Production of Urine (HL) (HL IB Biology)

Introduction to kidney structure

• Humans have two kidneys, which remove waste products from the blood and maintain the blood's balance of water and solutes
• The renal artery supplies blood to the kidneys, while the renal vein carries blood away
• The filtrate produced by the kidneys forms urine which is transferred to the bladder via a tube called the ureter

The urinary system diagram

The kidneys are supplied with blood by the renal artery, while the renal vein carries blood away. A ureter carries urine from each kidney to the bladder.

Kidney Structure & Function Table

• The kidney itself is surrounded by an outer layer known as the fibrous capsule
• Beneath the fibrous capsule, the kidney has three main regions
  • The cortex
  • The medulla
  • The renal pelvis

Kidney structure diagram

The kidney has three main regions; the cortex, the medulla, and the renal pelvis.

• Each kidney contains thousands of tiny tubes, or tubules, known as nephrons
• Nephrons are the functional unit of the kidney and are responsible for the formation of urine
• Different parts of the nephron are found in different regions of the kidney
  • The cortex
    • Location of the glomerulus, Bowman’s capsule, proximal convoluted tubule, and distal convoluted tubule
  • The medulla
    • Location of the loop of Henle and collecting duct
  • The renal pelvis
    • All kidney nephrons drain into this structure, which connects to the ureter

The nephron diagram

The nephron spans the three regions of the kidney.

The glomerulus and Bowman's capsule

• Within the Bowman’s capsule of each nephron is a structure known as the glomerulus
  • Each glomerulus is supplied with blood by an afferent arteriole which carries blood from the renal artery
  • The afferent arteriole splits into a ball of capillaries that forms the glomerulus itself
  • The capillaries of the glomerulus rejoin to form the efferent arteriole
• Blood flows from the glomerulus into a network of capillaries that run closely alongside the rest of the nephron and eventually into the renal vein

Glomerulus and Bowman's capsule diagram

The afferent arteriole supplies the capillaries of the glomerulus, which rejoin to form the efferent arteriole

Ultrafiltration

• The glomerulus sits within the Bowman's capsule; these two structures together carry out the process of ultrafiltration
• The blood in the glomerulus is at high pressure
  • The afferent arteriole is wider than the efferent arteriole, increasing the blood pressure as the blood flows through the glomerulus
  • Note that while all capillaries exert outward pressure, forcing tissue fluid out towards the surrounding cells, the outward pressure in the glomerulus is much higher than in other capillaries
• This high pressure forces small molecules in the blood out of the capillaries of the glomerulus and into the Bowman’s capsule
  • These small molecules include
    • Chloride ions
    • Sodium ions
    • Glucose
    • Urea
    • Amino acids
• The resulting fluid in the Bowman's capsule is called the glomerular filtrate
• Large molecules such as proteins remain in the blood and do not pass into the filtrate

High blood pressure in the glomerulus forces small molecules into the Bowman's capsule, forming glomerular filtrate; this is ultrafiltration

Composition of the blood plasma compared to glomerular filtrate table

• The structures within the glomerulus and Bowman's capsule are especially well adapted for ultrafiltration
• The blood in the glomerular capillaries is separated from the lumen of the Bowman’s capsule by two cell layers with a basement membrane in between them:
  • The first cell layer is the endothelium of the capillary
    • There are gaps between the cells of the capillary endothelium known as fenestrations; fluid can pass through these gaps but not blood cells
  • The next layer is the basement membrane
    • The basement membrane is made up of a network of collagen protein and glycoproteins
    • This mesh-like structure acts as a sieve, allowing small molecules through but preventing passage of large proteins from the blood plasma
  • The second cell layer is the epithelium of the Bowman’s capsule
    • The epithelial cells have many foot-like projections which wrap around the capillary; these cells are known as podocytes and the gaps between the projections allow the passage of small molecules
• As blood passes through the glomerular capillaries the fenestrations between the capillary endothelial cells, the mesh-like basement membrane, and the gaps between the podocyte projections allow substances dissolved in the blood plasma to pass into the Bowman’s capsule
  • The substances that pass into the Bowman’s capsule make up the glomerular filtrate
  • The main substances that form the glomerular filtrate are amino acids, water, glucose, urea and salts (Na⁺ and Cl^- ions)
• Red and white blood cells and platelets remain in the blood as they are too large to pass through the fenestrations between the capillary endothelial cells
• The basement membrane stops large protein molecules from getting through

The glomerular filtrate must pass through three layers during ultrafiltration; the capillary endothelium, the basement membrane, and the Bowman’s capsule epithelium

Selective reabsorption

• Many of the substances that pass into the glomerular filtrate are useful to the body
• These substances are therefore reabsorbed into the blood as the filtrate passes along the nephron
• This process is known as selective reabsorption since not all substances are reabsorbed
  • Reabsorbed substances include water, salts, glucose, and amino acids
• Most of this reabsorption occurs in the proximal convoluted tubule
  • Note that while most water and salts are reabsorbed in the proximal convoluted tubule, the loop of Henle and collecting duct are also involved in the reabsorption of these substances
• The lining of the proximal convoluted tubule is composed of a single layer of epithelial cells which are adapted to carry out reabsorption in several ways:
  • Microvilli
    • Microvilli are tiny finger-like projections on the surface of epithelial cells which increase the surface area for diffusion
  • Co-transporter proteins
  • Many mitochondria
  • Tightly packed cells
• Once useful substances are reabsorbed, the other unwanted solutes and toxins that remain in the filtrate will be excreted in urine

Proximal convoluted tubule cross-section diagram

 The proximal convoluted tubule, seen here in cross section, has several adaptive features to aid selective reabsorption

Adaptations for selective reabsorption table

The process of selective reabsorption

• Sodium ions (Na+) are transported from the proximal convoluted tubule into the surrounding tissues by active transport
• The positively charged sodium ions creates an electrical gradient, causing chloride ions (Cl-) to follow by diffusion
• Sugars and amino acids are transported into the surrounding tissues by co-transporter proteins, which also transport sodium ions
• The movement of ions, sugars, and amino acids into the surrounding tissues lowers the water potential of the tissues, so water leaves the proximal convoluted tubule by osmosis
• Urea moves out of the proximal convoluted tubule by diffusion
• All of the substances that leave the proximal convoluted tubule for the surrounding tissues eventually make their way into nearby capillaries down their concentration gradients

Cotransport in the proximal convoluted tubule diagram

Sodium ions, as well as sugars and amino acids, are reabsorbed by the action of cotransporter proteins

Note that while diffusion occurs during this process, cotransport is considered to be an active process

• Many animals deal with the excretion of the toxic waste product urea by dissolving it in water and excreting it
• While this method of excretion works well, it brings with it the problem of water loss
• The role of the loop of Henle is to enable the production of urine that is more concentrated than the blood, and to therefore conserve water
  • Note that it is also possible to produce urine that is less concentrated than the blood; this is important when water intake is high to prevent blood becoming too dilute

The process in the loop of Henle

• Sodium and chloride ions are pumped out of the filtrate in the ascending limb of the loop of Henle into the surrounding medulla region, lowering its water potential
  • The ascending limb of the loop of Henle is impermeable to water, so water is unable to leave the loop here by osmosis
  • The water potential of the ascending limb increases as it rises back into the cortex due to the removal of solutes and retention of water
• The neighbouring descending limb is permeable to water, so water moves out of the descending limb by osmosis due to the low water potential of the medulla created by the ascending limb
  • The descending limb has few transport proteins in the membranes of its cells, so has low permeability to ions
  • The water potential of the filtrate decreases as the descending limb moves down into the medulla due to the loss of water and retention of ions
• The low water potential in the medulla created by the ascending limb also enables the reabsorption of water from the collecting duct by osmosis
• The water and ions that leave the loop of Henle for the medulla make their way into nearby capillaries
  • The capillary that flows directly alongside the loop of Henle is known as the vasa recta
  • The vasa recta also supplies oxygen to and removes carbon dioxide from the respiring cells of the loop of Henle

Water reabsorption in the loop of Henle diagram

The loop of Henle generates a steep water potential gradient across the medulla, maximising the reabsorption of water

• Living organisms can maintain a safe balance of water and solutes in their bodies by osmoregulation
  • Osmoregulation is an example of homeostasis
• The kidneys play an important role in osmoregulation by altering the amount of water reabsorbed from the glomerular filtrate into the blood
• The amount of water reabsorbed by the kidneys can be regulated by changing the permeability of the walls of the distal convoluted tubule and collecting duct to water
• The permeability of these parts of the nephron is regulated by a hormone called antidiuretic hormone, or ADH
• ADH is released from the posterior section of the pituitary gland in the brain, which is regulated by a region of the brain called the hypothalamus
  • The hypothalamus monitors the composition of the blood as it flows past osmoreceptor cells in the brain, as well as receiving signals from receptors elsewhere in the body

Osmoreceptors in the hypothalamus diagram

Blood water content is monitored by osmoreceptor cells in the hypothalamus, which then regulates the release of ADH from the posterior pituitary gland into the blood

Low blood water content

• Blood water content might drop as a result of reduced water intake, sweating, or diarrhoea
  • Low blood water content can also be referred to as high blood solute concentration
  • If blood water content gets too low it can lead to dehydration
• A reduction of blood water content is detected by the hypothalamus in the brain
• The hypothalamus causes the pituitary gland to secrete ADH into the blood
  • The target cells of ADH are in the distal convoluted tubule and collecting duct in the kidneys
• ADH increases the permeability of the walls of the distal convoluted tubule and collecting duct in the kidneys to water
  • The permeability of the walls of the distal convoluted tubule and collecting duct are increased by increasing the number of channel proteins called aquaporins in the cell surface membranes of the cells lining the nephron lumen
  • Aquaporins are stored in the membranes of vesicles in the cells that line the collecting duct; ADH causes these vesicles to fuse with the cell surface membranes, incorporating the aquaporins into the cell surface membranes
• More water is reabsorbed into the blood via the distal convoluted tubule and collecting duct
  • The activity of the loop of Henle generates a concentration gradient across the medulla, meaning that as the collecting duct descends into the medulla the osmolarity of the tissues of the medulla increases; this means that water is reabsorbed by osmosis all the way down the length of the collecting duct
• The reabsorption of water leaves a concentrated filtrate that passes through the collecting duct and into the renal pelvis
  • This remaining filtrate is the urine; from the renal pelvis it passes along the ureter to the bladder
• The blood water content increases and a small volume of concentrated urine is produced

Aquaporin vesicles diagram

ADH causes vesicles containing aquaporins to fuse with the cell surface membrane of cells that line the collecting duct, increasing the permeability of the walls of the collecting duct to water

High blood water content

• Blood water content might increase due to increased water intake or loss of salts during sweating
  • High blood water content can also be referred to as low blood solute concentration
  • If blood water content gets too high it can lead to overhydration
• High blood water content is detected by the hypothalamus
• The hypothalamus no longer stimulates the pituitary gland to release ADH and ADH levels in the blood drop
• The distal convoluted tubule and collecting duct walls become less permeable to water
  • Fewer aquaporins are present
  • The cell surface membrane is pinched inwards to reform the vesicles in which aquaporins are stored
• Less water is reabsorbed from these regions of the nephron into the blood, and the water instead passes down the collecting duct into the renal pelvis along with the rest of the filtrate
• Blood water content decreases and a large quantity of dilute urine is produced

Osmoregulation diagram

Osmoregulation is an example of homeostasis; the volume of water reabsorbed by the kidneys into the blood is regulated