Functions of the kidney
- Primary function of the kidney are:
- Regulating blood volume
- Regulating the chemical composition of blood
- Secondary functions are:
- Metabolism of vitamin D
- Production of renin
- Production of erythropoietin
Functional anatomy of the kidney
- The kidney has three distinct regions
- Most superficial is the cortex
- Contains the glomeruli, proximal and distal convoluted tubules
- Deep to the cortex is the renal medulla
- Contains the loop of Henle and collecting duct
- The functional unit of the kidney is the nephron
The nephron
- Each kidney contains about one million nephrons
- There are two types of nephron
- Cortical nephrons - important in regulating chemical composition
of urine
- Juxtramedullary nephrons - important in concentrating urine
- Each nephron is made up of
- Glomerulus
- Bowman's capsule
- Proximal convoluted tubule
- Loop of Henle
- Distal convoluted tubule
- Several tubules enter one collecting duct
- A number of ducts run through the medullary pyramids and enter the
calyceal system
- Blood is filtered within the nephron
- Glomerular endothelium is fenestrated
- Solute-rich but protein-free fluid passes into Bowman's capsule
- Composition if the filtrate is modified as it passes through the
renal tubule
- Filtration occurs in the glomerular capillaries
- Composition is altered by absorption and secretion form the
peritubular capillaries
- This mainly occurs in the proximal convoluted tubule
- Loop of Henle has important role in water balance
Blood supply of nephron
- Glomerular capillaries
- Receive blood from afferent arteriole
- Drain into efferent arteriole
- High glomerular capillary pressure facilitates filtration
- Peritubular capillaries
- Receive blood from efferent arteriole
- Site of resorption and secretion
- Resorption assisted by low capillary pressure
- 99% of filter fluid is resorbed in the peritubular capillaries
Juxtaglomerular apparatus
- Abuts the afferent arteriole and distal convoluted tubule
- Important in regulating content of filtrate
- Cells of the distal convoluted tubule at this point are called the
Macula Densa
- Monitor sodium content of filtrate
- Juxtaglomerular cells are specialised smooth muscle cells in the
arteriole
- Act as baroreceptors
- Contain large amounts of renin
Control of renal function
Filtration
- About 20% of blood entering the glomerulus is filtered
- High pressure within the glomerular capillaries favours filtration
- Rate of filtration is known as the glomerular filtration rate (GFR)
- GFR is controlled by intrinsic and extrinsic mechanisms
- Intrinsic mechanisms include
- Myogenic regulation - changes in arteriolar smooth muscle
constriction
- Tubuloglomerular feedback - changes in response to sodium
concentration in distal convoluted tubule
- Extrinsic mechanisms include
- Sympathetic neural stimulation
- Renin-angiotensin system
Resorption
- Most resorption occurs in proximal convoluted tubule
- Primary chemical that drive most resorption is sodium
- Sodium passively diffused out of proximal convoluted tubule
- Actively transported in peritubular capillaries
- Movement of sodium has three important effects
- Creates osmotic gradient for water resorption
- Creates electrical gradient for negatively charged ions
- Allows secondary active transport in proximal convoluted tubule
Regulation of urine concentration and volume
- Maintaining the concentration of body fluids is integral to
homeostasis
- Concentration is measured in osmolarity
- A concentrated solution will have a high osmolarity
- A dilute solution will have a low osmolarity
- If blood osmolarity rises, the response will be for
- Water reabsorption to increase
- Urine volume to decrease
- If blood osmolarity falls, the response will be for
- Water reabsorption to decrease
- Urine volume to increase
Antidiuretic hormone
- Blood osmolarity is measured by specialized neurons in the
hypothalamus called osmoreceptors
- Will determine how much antidiuretic hormone (ADH) is secreted by
the posterior pituitary gland
- ADH increases water reabsorption in the collecting duct and
decreases urine volume
- When blood osmolarity rises, ADH release increases
- When blood osmolarity falls, ADH release decreases
- ADH works by increasing the permeability of the collecting ducts to
water
Aldosterone
- Produced in the renal cortex
- Increases sodium resorption and potassium excretion in distal
convoluted tubule
- Release is stimulated by:
- Low plasma sodium
- High plasma potassium
- Low blood volume and pressure
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