Urinary Tract Structure and Function 2.

Question 1

Kidney functions?

Answer 1

The kidneys have many functions:
-Excretion of metabolic waste products and chemicals.
-Regulation of electrolyte and water balance.
-Regulation of body fluid osmolality and electrolyte concentration.
-Regulation of arterial pressure.
-Regulation of acid-base balance.
-Secretion, metabolism, and excretion of hormones.
-Gluconeogenesis.

Question 2

Bloody supply of the kidneys?

Answer 2

Simplistically, the branches of the blood supply are:
-Renal artery: enters the kidney through the hilus.
-Interlobar artery.
-Arcuate artery.
-Interlobular artery.
- Afferent arteriole.
-Glomerular capillaries (Renal circulation is unique in that is had two capillary beds).
-Efferent arteriole.
-Peritubular capillaries (Renal circulation is unique in that is had two capillary beds).
-Interlobular veins.
-Arcuate vein.

Question 3

Kidney capillaries?

Answer 3

-The renal circulation has two capillary beds, the glomerular and the peritubular.
-These are arranged in series (i.e., one after the other), and separated by the efferent arterioles.
-The efferent arteriole helps to regulate the hydrostatic pressure in both sets of capillaries.

Question 4

Capillary hydrostatic pressure?

Answer 4

-The high hydrostatic pressure of the glomerular capillaries causes rapid filtration.
-Whereas a much lower hydrostatic pressure in the peritubular capillaries permits rapid fluid reabsorption.
-By adjusting the resistance of the afferent and efferent arterioles, the kidneys can regulate the hydrostatic pressure in both the glomerular and peritubular capillaries.
-Thereby, changing the rate of glomerular filtration, tubular reabsorption, or both in response to the body’s needs.

Question 5

Urine formation- introduction?

Answer 5

-Urine formation results from:
1.) Glomerular filtration.
2.) Tubular reabsorption and tubular secretion.
-Tubular reabsorption: reabsorption of substances from the renal tubules into the blood (i.e., reabsorbed back into the blood).
-Tubular secretion: secretion of substances from the blood into the renal tubule (i.e., secreted from the blood).
-Urine formation begins with a large amount of fluid being filtered from the glomerular capillaries into Bowman’s capsule, which is virtually protein-free.
-Most substances in the plasma, except for proteins, are freely filtered.
-Thus, the concentration of most substances in Bowman’s capsule is the same as the plasma.
-As the filtered fluid leaves Bowman’s capsule and passes through the tubules, it is modified by reabsorption and secretion.

Question 6

Hypothetical substance to explain filtration, reabsorption, and secretion?

Answer 6

1.) Filtration only: creatinine.
2.) Filtration and partial reabsorption: electrolytes.
3.) Filtration and complete absorption: amino acids and glucose.
4.) Filtration and secretion: organic acids and bases.

Question 7

Structure of capillaries?

Answer 7

-All blood vessels have an internal lining of endothelial cells (epithelial tissue).
-Capillary walls are thin and consist of a single layer of endothelial cells supported by a basement membrane.
-Between the basement membrane and the endothelial cells are pericapillary cells such as fibroblasts, macrophages, and smooth muscle cells.
-Capillaries can be classified based on their permeability into:
1.) Continuous: no gaps between endothelial cells, less permeable to large molecules.
2.) Fenestrated: Endothelial cells have numerous fenestrae (cytoplasm absent and plasma membrane thinner), highly permeable.
3.) Sinusoidal: fenestrae large and gaps can exist between endothelial cells, allows large molecules to move into the blood.

Question 8

Structure of the glomerulus?

Answer 8

-The glomerular capillaries are like most other capillaries, except there are three major layers (rather than two):
1.) Endothelium: perforated with fenestrae, but does not allow passage of proteins, because endothelial cells have fixed negative charges.
2.) Basement membrane: consists of collagen and proteoglycan fibrillae that have large spaces through which water and solutes move.
3.) Epithelial cells: not continuous, but have footlike processes (podocytes) encircling the capillaries. The podocytes are separated by slit pores through which the glomerular filtrate moves.

Question 9

Composition of the glomerular filtration?

Answer 9

-Urine formation begins with filtration of large amounts of fluid through the glomerular capillaries into Bowman’s capsule.
-As the glomerular capillary membrane is relatively impermeable to proteins, the glomerular filtrate is protein-free and devoid of cellular elements, including red blood cells.
-The concentrations of other constituents of the glomerular filtrate are similar in concentration to those in plasma.

Question 10

Glomerular filtration rate (GFR)?

Answer 10

-The filtration rate of the glomerulus is determined by:
1.) Sum of the hydrostatic and colloid osmotic pressures acting across the glomerular membrane, giving the net filtration pressure.
2.) The glomerular capillary filtration coefficient.

Question 11

Determinants of GFR?

Answer 11

Glomerular capillary filtration coefficient:
1.) Increased coefficient increases GFR and vice versa.
2.) Unlikely to provide day-to-day regulation of GFR.
3.) Some diseases lower the coefficient (e.g., diabetes).
Bowman’s hydrostatic pressure:
1.) Increased Bowman’s hydrostatic pressure decreases GFR (inversely related).
2.) Not normally a primary means of regulating GFR.
3.) Blockage of the urinary tract can markedly increase the pressure.
Glomerular capillary colloid osmotic pressure:
1.) Increasing glomerular capillary colloid osmotic pressure decreases GFR.
2.) As blood passes from the afferent arteriole through the glomerulus to the efferent arterioles, the colloid osmotic pressure increases.
3.) The pressure increases because fluid from the glomerulus filters into Bowman’s capsule, thus concentrating the glomerular plasma proteins.
4.) Two factors affect the glomerular capillary colloid osmotic pressure: The arterial colloid osmotic pressure. An increase in pressure results in a decrease in GFR. The fraction of plasma filtered, an increased fraction means an increased concentration of plasma proteins, meaning decrease GFR.
Glomerular capillary hydrostatic pressure:
1.) Increased glomerular hydrostatic pressure increases GFR.
2.) Changes in the glomerular hydrostatic pressure serve as the primary means for physiological regulation of GFR.
3.) The glomerular hydrostatic pressure is determined by 3 variables: Arterial pressure, Afferent arteriolar resistance, and Efferent arteriolar resistance.
4.) Constriction of afferent arterioles reduces GFR.
5.) The effect of the efferent arterioles depends on the severity of the constriction: severe constriction reduces GFR, and modest constriction increases GFR.
6.) Increased arterial pressure increases GFR: refer to the autoregulatory mechanisms.

Question 12

Homework: look into the control of renal blood flow and glomerular filtration?

Question 13

Tubular function- introduction?

Answer 13

-Next, the glomerular filtrate enters the renal tubules and moves sequentially through each successive part:
1.) Proximal convoluted tubule.
2.) Loop of Henle.
3.) Distal convoluted tubule.
4.) Collecting tubule.
5.) Collecting duct.
-Along this course, substances are selectively reabsorbed and secreted.
-Tubular reabsorption is highly selective, whereas glomerular filtration is not selective except for proteins.

Question 14

How are substances reabsorbed?

Answer 14

-Reabsorption involves passive and active mechanisms.
-For a substance to be reabsorbed, it must:
1.) Be transported across the tubular epithelium membranes into the renal interstitial fluid.
2.) Be transported through the peritubular capillary membrane back into the blood.

Question 15

Passive transport (diffusion) of substances across the cell membrane?

Answer 15

-Simple diffusion through lipid bilayer (e.g., Oxygen and Carbon dioxide).
-Simple diffusion through channel proteins (gated or non-gated, e.g., ions).
-Facilitated diffusion through carrier proteins (e.g., Glucose).
-Hydrophilic head: Polar head attracted to water.
-Hydrophobic tail: nonpolar tail not attracted to water.

Question 16

Active transport of substances across the cell membrane?

Answer 16

-Simple diffusion eventually equilibrates concentration on either side of the membrane (i.e., equilibrium is reached).
-Therefore, active transport pumps are required to alter the concentration of substances on either side of the membrane.
-Primary active transport (ATP pump) of substances against a concentration gradient (e.g., sodium-potassium pump, calcium pump, and hydrogen pump).

Question 17

Reabsorption and secretion along different parts of the nephron: PCT?

Answer 17

-Reabsorb:
1.) Sodium.
2.) Chloride.
3.) Bicarbonate.
4.) Potassium.
5.) Glucose.
6.) Amino acids.
-Secrete:
1.) Organic acids.
2.) Bases.
3.) Hydrogen ions.

Question 18

Reabsorption and secretion along different parts of the nephron: Loop of Henle?

Answer 18

-Thin descending segment:
1.) Highly permeable to water.
2.) Moderately permeable to most solutes.
-Thin ascending segment:
1.) Lower resorptive capacity.
-Thick ascending segment:
Reabsorbs:
1.) Sodium.
2.) Chloride.
3.) Potassium.
4.) Calcium.
5.) Bicarbonate.
6.) Magnesium.
Secretes:
1.) Hydrogen.

Question 19

Reabsorption and secretion along different parts of the nephron: DCT?

Answer 19

-The first portion forms part of the juxtaglomerular complex, providing feedback control on GFR and blood flow.
- The second part of the dital tubule is highly convoluted and has similar reabsorptive characteristics as the ascending limb of LoH: impermeable to water and urea.

Question 20

Reabsorption and secretion along different parts of the nephron: Late DT and cortical CT?

Answer 20

-Two distinct cell types:
1.) Principal cells.
2.) Intercalated cells.
-Principal cells:
1.) Reabsorb: sodium and water.
2.) Secrete: potassium.
-Intercalated cells:
1.) Reabsorb: potassium and bicarbonate.
2.) Secrete: hydrogen.