Term 2 Lecture 8: Renal Physiology Flashcards
Multiple functions of the kidneys
Excretion of metabolic waste products (ingestion/metabolism) and foreign chemicals
Regulation of water and electrolyte balance
Regulation of body fluid osmolality and electrolyte concentration
Regulation of acid-base balance- important for cardiac function and crucial to make sure respiration occurs efficiently
Also (but not covered here):
-regulation of arterial pressure
-regulation of erythrocyte production
- secretion, metabolism and excretion of hormones
- gluconeogenesis
Regulation of water balance in the kidney is directly related to regulation of blood pressure
General organisation of the kidney
See diagram end of notebook 2
2 kidneys each the size of a clenched fist (~12 cm)
Each weighing ~150g
Hilum region:
-renal artery/vein
-lymphatics
- nerve
-ureter
- tough protective fibrous capsule
Kidneys are linked to the ureter that links to the bladder from which urine is excreted via the urethra
The human left kidney is slightly higher than the right to give space to the stomach
Kidneys are situated at the back of the body so to operate on them surgeons usually enter through the back avoiding the need to move the guts to reach them.
The outer cortex surrounds the inner medulla that contains the nephrons that form the renal pyramids (the functional units of the kidneys.) Nephrons feed into the papillae and on into the renal pelvis through the minor and major calyces from which urine is transported to the urethra
Renal blood supply
Renal artery and renal vein
Renal artery is innervated by the sympathetic nervous system - activation causes construction of the renal artery and reduced blood flow to the kidneys.
Renal arteries enter the kidneys and divide into interlobular arteries.
Interlobular veins take blood away from the kidneys to the renal vein feeding into the Vena Cava and back to the heart
Interlobular veins and arteries divide into arcuate arteries and veins. They surround the renal pyramids and are close to the Bowman’s capsule - where filtration starts in the kidneys
Arcuate arteries link to the afferent arteries which move into a bolus of capillaries (the glomerular capillaries) that links to efferent arterioles and onto the peritubular capillaries surrounding the kidney tubule
Two capillary beds: glomerular and peritubular, separated by the efferent arteriole
The kidneys use a lot of O2 so need a high % of the CO ~22% ~1100ml/min
(See renal blood supply glomerular diagram)
Afferent and efferent arterioles can autoregulate, they are not innervated by the sympathetic nervous system and have no neural supply.
They have a property allowing them to autoregulate their diameter controlling blood flow to the capillary beds and maintaining glomerular flow rate.
The nephron is the functional unit of the kidney
800-1000 nephrons per kidney
Each nephron has:
1) glomerulus (glomerular capillaries)
2) long tubule segments - where fluid exchange and urine reduction happen
3) cortex and medulla
A number of nephrons feed into the same collecting duct.
No regeneration of nephrons
- ageing disease and injury can cause nephron loss
- after age 40 humans lose ~10% of their nephrons every 10 years
- in natural ageing the kidneys are able to make adaptive changes allowing the remaining nephrons to excrete proper amounts of water, electrolytes and waste products
Medulla densa
A short segment of the loop of Henle,
This group of specialised epithelial cells near the Bowman’s capsule produce the enzyme renin to convert angiotensinogen to angiotensin 1
This is important in regulation of blood pressure.
Distal nephron
-distal tubule
- connecting and cortical collecting tubules
- cortical collecting duct
- 8-10 cortical collecting ducts join to form a single collecting duct leading to the medullary collecting duct.
~25 large collecting ducts per kidney
Collect urine from ~4000 nephrons
2 types of nephron
Cortical
-glomerulus in the cortex
- short loop of Henle
-shoet distance from medulla
- makes up 70-80% of nephrons in humans
- peritubular capillaries
Juxtamedullary nephrons
- long loop of Henle
-deep into the medulla
-20-30% of nephrons
- long efferent arterioles
- specialised peritubular capillaries
- Vasa Recta
- produce concentrated urine
Desert mammals have only juxtamedullary type as their environment lacks water so they need to conserve water by producing as little urine as possible - very concentrated
Three basic renal processes
GF- glomerular filtration
Movement of fluid from blood into the lumen of the nephron
125ml of glomerular filtrate is formed collectively through all glomeruli per minute
180L (47.5 gallons) each day.
Plasma volume of an adult is 2.75 litres therefore the kidneys filter the entire plasma volume 65 times per day.
- if everything filtered was passed out through the urine the total plasma volume would be excreted in 30 mins
TR- tubular reabsorption
Substances of value to the body in the filtrate from the lumen of the tubule move back into blood flowing through the peritubular capillaries
-178.7L reabsorbed
-1.5L eliminated as urine
TS- tubular secretion
Selected transfer of substances from peritubular capillaries into the lumen - second route for substances to enter the renal tubules
(See diagram start of notebook 3)
80% of the plasma that enters the glomerulus is not filtered and leaves through the efferent arteriole
All fluid in the tubule is referred to as filtrate
GF / TR / TS summary
GF- discriminant filtration of a protein-free plasma from the glomerulus in the Bowman’s capsule
TR- selective movement of flitered substances from tubular lumen into the peritubular capillaries and into the lumen
Glomerular filtration is an indiscriminate process
(no carrier mechanisms just diffusion)
Filtrate contains water, electrolytes, waste, nutrients (but NOT plasma proteins)
Filtrate = water + dissolved solutes
Filtration fraction is 20% if 100% was filtered then there would be no fluid in the vessels for RBCs to circulate
Tubular processes work on the filtrate to return to the bloody fluid that has the necessary composition and volume to maintain internal fluid homeostasis
Glomerular filtration process
Fluid filtered from the glomerulus into the Bowman’s capsule must pass through three layers that make up the glomerular membrane
1) the fenestrated capillary wall
2) the basement membrane
3) the inner layer of the Bowman’s capsule
3 layers function as a fine molecular sieve that retains the blood cells and plasma proteins but permits water and solutes of small molecular diameter to pass through
The 3 layers of the glomerular membrane
1) glomerular capillary wall
-single layer of flattened epithelial cells
-perforated with large pores (fenestrated)
-100x more permeable to water and solutes than capillaries elsewhere in the body.
2) basement membrane
-acellular (lacks cells)
- collagen for strength and glycoproteins to discourage filtration of small plasma proteins
- only albumin (smallest plasma protein) can pass through
- glycoproteins are negatively charged and they repel albumin and other plasma proteins (neg charged also)
- plasma proteins are almost completely excluded from the filtrate
- urine is normally protein free
3) inner layer - bowman’s capsule
-podocytes
- filtration slits
- fluid leaving the glomerular capillaries enters the lumen of the Bowman’s capsule
This fluid then enters the proximal tubule and is modified via many reabsorption and secretion mechanisms as it journeys through the rest of the nephrom
Proteinuria (albumin)
If your kidneys are damaged and they’re not filtering properly at the glomerular network albumin can get through due to disruption of negative charge and this can be indicative of diseases such as nephritis (kidney disease)
Also if you exercise intensely this can cause a transient disruption of this barrier allowing albumin into the urine - this is harmless, transient and reversible (lasts less than 6 hours normally) this is sometimes referred to as psuedonephritis.
Filtration pressure (starlings force) of the glomerular capillary blood is the major force that causes glomerular filtration
1) glomerular capillary blood pressure
= Blood pressure+resistance
Favours filtration (55mmHg)
2) plasma-colloid osmotic pressures
-plasma proteins in capillaries
- water down concentration gradient (osmosis) out of Bowman’s capsule
Opposes filtration (30mmHg)
3) Bowman’s capsule hydrostatic pressure - pressure exerted by the fluid in this initial part of the tubule out of the Bowman’s capsule
Opposes filtration (15mmHg)
Net filtration pressure=
The difference between force favouring filtration and force opposing it
55mmHg - (30mmHg+15mmHg) = 10mmHg
^ as this value is positive (normally) filtration is favoured
