WEEK 11 - RENAL SYSTEM & pH Flashcards
Functions of the Renal System
The kidneys are very important. (You can not live without them). They perform the overall function of eliminating wastes from the blood and regulating water levels.
What are the major functions of the renal system?
Major functions:
Regulation of water and electrolyte balance
H+ ion regulation (long term regulation of pH)
Long term regulation of BP
Excretion of wastes (metabolic and bioactive substances)
Regulation of RBC production (by releasing erythropoietin)
Conversion of vitamin D to its active form. (Vitamin D stimulates Ca2+ uptake).
Renal System
Kidneys are the body’s major excretory organs
Filter fluid from blood (~180 litres per day)
Form urine (~1.5 litres per day)
Two kidneys - forms urine
Two ureters - transport urine from kidney to bladder
Urinary bladder - stores urine
Urethra (‘aaaa’) - transports urine from bladder out of body.
Micturition is the process of releasing urine from the bladder through the urethra.
NEPHRON TUBULES:
Structural and functional units of the kidneys that carry out processes that form urine ~1 million per kidney
Renal tubule:
Bowman’s capsular space (renal or glomerular capsule)
Proximal convoluted tubule (PCT)
Loop of Henle (nephron loop)
Descending limb
Ascending (thick) limb
Distal convoluted tubule (DCT)
Collecting duct
Nephron glomerulus and peritubular capillaries
Glomerulus - a tuft of capillaries = ‘glomerular capillaries’
Renal corpuscle: glomerulus and Bowman’s capsule
Blood enters the nephron via the afferent arteriole and flows through the glomerulus
Around 20% of plasma volume filtered into glomerular capsule
If plasma is not filtered, blood flows through to the efferent arterioles onto the peritubular capillaries which wrap around the nephron
ANATOMY OF A KIDNEY
Renal cortex - ‘outer region’ lighter in colour
Most of nephrons are found here
Renal medulla – ‘inner region’ (darker)
Contains renal pyramids (cone-shaped), structures that contain collecting ducts and part of nephron
Renal pelvis - innermost portion (yellow)
Accepts urine from collecting ducts and sends it to ureter
Hilum - ‘root’ of the kidney, medial border (hilum = fissure where structures enter/exit)
Renal artery and vein enter and exit kidney
Ureter exits kidney and transports urine to urinary bladder
THREE MAJOR RENAL PROCESSES
- Filtration: filtration of fluid - water and small solutes from blood plasma (glomerulus → ‘Bowmans’ capsular space)
- Initial ‘filtrate’ has same composition as plasma but does NOT contain cells or proteins - Reabsorption: movement of substances from tubule lumen to peritubular capillary plasma
- Different substances are reabsorbed at different sites, based on body’s needs - Secretion: some solutes and toxins are secreted from the peritubular capillaries into tubules (i.e. H+)
- Excretion: sending waste - now called urine that has been produced from the above 3 processes to the bladder
The amount of any substance excreted in the urine can be calculated as: amount excreted = amount filtered + amount secreted - amount reabsorbed!
THREE MAJOR RENAL PROCESSES
- GLOMERULAR FILTRATION
- TUBULAR REABSORPTION
- TUBULAR SECRETION
RENAL PROCESS - QUESTION
Substance X:
Is filtered
A large portion is reabsorbed
That which is not reabsorbed is excreted in urine
(NORMAL PROCESS IMAGE)
Filtration membrane
Filtration barrier/membrane ensures that filtrate is free of cells and proteins.
Filtration membrane consists of glomerular endothelial wall, basement membrane and podocytes (cells with elaborate ‘foot processes’).
If protein or cells are found in the urine, it usually means that there is a breakdown in the filtration barrier.
Forces involved in glomerular filtration
The glomerular filtration rate (GFR) is the amount of blood filtered by the kidney’s glomerulus into the Bowman’s capsular space per unit of time. Hydrostatic pressure and oncotic pressure help to increase and decrease the GFR.
If the afferent arteriole is constricted = ↓ GFR (i.e. ↓ blood flow)
If the efferent arteriole is constricted = ↑ GFR (i.e. blood flow out of glomerulus is slowed down)
Chronic hypertension and diabetes can lead to damage of the glomerular capillaries which can lead to scarring and chronic renal failure
Kidney Volumes
Remember: tubular reabsorption occurs when substances are reabsorbed from the kidney tubule into the peritubular capillaries.
In essence, substances are added back to the blood.
Q). Why filter so much when the body reabsorbs 99% anyway?
A). Wastes and foreign substances can be cleared rapidly.
Allows constant and rapid adjustments to maintain homeostasis for a given substance.
Tubular Reabsorption
Isosmotic fluid (300mOsm) leaves the PCT
Filtrate becomes progressively more concentrated (↑ osmolarity, up to 12OOmOsm) as only water can leave the descending limb
Filtrate becomes progressively more dilute (↓ osmolarity, 100mOsm) as only solute can leave the ascending limb
Hormones control permeability to solute and water reabsorption in the DCT
Urine osmolarity depends on reabsorption in the collecting duct (under the control of the hormone ADH)
The combined but asymmetrical activities of the two limbs (of the Loop of Henle) create the medullary osmotic gradient
Tubular Reabsorption
Proximal Convoluted Tubule (PCT)
~65% of Na+ and H2O reabsorbed here
Virtually all nutrients reclaimed (i.e. K+, Mg+, Ca2+, Cl-)
Osmolarity of filtrate at end of PCT remains at 300 mOsM
Tubular Reabsorption cont.
Loop of Henle
Na+ and H2O are NOT coupled here
Descending limb: reabsorption of H2O only
Ascending limb: reabsorption of Na+ & other solutes but NOT H2O
Filtrate at end of loop is ~100 mOsM
