Lecture 1: The renal system
Components of renal system
Structure of nephron
Basic renal functions
- Filtration
- Reabsorption
- Secretion
Functions of kidney
– Maintain H2O and salt balance in the body
– Maintain proper osmolarity of body fluids, primarily
through regulating H2O balance
– Regulate the quantity and concentration of most ECF
ions
– Maintain proper plasma volume
– Help maintain proper acid-base balance in the body
– Excreting (eliminating) the end products (wastes) of
bodily metabolism and drug metabolites
– Producing erythropoietin (rbc formation)
– Producing renin
– Converting vitamin D into its active form
– Urine forming organs
• Kidneys
– Structures that carry urine from the kidneys to the
outside for elimination from the body
• Ureters
• Urinary bladder
• Urethra
Ureters
• Smooth muscle-walled duct
• Exits each kidney at the medial border in close proximity to renal
artery and vein
• Carry urine to the urinary bladder
Urinary Bladder
• Temporarily stores urine
• Hollow, distensible, smooth muscle lining wall
• Periodically empties to the outside of the body through the urethra
Urethra
• Conveys urine to the outside of the body
• Urethra is straight and short in females
• In males
– Much longer and follows curving course from
bladder to outside
– Dual function
• Provides route for eliminating urine from bladder
• Passageway for semen from reproductive organs
NEPHRON
• Functional unit of the kidney
• ~ 1 million nephrons/kidney
• Each nephron has two components
– Vascular component
– Tubular component
• Arrangement of nephrons within kidney gives rise to two
distinct regions
– Outer region
• Renal cortex
– Inner region
• Renal medulla
Nephron
• Vascular component
Glomerulus
• Ball-like tuft of capillaries
– From renal artery, inflowing blood passes through
afferent arterioles which deliver blood to glomerulus
– Efferent arteriole transports blood from glomerulus
– Efferent arteriole breaks down into peritubular capillaries
which surround tubular part of nephron
– Peritubular capillaries join into venules which transport
blood into the renal vein
• Water and solutes are filtered through glomerulus
as blood passes through it
Renal artery
Efferent arteriole
Afferent arteriole
Glomerulus
FILTRATION
PTC
Renal vein
2
1
4
5
• Tubular component
– Hollow, fluid-filled tube formed by a
single layer of epithelial cells
– Components
1.Bowman’s capsule
2. Proximal tubule
3. Loop of Henle
– Descending limb
– Ascending limb
4. Juxtaglomerular apparatus
5. Distal tubule
6. Collecting duct or tubule
3
6
Nephron
• Two types of
nephrons
• Distinguished by
location and length
of their structures
– Juxtamedullary
nephrons
– Cortical
nephrons
Basic Renal Processes
• Glomerular filtration
• Tubular reabsorption
• Tubular secretion
1. Glomerular Filtration
• Fluid filtered from the glomerulus into Bowman’s
capsule pass through three layers of the glomerular
membrane
– Glomerular capillary wall
• Single layer of endothelial cells
• More permeable to water and solutes than capillaries
elsewhere in the body
– Basement membrane
• Acellular gelatinous layer
• Composed of collagen and glycoproteins
– Inner layer of Bowman’s capsule
• Consists of podocytes that encircle the glomerulus tuft
Glomerular Filtration
-The glomerular filtration rate (GFR) is about 125 ml/min in a normal adult
- Ultrafiltrate is cell and protein-free and the concentration of small solutes are
the same as in plasma
- In some diseases the negative charge on the filtration barrier is lost so that
proteins are more readily filtered - a condition called proteinuria
Forces Involved in Glomerular Filtration
• Three physical forces
– Glomerular capillary
pressure
– Plasma-colloid
pressure
– Bowman’s capsule
pressure
involved
blood
osmotic
hydrostatic
Net Filtration Pressure
• Net filtration pressure = glomerular capillary
blood pressure – (plasma-colloid osmotic
pressure + Bowman’s capsule hydrostatic
pressure)
For example:
55 mm Hg – (30 mm Hg + 15 mm Hg) = 10
mm Hg
RENAL BLOOD FLOW (RBF)
Renal blood flow is 25% of the cardiac output (1.25 l/min )
Flow l/min
1.5
- RBF determines GFR
1.0
- Renal blood flow is
autoregulated between 90
and 180 mm Hg
Renal blood flow
0.5
GFR
0
0
100
200
Arterial blood pressure, mm Hg
RENAL BLOOD FLOW - AUTOREGULATION
Autoregulation uncouples renal function from arterial blood pressure
and ensures that fluid and solute excretion is constant.
1. Myogenic hypothesis
When arterial pressure increases the renal afferent arteriole is stretched
Increase of
Flow
arterial pressure
increases
Vascular smooth muscle contracts to increase resistance
Flow
Increase of
vascular tone
returns to
normal
2. Renal blood flow- autogreulation-Tubuloglomerular feedback
Alteration of tubular flow is sensed by the macula densa of the
juxtaglomerular apparatus (JGA) and produces a signal that alters
GFR.
Juxtaglomerular
(granular) cells
Efferent
Arteriole
Macula
Densa
Afferent
Arteriole
Distal
tubule
capillaries
2. REABSORPTION
Reabsorption
Reabsorb:
99% H2O
100% glucose, amino acids
99.5% Na+
50% urea.
Most of this occurs at proximal convoluted tubule.
Tubular Reabsorption
• Passive reabsorption
– No energy is required for the substance’s net
movement
– Occurs down electrochemical or osmotic
gradients
• Active reabsorption
– Occurs if any one of the steps in
transepithelial transport of a substance
requires energy
– Movement occurs against electrochemical
gradient
• An active Na+ - K+ ATPase pump in
basolateral membrane is essential
for Na+ reabsorption
• Of total energy spent by kidneys,
80% is used for Na+ transport
• Water follows reabsorbed sodium by
osmosis which has a main effect on
blood volume and blood pressure
3. SECRETION
Tubular Secretion
• Transfer of substances from peritubular
capillaries into the tubular lumen
• Involves transepithelial transport
• Kidney tubules can selectively add some
substances to the substances already
filtered
Tubular Secretion
• Most important secretory systems are for
– H+
• Important in regulating acid-base balance
• Secreted in proximal, distal, and collecting tubules
– K+
• Keeps plasma K+ concentration at appropriate level to maintain
normal membrane excitability in muscles and nerves
• Secreted only in the distal and collecting tubules under control of
aldosterone
– Organic ions
• Accomplish more efficient elimination of foreign organic compounds
from the body
• Secreted only in the proximal tubule