Anatomy and Physiology II Ch. 26 - Urinary System (Part 2)
Tubular Re-absorption: in PCT
- 65% of re-absorption happens in PCT Substances Reabsorbed: - Na+: Active (energy-dependent, transport) - Glucose, Amino Acids - Cl, HCO3: Passive (Diffusion) - Water: Passive (Osmosis) - Urea: Passive (Diffusion) In Diabetes mellitus: - Concentration of glucose in filtrate exceeds rate of transport - some glucose gets left behind in the urine formed - presence detected in blood - High concentration of glucose in blood plasma (and thus in filtrate) reflected in glucose in the urine
Hormonal Regulation of urine volume: ADH
- Antidiuretic hormone (Vasopressin) - Secreted by the posterior pituitary Function: water retention Mechanism and results: - ADH stimulates the insertion of aquaporins into the collecting ducts (CD) allowing water to flow back into the blood -ADH acts on DT & CD causing both to become permeable to water (water reabsorbed from DT and CD) - Urine volume decreases
Regulation of urine volume: Renin-Angiotensin-Aldosterone System
- Decreased blood pressure in the afferent arteriole causes the JG cells to secrete Renin Renin: produced by the kidneys is an enzyme that acts on angiotensinogen to produce angiotensin I - Angiotensin I is further modified to active Angiotensin II by ACE (Angiotensin Converting Enzyme) - Angiotensin II which is the active form of angiotensin causes vasoconstriction and the secretion of aldosterone - This leads to an increase in blood pressure Urine volume decrease
Glomerular filtration-mechanism
- Movement of substances from blood (glomerulus) into Bowman's capsule- product in the tubule is called Filtrate - Filtrate: water, small molecules, ions that can pass through membrane into the BC How? - Due to pressure differences What creates pressure differences? - Glomerulus capillary network - Afferent arteriole has larger diameter than efferent arteriole - Pressurized blood enters the glomerulus through a relatively wide tube (afferent), but is forced to exit through a narrower tube (efferent) What's filtered? - 99% Water & small solutes (glucose, amino acids, electrolytes, urea, toxins, drugs) = Renal filtrate What's not filtered: based on size? - Large substances (blood cells, plasma proteins) Glomerular filtration rate (GFR): amount of filtrate produced/minute = 180L/day Average urine production/day: = 1-2L (means that most of filtrate is reabsorbed) Renal filtrate= Plasma - (Blood cells + Proteins)
Tubular secretion: Mechanism
- Potassium (K+) can be actively secreted in the DCT and CD if aldosterone is present - Hydrogen (H+) can be actively secreted into PCT, DCT, and CD to regulate pH - Excessive elimination of hydrogen can raise pH and cause metabolic alkalosis - Too little hydrogen elimination can decrease pH and cause metabolic alkalosis - Regulated secretion of H+ helps maintain blood pH at normal levels Excretion: - Anything that remains in the collecting duct will be excreted in the urine as waste during micturition - Only about 1% of the filtrate is excreted as waste - Contains urea, uric acid, creatinine, K+ toxins, drugs, etc.
Regulation of urine volume by ANP/ANH
- Produced by heart when BP increases - Inhibits ADH production - Reduces kidney's ability to concentrate urine Function: Na+ loss (also water loss) Mechanism and results: ANH acts on DT & CD to: - Decrease Na+ reabsorption - Inhibits Aldosterone - Inhibit ADH secretion - Na is excreted out and water follows Urine volume increases - Venous return is lowered, volume in right atrium decreases, ANH stops being secreted- homeostasis Atrial natriuretic hormone: Produced by right atrium of heart when stretch receptors indicate that blood volume has increased
Glomerular Filtration (GF)
- Site of GF- Renal corpuscle - Movement of materials across the filtration membrane into the Bowman's capsule to form filtrate - Filtration membrane is much more permeable than a capillary membrane - This filtration is non-specific (based on size & charge) and is driven by the blood pressure - Passive transport: along concentration gradient - GF occurs as a result of pressure gradient in the renal corpuscle called Filtration Pressure
Tubular Re-absorption (from urine back into blood)
- Tubular reabsorption is the process that returns solutes and water out of the filtrate and back into the blood - Direction: from the renal tubule into the peritubular capillaries - 99% of water & solutes from tubule fluid (renal tubules) return into blood (peritubular capillaries) - Only toxins, drugs & other chemicals remain in the filtrate - More specific: can be either active or passive - Occurs in all parts of the renal tubules -PCT, Loop of Henle and DCT
Filtration Pressure
1) Glomerular Capillary pressure, the blood pressure (50 mm Hg) within the glomerulus, moves fluid from the blood into the Bowman's capsule. 2) Capsule Hydrostatic pressure, the pressure inside Bowmans capsule (10 mm Hg) moves fluid from Bowmans capsule into the blood. 3) Blood colloid osmotic pressure, produced by the concentration of blood proteins ( 30 mm Hg) moves fluid from the bowman's capsule to the blood by osmosis. 4) Filtration pressure is equal to the glomerular capillary pressure minus the capsular hydrostatic and blood colloid osmotic pressure.
Hormonal Regulation of urine volume: Aldosterone
Function: - Primary is Na+ Retention - Leads to retaining water - Eliminates K+ Mechanism and Results: Aldosterone acts on DCT & CD to: - Increase Na+ reabsorption by increasing the Na+/K+ pump activity - Increase K+ secretion (loss) Urine volume decreases
Tubular Secretion: from blood into filtrate
Movement of non-filtered substances, toxic byproducts, drugs, molecules not produced normally by the body from blood into the urine Ex. Ammonia, urea, creatinine, penicillin, H+, etc. are secreted - Movement of substances from the blood (peritubular capillaries) into tubule fluid (urine in tubule) - Takes place in PCT, DCT, & CD - Either active or passive transport - H+ ions secreted- help in regulating pH of body fluid - When CO2 combines with water to form H+ and HCO3- - Excess H+ is exchanged for Na+ (in PCT & DCT)
Tubular Reabsorption: Distal tubule & Collecting duct
Site of action of hormones like ADH & Aldosterone - Substances reabsorbed generally under hormonal control (ADH & Aldosterone) and current conditions of the body Another 19% may be re-absorbed - Na+: active (amount depends upon aldosterone) - Cl-: passive (diffusion) - Water: passive (osmosis) -- In the presence of ADH- the tubules become more permeable to water by insertion of aquaporins
Tubular Reabsorption: Loop of Henle
The filtrate then enters the loop of Henle (descending and ascending limbs), which is responsible for first concentrating & then diluting the tubular fluid Descending limb (thin-walled): - Water: passive (osmosis) - 15% filtrate volumes gets re-absorbed - Urine becomes concentrated (from 300-1200mOsm/L) at the tip of the LoH - Urine is most concentrated at the tip of the LoH - But as it goes up the ascending loop, the concentration of urine reduces Ascending limb (thick-walled): - Na+: active - Cl-: passive (diffusion) - Concentration reduces even more to 100mOsm/L
Stages of Urine Production in Nephrons
urine formation results from the following three processes: 1. Glomerular Filtration: Filtration is the movement of materials across the filtration membrane into the Bowman capsule to form filtrate 2. Tubular reabsorption: Solutes are reabsorbed across the wall of the nephron into the interstitial fluid by transport processes, such as active transport and cotransport. Water is reabsorbed across the wall of the nephron by osmosis. Water and solutes pass from the interstitial fluid into the peritubular capillaries 3. Tubular secretion: Solutes are secreted across the wall of the nephron into the filtrate
