Mechanisms

MECHANISMS Sympathetic Division Response ReninAngiotensinAldosterone mechanism

Atrial Natriuretic Hormone (ANH) mechanism Antidiuretic Hormone (ADH) mechanism

INCREASED ↓sympathetic stimulation → vasodilation of renal arteries→ ↑renal blood flow→ ↑filtrate formation and more water is lost in the urine Inhibition of renin→ angiotensinaldosterone mechanism: ↓ADH also ↑ANH → ↓Na+ reabsorption in the distal tubules and collecting ducts → ↑Na+ and H2O in the urine

DECREASED ↑sympathetic stimulation → vasodilation of renal arteries→ ↓renal blood flow→ ↓filtrate formation and less water is lost in the urine Stimulation of renin→ angiotensin and aldosterone mechanism: ↑ADH also ↓ANH → ↑Na+ reabsorption in the distal tubules and collecting ducts → ↓Na+ and H2O in the urine ↑ANH secretion → angiotensin↓ANH secretion → angiotensin aldosterone mechanism: ↓ADH also and aldosterone mechanism: ↑ADH and ↓ANH → ↑Na+ ↑ANH → ↓Na+ reabsorption in the reabsorption in the distal tubules distal tubules and collecting ducts + and collecting ducts → ↓Na+ and → ↑Na and H2O in the urine H2O in the urine ↓ADH secretion→ ↓permeability of ↑ADH secretion and ↑thirst → distal tubules and collecting ducts ↑permeability of distal tubules to water → less water returns to and collecting ducts to water → the blood and more water is lost in more water returns to the blood the urine and less water in the urine

Effects of Blood Volume to Urine Volume and Concentration

Urine Production 21% of the blood pumped by the heart/minute flows through the kidneys • 19% of the total volume of blood plasma that flows through the glomerular capillaries passes through the filtration membrane into Bowman’s capsule to become filtrate. • 180 L of filtrate are produced/day but only 1% or less of the filtrate becomes urine •

Definition of Terms Capsular Pressure - opposes the movement of fluid into the lumen of Bowman’s capsule; caused by the pressure of filtrate already inside the Bowman’s capsule Colloid osmotic pressure - also opposes the movement of fluid into the Bowman’s capsule; caused by plasma proteins which are not allowed to enter Bowman’s capsule. Filtration - Movement of plasma through filtration membrane of renal corpuscle. Filtration pressure - forces fluid from the glomerular capillary across the filtration membrane into Bowman’s capsule Glomerular capillary pressure - Capsular Pressure - Colloid osmotic pressure Filtration Pressure Glomerular Capillary Pressure - blood pressure in the glomerular capillary which causes fluid to move from the glomerular capillary across the filtration membrane into Bowman’s capsule Reabsorption - Movement of substances from filtrate back into the blood Secretion - Transport of substances, usually waste products, into the filtrate. Urine - produced by the nephrons; consists of the substances that are filtered and secreted into the nephron minus those substances that are reabsorbed.

Filtration Glomerular Capillary Pressure is greater than both the Capsular and colloid osmotic pressure. ↑ Filtration Pressure ↑ Urine Volume ↓ Filtration Pressure ↓ Urine Volume Factors that influence FP A. BP in the glomerular capillaries B. Blood protein concentration C. Pressure in the Bowman’s Capsule A. BP in the glomerular capillaries ↑ BPGC ↑ FP

↓BPGC ↓FP

BP is normally higher in the glomerular capillaries than it is in most capillaries.  BP in the glomerular capillaries is constant because the efferent and afferent arterioles either dilate or constrict to REGULATE THE BP IN THE GLOMERULAR CAPILLARIES even though the systemic BP may fluctuate. 

B.

Blood protein concentration - Colloid osmotic pressure ↑ Blood protein concentration ↑ water movement ↓ FP ↓ Blood protein concentration ↓ water movement ↑ FP

↑ Blood protein concentration ↓ solvent, water will move to the blood through osmosis bec. it has more solute and less water molecules. C.

Pressure in the Bowman’s capsule - Capsular Pressure The concentration of blood proteins and the pressure inside the Bowman’s capsule are fairly constant. ↑ Pressure of filtrate inside the Bowman’s capsule ↓ FP

Reabsorption About 99% of the original filtrate volume is reabsorbed and enters the peritubular capillaries to the renal veins for the general circulation of blood. 1.

Proximal tubule (figure 18.11) • 65% of the filtrate is reabsorbed here. • Proteins, sugar, Sodium, Potassium, Calcium, Hydrochloric Acid, and Chlorine ions, amino acids, fructose molecules plus water through osmosis. • Cuboidal cells have numerous microvilli and mitochondria

2.

Descending Limb (Fig. 18.12a) • Functions to concentrate the filtrate by removing water and adding solutes • 15% of the filtrate is reabsorbed and the filtrate is as concentrated as the interstitial fluid of the medulla. • Permeable to water and moderately permeable to solutes • As the filtrate passes through the descending loop, water moves out of the nephron wall by osmosis, and some solutes move into the nephron by diffusion.

3. Ascending Limb (Fig. 18.12b) • Functions to dilute the filtrate by removing the sodium and chloride ions from the filtrate • Water is not permeable • Removes solutes in the solution to dilute it • Thick segment of the ascending limb actively transport sodium out of the nephron, and K+ and Cl- are cotransported with Na+ (Fig. 18.13) • Only 20% of the original filtrate volume is left • By the time the filtrate reaches the cortex of the kidney, the concentration of the filtrate is approx. 100 mOsm/L. • The filtrate is more dilute than the interstitial fluid (300 mOsm/L) of the renal cortex.

4. Distal tubule and collecting duct • Here, sodium ions and chloride are removed from the filtrate and water follows through osmosis. • 19% of the filtrate is reabsorbed, leaving only 1% of the original filtrate volume. Summary Proximal tubule • Most of the useful solutes that pass through the filtration membrane to the Bowman’s capsule are reabsorbed • 65% - 15% in the descending loop of Henle • Na+, K+, Cl+ are reabsorbed leaving the filtrate less concentrated (dilute) Distal tubule… • Furthermore, Na+ and Cl+ ions are removed, water moves out by osmosis and the filtrate volume is reduced by 19% leaving only 1% of the original filtrate concentration Secretion • Transport of substances usually, waste products, into the filtrate. • Hydrogen ions are actively secreted in the proximal tubule of the nephron. • Potassium ions are secreted in the distal tubule and collecting duct of the nephron. Potassium ions play an important role in the regulation of the body fluid pH.

Regulation of Urine Concentration and Volume Kidneys function to maintain the concentration of the body fluids by increasing water reabsorption from the filtrate when the body fluid concentration increases and by reducing water reabrsorption from filtrate when the body fluid concentration decreases. o ↑ above normal body fluid concentration : the kidney produce a smaller than normal amount of concentrated urine. o ↓ below normal body fluid concentration : the kidney produce a large volume of diluted urine. • Urine formation maintains blood volume and therefore BP. o ↑ blood volume: large volume of urine o ↓ blood volume: small volume of urine •