultrafiltration, water and most small, soluble molecules and ions are filtered out of the blood
• Many useful substances need to be reabsorbed, while urea needs to remain in the filtrate for excretion