Structure And Function Of The Kidney Flashcards
What is the gross anatomy of the kidney?
In the retroperitoneal space
Between 11th thoracic and 3rd lumbar vertebra
Either side of vertebral column
Right situated slightly lower than left - to make room for the liver
~12cm long (x6x4cm)
~150g male, ~135g female
What is the renal blood supply?
25% cardiac output - mainly toward the glomerelulus, form a single abdominal aorta and out via the vena cava
90% of which supplies the renal cortex
Required to maintain highly active tubular cells
What is the microstructure of the kidney?
2 main parts, the outer part is the renal cortex and the inner part called the renal medulla.
Medulla is made up of pyramid structures where the functional parts of the kidneys are - the nephrons, they extend down the renal cortex into the medulla and out again.
There are long and short nephrons - same function just different reach into the medulla.
The nephrons feed into the bulb-like glomerulus where the blood is filtered.
The nephron acts to reabsorb anything we need back into the blood and secrete any waste.
Any waste goes into the vena cava that then feeds into the ureta and then the bladder to be excreted through the urethra.
What are the features of the urinary tract?
Filtered toxins and water leave each kidney through the ureter.
Each ureter drains into the urinary bladder.
Urine is then excreted through the urethra
What are the major functions of the kidney?
Maintains water and electrolyte homeostasis:
- Maintenance of extracellular fluid (ECF) volume and composition
Removes water soluble waste products and toxins
- Nitrogenous compounds, eg urea and creatinine
- Maintains acid-base balance - body produces a net of acid, kidney
excretes H ions, to maintain blood ph (approx 7.5)
Produces/Responds to chemical messengers (hormones)
- Role in vitamin D metabolism (1-alpha hydroxylase converts vit D to the
active form which controls calcium absorption)
- Role in haemoglobin synthesis
What are the features of the nephrons?
0.4-1.2 million per kidney
Each nephron consists of five main functional units:
- Glomerulus
- Proximal convoluted Tubule
- Loop of Henle
- Distal Tubule
- Collecting Duct
Each are structurally distinct
What is the nature and functions of the glomerulus?
Formed from specialised capillary network enclosed within the Bowman’s capsule ( outer bulb like structure)
Function: To form an ‘ultra-filtrate’ of plasma
- Blood is filtered to maintain important constituents like blood
cells in the blood but remove excess fluid, waste products and regulate
H+, Na+, K+ (filtered out across filtration barrier into renal lumen)
What is the Glomerular Filtration Barrier?
Endothelium (barrier to RBCs and WBCs)
Basement membrane (filters based on size and charge, excluding large molecular weight species (68 KDa, eg albumin), anything positively charged will be more easily filtered out)
Podocytes (also regulate on size)
Waste is glomereluar filtrate that is then passed through the nephron.
What is the glomerular filtrate?
An ‘ultra-filtrate’ of the blood enters the lumen of the glomerulus.
Composition similar to plasma except blood cells and molecules of protein >50kDa are absent.
Molecules around the size of albumin (68kDa) and larger are prevented from entering the lumen.
Proteins prevented according to charge as well as size (more negatively charged proteins retained in blood).
What are the functions of the Proximal Convoluted Tubule?
Function: Bulk reabsorption of electrolytes from the glomerular filtrate back into circulation.
- Active reabsorption:
- Na+ (~75%) and therefore chloride (through sodium potassium
ATPase pump)
- Na+ (~75%) and therefore chloride (through sodium potassium
- Passive reabsorption:
- Glucose
- Amino acids
- HCO3-
- K+
- HPO4
Secretion: - H+ (from circulation into proximal convoluted tubule, amount depends on the acid-base balance of blood) - Organic anions - Organic cations
What is the nature and functions of the Loop Of Henle?
Consists of a descending and ascending limb.
Extends from the cortex down into the medulla and back up again.
Function: Responsible for creating a hyperosmolar medulla - necessary for the production of a concentrated urine.
- Counter-current Multiplication System:
- Allows formation of a dilute urine after a H2O load
OR
- Allows formation of a concentrated urine after H2O restriction via the
action of ADH (anti-diuretic hormone) on the collecting duct (final
part of nephron)
What is counter-current multiplication?
- Descending limb is highly permeable to water, so water can easily move out into the medulla. This water is drawn out of the filtrate down a concentration gradient due to the surrounding medulla tissue being hypoosmotic. So as you go down the filtrate becomes more concentrated.
- Ascending limb is impermeable to water, but allows the blood supply to reabsorb solutes from the tubular filtrate (~25% of sodium/chloride), by pumping out Na/Cl into the vasa recta. This makes the tubular filtrate more dilate (lower osmolality). The blood supply then carries solutes into the medulla in order to retain the hypoosmolality that causes water to freely move in the descending limb (1).
- Owing to NaCl reabsorption by the thick ascending limb, tubular filtrate reaching the collecting duct is hyposmotic
- Vasa recta capillary quickly removes water that is reabsorbed into the medulla from the descending limb to maintain a high medullary osmolality. This blood supply runs in the opposite direction to the tubular filtrate. So after reabsorbing solutes from the tubular filtrate in the ascending limb the blood ascends parallel to the descending limb (2), which is water permeable, so water passes into the concentrated ascending vasa recta, reducing the osmotic tension in the blood to normal (1). This interplay between the loop of henle and the vasa recta is the counter current multiplier.
- The result of the counter-current multiplier is a hypertonic medullary fluid compared to filtrate going into the distal convoluted tubule.
- Dilute fluid enters the DCT and CD where water can be reabsorbed by passive diffusion down the concentration by the medullary hyperosmolality that has been created.
- ADH makes collecting duct permeable to water making the water move into the hyperosmotic medulla where it is drawn up by the vasa recta and returned to the body.
What is are the functions of the distal convoluted tubule?
Function: To carry out the ‘fine-tuning’ of electrolyte reabsorption or secretion.
Specifically Na+, K+, H+
Amount that occurs depends on concentration of electrolytes in plasma
Under hormonal control (e.g. Aldosterone - acts to increase sodium reabsorotion, in exchange for K and H ions)
What is are the functions of the collecting duct?
Function: To carry out the reabsorption of water
Naturally impermeable to water
If there is a need to conserve water: ADH is stimulated
- Causes aquaporins ‘water transporters’ to move into the impermeable
membrane to allow water to pass through.
- Movement of water is via passive diffusion - under the control of
osmolar difference between tubular cells and the lumen (created by the
counter-current multiplication system of the Loop of Henle)
What does the kidney need in order to function?
The kidney needs….
- Adequate perfusion (rich blood supply)
- Positive partial pressure at the glomerulus (across filtration barrier is
passive dependent on hydrostatic pressure maintained between
glomerulus and lumen)
- Viable, semi-permeable glomular membrane
- Intact functioning tubular endothelium
- Channels, transporter, gradients
- Clear passage for filtrate to travel
- Appropriate hormonal activity/ability to respond
