Ch. 17; Power point

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The urinary System: Filtration and Fluid Balance

Urine Formation

* The kidney controls fluid and electrolyte balance by controlling urine volume and composition.
* In order to form urine, the nephron must perform three processes:
-Glomerular filtration
- Tubular re-absorption
- Tubular secretion

Filtration

* Fluid and molecules pass from the glomerular capillaries into the glomerular capsule.
-Passes across a filter composed of the walls of the capillaries and the podocytes of the glomerular capsule.
-Flow into the renal tubule where the chemistry is controlled by reaborption and secretion.

Filtration

*Fluid and molecules pass from the glomerular capillaries into the glomerular capsule.
- Move fluid and chemicals into the nephron from blood.
-Glomerular filtrate is chemically similar to blood.

Reabsorption and Secretion

* Urine is chemically different from plasma because reabsorption and secretion control the concretion of chemicals and volume of urine
-Reabsorption- substances pass from the renal tubule into the peritubular capillaries and return to the blood stream.
-Secretion- Substances pass through the peritubular capillaries into the renal tubule and eventully leave the body as urine

Reabsorption and Secretion

* Urine is chemically different from plasma because reabsorption and secretion control the concentration of chemicals and volume of urine.
-Some substances, like glucose, are completely reabsorbed while substances like metabolic waste products (urea and creatinine) are almost completely secreted in urine.

Control of Filtration

*Filtration is controlled by several factors.
-Filter size- determines what gets through the filter
*Podocytes and capillary walls of the renal corpuscle create a filter with fixed openings.
* Plasma, and many of the substances dissolved in plasma, pass through the filter.
*Blood cells, platelets and large molecules, can't get into a healthy kidney,

Control of filtration

* Filtration is controlled by several factors
- Pressure
- Higher pressure on side of the filter allows chemicals to be pushed though the filter more quickly.
- Higher blood pressure in the glomerular capillaries increases filtration, while lower pressure decreases filtration.

Control of Filtration

*Changes in blood pressure change filtration rate
- Minor changes in systolic blood pressure do not change glomerular pressure because it is protected by a mechanism called autoregulation.
- As systemic BP increases, the afferent arterioles leading into the glomerulus constrict, decreasing the amount of blood getting into the glomerulus.

Control of Filtration

*Changes in blood pressure change filtration rate
-Autoregulation can be overridden.
-Since the kidney regulates fluid volume, the kidney can work with the cardiovascular system to regulate blood pressure.
- Glomerular filtration can decrease to conserve fluid when blood pressure falls, or increase filtration if blood pressure rises.
-The sympathetic nervous system can control urine production through the adrenal medulla.

Control of filtration

*Tubular reabsorption and secretion control the chemistry and volume of urine.
*Anything that affects reabsorption and secretion affects urine chemistry.

Control of Tubular Reabsorption and secretion

*The first thing that affects tubular reabsorption and secretion is tubule permeability.
-Each section of the tubule can reabsorb and secrete different substances.
-Molecules move across membranes through several different methods including diffusion and active transport.

Control of Tubular Reabsorption and secretion

*The first thing that affects tubular reabsorption and secretion is tubule permeability.
-Differences in tubular permeability result in dramatic differences in what molecules are reabsorbed or secreted in each part of the tubule.

Control of Tubular Reabsorption and secretion

*Special type of circulation around the nephron loop, called counter current circulation
-Ions move across cell membranes from higher to lower areas of concentration

Control of Tubular Reabsorption and secretion

Special type of circulation around the nephron loop, called countercurrent circulation
-Water and ions tend to move in opposite directions, making it impossible to move both without the special environment around the nephron loop.
-Concentration gradient, with low ion concentration at the beginning of the descending loop and high concentration at the tip of the loop

Control of Tubular Reabsorption and secretion

*Special type of circulation around the nephron loop, called countercurrent circulation
-Water and ions tend to move in opposite directions, making it impossible to move both without the special environment around the nephron loop.
-Differences in permeability between the descending loop (water) and ascending loop (ions)

Control of Tubular Reabsorption and secretion

*Countercurrent circulation
-Filtrate flows into the descending loop, reabsorbing water and increasing the concentration of ions.
-As the filtrate enters the ascending loop, fluid is concentrated because of water loss.
-The membrane is permeable to only ions, and ions move across the membrane.

Control of Tubular Reabsorption and secretion

*The third factor that affects reabsorption and secretion are several hormones that regulate blood pressure.
-Antidiuretic Hormone
-Aldosterone
-Atrial Natriuretic Peptide
-Renin-Angiotensin-Aldosterone

Control of Tubular Re absorption and secretion

*Hormonal control
-Antidiuretic hormone (ADH) is made by the hypothalamus and secreted from the posterior pituitary when BP decreases or ionic concentration increases.
-ADH increases permeability of distal tubules and the collecting duct.
More water is reabsorbed, increasing blood volume, increasing blood pressure, and diluting the ionic concentration

Control of Tubular Re absorption and secretion

*Hormonal control
-The renin-angiotensin-aldosterone system is a series of chemical reactions that regulate blood pressure.
-Decrease in blood flow to the kidney:
-Causes a special group of cells near the glomerulus, the juxtaglomerular apparatus, to secrete renin into the blood stream.
-The liver secretes a chemical called angiotensinogen.
e production

Control of Tubular Re absorption and secretion

*Hormonal control
-Decrease in blood flow to the kidney:
Renin converts angiotensinogen into angiotensin I.
Another enzyme made by the lungs, angiotensin converting enzyme (ACE), converts angiotensin I to angiotensin II,
This increases thirst, increasing ADH secretion, increasing aldosterone secretion, and causing vasoconstriction. These increase blood pressure by increasing fluid volume.

The Urinary Bladder and urination Reflex

* Glomerular filtrate flows out the collecting duct, into the minor calyces, and then into the major calyces, forming the renal pelvis.
*Once the glomerular filtrate leaves the collecting ducts, its concentration can't be changed and it is urine.
*Urine collects in the renal pelvis and flows down the ureters to the urinary bladder, where it is stored.

Urinary Bladder

*The urinary bladder is a small hollow organ posterior to the pubic symphysis and behind the peritoneum.
- It is lined with transitional epithelium, the only epithelium stretchy enough to expand as the bladder fills.
- The ability to stretch is enhanced by a series of pleats called rugae.

Urinary Bladder

*The urinary bladder is a small hollow organ posterior to the pubic symphysis and behind the peritoneum.
-Muscular wall consisting of several layers of circular and longitudinal smooth muscle
-Covered by connective tissue and parietal peritoneum

Urination Reflex

*As urine accumulates, the bladder fills and stretches.
-This stretch triggers the urinary reflex and the need to void to empty the bladder.
-Urination had been thought to be a spinal reflex, but new research indicates it is controlled by the brain.
-When the bladder is full signals are sent from the bladder to the spinal cord to the pons.

Urination Reflex

*As urine accumulates, the bladder fills and stretches.
-The pons sends parasympathetic signals down the spinal cord.
-This causes contraction of the muscular walls of the bladder.
-The bladder empties.

Urination

*Urine leaves the bladder via the urethra, a thin muscular tube lined with several different types of epithelium along its length.
-The brain can inhibit urination by controlling the internal urethral sphincter and external urethral sphincter.
-Sympathetic stimulation of these sphincters prevents urine from leaving the body.

Urination

Urine leaves the bladder via the urethra, a thin muscular tube lined with several different types of epithelium along its length.
Although you have little control over bladder contraction, you have good control over the sphincters starting from age 2, or slightly later in boys.

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