Renal System Flashcards
Describe the structure of the kidney
Internal structure: 3 key regions:
- cortex
- The outer layer of the kidney
is termed the renal cortex.
- the site of glomerular filtration
and the convoluted tubules. - medulla
- The inner part of the kidney is
the renal medulla.
- the location of the longer loops of Henle, and the drainage of the collecting ducts into the renal pelvis
and ureter. - pelvis
- the broadened top part of
the ureter into which the kidney
tubules drain
Pyramid shaped structures
contain kidney tubules
BLOOD VESSELS IN THE KIDNEY – renal artery divides into smaller and smaller blood vessels ending in glomerular capillaries
Describe the function of the kidney
The paired kidneys form a filtrate of the blood that is modified
by reabsorption and secretion; urine designated for excretion
moves along the ureters to the bladder.
Function of kidney!
– Regulation of water & electrolyte volume & osmolarity
* Sodium, chloride, potassium, calcium, phosphate, magnesium
– Regulation of acid-base balance
* Hydrogen ions (pH)
– Excretion of metabolic waste products and foreign chemicals
* Nitrogen
– Removal of foreign chemicals from blood
* drugs, pesticides and food additives
– Secretion of hormones:
* renin = controls the formation of angiotensin
* erythropoietin = stimulates red blood cell production
* 1,25-dihydroxy vitamin D3
Structure of the nephron
The kidney is made of individual functional units called “nephrons”
Each kidney has about 1 million nephrons
- The nephron is made up of – a filtration component (renal corpuscle = the glomerulus and Bowmans
capsule) and
– a complex set of renal tubules, which are further divided into structural and functional regions
The nephron is made up of
– Vascular components (blood vessels)
and
– a complex set of renal tubules which carries the fluid filtered by glomeruli termed the tubular filtrate
Vascular elements of the nephron and their function
- Afferent arteriole
- Supplies blood to the glomerulus - Glomerulus
- Ball of capillaries where filtration takes place - Efferent arteriole
- Carries blood away from the glomerulus - Peritubular capillaries
- Involved in exchange with tubular fluid in lumen (i.e reabsorption and secretion of substances)
———–
Renal vein (not a vasc element)
Afferent arteriole
Afferent arteriole brings blood into the glomerulus. Glomerulus is the filtration unit of the kidney. (glomerulus = Ball of capillaries that filter plasma)
Afferent arteriole gives rise to a ball of small blood vessels called capillaries. These are termed glomerular capillaries
Glomerular capillaries
Glomerular capillaries then form the efferent arteriole which then becomes peritubular capillaries. (after afferent). Blood exists the glomerulus via the efferent arteriole.
STRUCTURE OF THE NEPHRON
Tubular system
- Proximal (convoluted and straight) tubule
– drains Bowmans capsule - Loop of Henle
– thin descending limb
– thin ascending limb
– thick ascending limb
– Urinary concentration - Distal tubule
– macula densa - Collecting duct
– cortical and medullary
Function of different tubular segments
Bowman’s capsule
- Surrounds the glomerulus; collects filtrate
Proximal tubule
- Where the majority of reabsorption
(unregulated) takes place
Loop of Henle
- Important for producing urine of varying concentrations
Distal tubule/collecting
duct
- Controlled reabsorption (esp Na+ and H2O) and secretion (K+ and H+)
Renal pelvis
- Collects urine and funnels to ureter/bladder
Juxtaglomerular apparatus
Juxtaglomerular apparatus (indicated
within the blue box in the diagram) is a specialised structure formed by the distal convoluted tubule and the glomerular afferent arteriole.
its main function is to regulate blood
pressure and the filtration rate of the
glomerulus
Overview of Urine Formation in the Nephron - Summary
- afferent arteriole
- glomerulus
- efferent arteriole
- peritubular capillary
glomerular capsule
nephron tubule
The glomerulus
The glomerulus is the filtration
unit of the nephron
- The GLOMERULUS consists
– of a tuft of interconnected
capillaries (glomerular capillaries)
– a fluid filled capsule (Bowmans
capsule) - Two capillary beds in series
– glomerular capillaries
– peritubular capillaries
excretion equation
Excretion = Filtration - Reabsorption + Secretion
- Glomerular filtration
– the movement of fluid and
solutes from the glomerular
capillaries into Bowman’s space. - Tubular reabsorption
– the movement of materials from
the filtrate in the tubules into the
peritubular capillaries.
– Taking fluid back into the body! - Tubular secretion
– the movement of solutes from the peritubular capillaries into the tubules
– Removing fluid from the body
Reabsorption
- Quantitatively large
– Plasma volume ~3L
– GFR 125ml/min or 180L/day
– -> without tubular reabsorption → whole plasma volume & essential solutes excreted within 30min - Selective
– some substances almost completely reabsorbed
– many ions highly reabsorbed
– Waste products poorly reabsorbed
Where is sodium reabsorbed?
Proximal tubules – 64%
Loop of Henle – 25%
- Descending - impermeable
- Ascending- Active transport
Distal tubule & Collecting Duct – 7%
-Fine control
-aldosterone
Excreted – 1%
Where is water reabsorbed?
Proximal tubules - 67%
- Osmosis
Distal tubule & Collecting Duct – 9%
- Fine control ADH
Loop of Henle
Descending- osmosis
Ascending- impermeable
Excreted – <1%
Vasopressin (or antidiuretic hormone) increases water retention in the kidney
Vasopressin (another name antidiuretic hormone (ADH)) increases the tubular permeability in the collecting duct. Thus, the presence or the absence of vasopressin can influence the osmolarity of urine.
- ADH is synthesized in the hypothalamus.
- ADH then travels down to the posterior pituitary where it is stored until released to circulation in response to dehydration (hyperosmolarity)
- ADH is delivered via the blood
(peritublar capillaries) and stimulates V2 receptors on the basolateral membrane of the collecting duct. - This activates adenyl cyclase causing a cascade of events that results in the
insertion of ready made aqua pores into the luminal membrane of the collecting duct increasing water reabsorption.
Pathway by which ADH secretion is lowered upon drinking
a litre of soft drink and water excretion raised
Drinking a litre of soft drink
->
decrease ECF osmolarity
->
decrease ADH secretion
->
decreasing collecting duct
permeability to H20
->
decrease H20 reabsorption
->
^ H20 excreted
Situation: Dehydration
Bushwalking for 8 hour in hot weather with no water
Problem
* No water intake
* sweating
* Remember the kidney must make some urine.
– 1200 mOsm/l maximum concentrating ability of the kidney
Solution
* retain sodium and water to minimise ECF osmolarity and volume loss
* intake fluid
Body’s response
DEHYDRATION
->
^ ECF osmolarity
->
Osmoreceptors (thirst)
->
^ ADh
->
^ water reabsorption
->
decrease water excretion
->
Minimise fluid losses
Adrenal gland - zona glomerulosa cells
Mineralocorticoid - Aldosterone
* Secreted from the adrenal cortical zona glomerulosa cells
In response to
– increased plasma K+
– angiotensin II
– but plasma Na+ has little effect
Acts on renal distal tubule to increase reabsorption of Na+ from the tubule
Aldosterone increases sodium retention in the kidney
Acts on renal distal tubule to increase reabsorption of Na+ from the tubule
Transport Maximum (Tm)
For most substances that are actively reabsorbed (or secreted) there is a limit to the rate the solute can be
transported
– Transport Maximum (Tm;
mg/min)
* Due to saturation of available
carrier proteins
* Excess excreted in urine
eg Glucose Transport
– Tm 320mg/min
– Normal filtered load: 125mg/min
* no glucose in urine
– Uncontrolled diabetes mellitis
– filtered load: >320mg/min
* glucose in urine
Tubular Secretion
Tubular secretion is reabsorption in reverse which is the movement of
solutes from the peritubular capillaries into the tubules
Occurs almost completely in PCT (Proximal convoluted tubule)
Selected substances are moved from peritubular capillaries through
tubule cells out into filtrate
– K+, H+, NH4+, creatinine, organic acids and bases
– Substances synthesised in tubule cells also are secreted (example:
HCO3–)
Tubular secretion is important for:
– Disposing of substances, such as drugs or metabolites, that are bound to plasma proteins
– Eliminating undesirable substances that were passively reabsorbed (example: urea and uric acid)
– Ridding body of excess K+
(aldosterone effect)
– Controlling blood pH by altering amounts of H+ or HCO3– in urine
Hormones that affect tubular sodium and water reabsorption
…
