1. Kidney Transport
Reabsorption of filtered water and solutes from the tubular lumen across the tubular epithelial cells, through the renal interstitium, and back into the blood. Solutes are transported through the cells (transcellular route) by passive diffusion or active transport, or between the cells (paracellular route) by diffusion. Water is transported through the cells and between the tubular cells by osmosis. Transport of water and solutes from the interstitial fluid into the peritubular capillaries occurs by ultrafiltration (bulk flow).
Reabsorption of filtered water and solutes from the tubular lumen across the tubular epithelial cells, through the renal interstitium, and back into the blood. Solutes are transported through the cells (transcellular route) by passive diffusion or active transport, or between the cells (paracellular route) by diffusion. Water is transported through the cells and between the tubular cells by osmosis. Transport of water and solutes from the interstitial fluid into the peritubular capillaries occurs by ultrafiltration (bulk flow).

2. Reabsorption
Principles governing the tubular reabsorption of solutes and water
Na+ is reabsorbed
by active transport.
Na+
Tubular
epithelium
Extracellular fluid
Tubule lumen
Filtrate is
similar to
interstitial fluid. 1
Figure 19-11, step 1

3. Reabsorption Figure 19-11, steps 1–2 1 Na+ is reabsorbed
by active transport.
Electrochemical
gradient drives
anion reabsorption.
Na+
Anions
Tubular
epithelium
Extracellular fluid
Tubule lumen
Filtrate is
similar to
interstitial fluid. 1 2
Figure 19-11, steps 1–2

4. Reabsorption Figure 19-11, steps 1–3 1 Na+ is reabsorbed
by active transport.
Electrochemical
gradient drives
anion reabsorption.
Water moves by
osmosis, following
solute reabsorption.
Na+
Anions
H2O
Tubular
epithelium
Extracellular fluid
Tubule lumen
Filtrate is
similar to
interstitial fluid. 1 2 3
Figure 19-11, steps 1–3

5. Reabsorption Figure 19-11, steps 1–4 1 Na+ is reabsorbed
by active transport.
Electrochemical
gradient drives
anion reabsorption.
Water moves by
osmosis, following
solute reabsorption.
Concentrations of
other solutes increase
as fluid volume in lumen
decreases. Permeable
solutes are reabsorbed
by diffusion.
Na+
Anions
H2O
K+, Ca2+,
urea
Tubular
epithelium
Extracellular fluid
Tubule lumen
Filtrate is
similar to
interstitial fluid. 1 2 3 4
Figure 19-11, steps 1–4

6. Reabsorption Transepithelial transport Paracellular pathway
Substances cross both apical (lumen) and basolateral membrane (interstitial space before capillary)
Paracellular pathway
Substances pass through the junction between two adjacent cells

7. Reabsorption
Sodium reabsorption in the proximal tubule
Figure 19-12

8. Basic mechanism for primary active transport of sodium through the tubular epithelial cell.
The sodium-potassium pump transports sodium from the interior of the cell across the basolateral membrane, creating a low intracellular sodium concentration and a negative intracellular electrical potential. The low intracellular sodium concentration and the negative electrical potential cause sodium ions to diffuse from the tubular lumen into the cell through the brush border.

9. Reabsorption Sodium-linked glucose reabsorption in the proximal tubule
Figure 19-13

10. Mechanisms of secondary active transport
The upper cell shows the co-transport of glucose and amino acids along with sodium ions through the apical side of the tubular epithelial cells, followed by facilitated diffusion through the basolateral membranes. The lower cell shows the counter-transport of hydrogen ions from the interior of the cell across the apical membrane and into the tubular lumen; movement of sodium ions into the cell, down an electrochemical gradient established by the sodium-potassium pump on the basolateral membrane, provides the energy for transport of the hydrogen ions from inside the cell into the tubular lumen.
The upper cell shows the co-transport of glucose and amino acids along with sodium ions through the apical side of the tubular epithelial cells, followed by facilitated diffusion through the basolateral membranes. The lower cell shows the counter-transport of hydrogen ions from the interior of the cell across the apical membrane and into the tubular lumen; movement of sodium ions into the cell, down an electrochemical gradient established by the sodium-potassium pump on the basolateral membrane, provides the energy for transport of the hydrogen ions from inside the cell into the tubular lumen.

11. Reabsorption
Urea
Passive reabsorption
Plasma proteins
Transcytosis

12. Reabsorption
Saturation of mediated transport
Figure 19-14

13. Reabsorption
Glucose handling by the nephron
Figure 19-15a

14. Reabsorption
Figure 19-15b

15. Reabsorption
Figure 19-15c

16. Reabsorption
Figure 19-15d

17. Secretion
Transfer of molecules from extracellular fluid into lumen of the nephron
Active process
Secretion of K+ and H+ is important in homeostatic regulation
Enables the nephron to enhance excretion of a substance
Competition decreases penicillin secretion

18. Cellular ultrastructure and primary transport characteristics of the proximal tubule
The proximal tubules reabsorb about 65 per cent of the filtered sodium, chloride, bicarbonate, and potassium and essentially all the filtered glucose and amino acids. The proximal tubules also secrete organic acids, bases, and hydrogen ions into the tubular lumen.

19. Transport characteristics of the proximal tubule
Figure 27-7 Changes in concentrations of different substances in tubular fluid along the proximal convoluted tubule relative to the concentrations of these substances in the plasma and in the glomerular filtrate. A value of 1.0 indicates that the concentration of the substance in the tubular fluid is the same as the concentration in the plasma. Values below 1.0 indicate that the substance is reabsorbed more avidly than water, whereas values above 1.0 indicate that the substance is reabsorbed to a lesser extent than water or is secreted into the tubules.

20. Transport in loop of Henle
Cellular ultrastructure and transport characteristics of the thin descending loop of Henle (top) and the thick ascending segment of the loop of Henle (bottom). The descending part of the thin segment of the loop of Henle is highly permeable to water and moderately permeable to most solutes but has few mitochondria and little or no active reabsorption. The thick ascending limb of the loop of Henle reabsorbs about 25 per cent of the filtered loads of sodium, chloride, and potassium, as well as large amounts of calcium, bicarbonate, and magnesium. This segment also secretes hydrogen ions into the tubular lumen.

21. Cellular ultrastructure and transport characteristics of the early distal tubule and the late distal tubule and collecting tubule.
The early distal tubule has many of the same characteristics as the thick ascending loop of Henle and reabsorbs sodium, chloride, calcium, and magnesium but is virtually impermeable to water and urea. The late distal tubules and cortical collecting tubules are composed of two distinct cell types, the principal cells and the intercalated cells. The principal cells reabsorb sodium from the lumen and secrete potassium ions into the lumen. The intercalated cells reabsorb potassium and bicarbonate ions from the lumen and secrete hydrogen ions into the lumen. The reabsorption of water from this tubular segment is controlled by the concentration of antidiuretic hormone.

22. Cellular ultrastructure and transport characteristics of the medullary collecting duct.
The medullary collecting ducts actively reabsorb sodium and secrete hydrogen ions and are permeable to urea, which is reabsorbed in these tubular segments. The reabsorption of water in medullary collecting ducts is controlled by the concentration of antidiuretic hormone.

23. Excretion Excretion = filtration – reabsorption + secretion Clearance
Rate at which a solute disappears from the body by excretion or by metabolism
Non-invasive way to measure GFR
Inulin or creatinine used to measure GFR

24. Inulin Clearance Figure 19-16, step 1 Glomerulus Peritubular
capillaries
Afferent
arteriole
Nephron
Filtration
(100 mL/min)
= 100 mL of plasma or filtrate
Inulin concentration is 4/100 mL
KEY
Inulin
molecules
Efferent 1
Figure 19-16, step 1

25. Inulin Clearance Figure 19-16, steps 1–2 Glomerulus Peritubular
capillaries
Afferent
arteriole
Nephron
Filtration
(100 mL/min)
= 100 mL of plasma or filtrate
Inulin concentration is 4/100 mL
GFR = 100 mL /min
KEY
Inulin
molecules
Efferent 1 2
Figure 19-16, steps 1–2

26. Inulin Clearance Figure 19-16, steps 1–3 Glomerulus Peritubular
capillaries
Afferent
arteriole
Nephron
Filtration
(100 mL/min)
100 mL,
0% inulin
reabsorbed
= 100 mL of plasma or filtrate
Inulin concentration is 4/100 mL
GFR = 100 mL /min
100 mL plasma is reabsorbed.
No inulin is reabsorbed.
KEY
Inulin
molecules
Efferent 1 2 3
Figure 19-16, steps 1–3

27. Inulin Clearance Figure 19-16, steps 1–4 Glomerulus Peritubular
capillaries
Afferent
arteriole
Nephron
Filtration
(100 mL/min)
100 mL,
0% inulin
reabsorbed
Inulin clearance
= 100 mL/min
= 100 mL of plasma or filtrate
100% inulin
excreted
Inulin concentration is 4/100 mL
GFR = 100 mL /min
100 mL plasma is reabsorbed.
No inulin is reabsorbed.
100% of inulin is excreted so
inulin clearance = 100 mL/min
KEY
Inulin
molecules
Efferent 1 2 3 4
Figure 19-16, steps 1–4

28. GFR Filtered load of X = [X]plasma  GFR
Filtered load of inulin = excretion rate of inulin
GFR = excretion rate of inulin/[inulin]plasma = inulin clearance
GFR = inulin clearance

29. Excretion

30. Excretion The relationship between clearance and excretion
Figure 19-17a

31. Excretion
Figure 19-17b

32. Excretion
Figure 19-17c

33. Micturition The storage of urine and the micturition reflex
Figure 19-18a

34. Micturition Figure 19-18b, step 1 Stretch receptors fire. Stretch
Internal
sphincter
External
Sensory neuron 1
Figure 19-18b, step 1

35. Micturition Figure 19-18b, steps 1–2 Stretch receptors fire. Stretch
Parasympathetic neurons fire.
Motor neurons stop firing.
(b)
Micturition
Internal
sphincter
External
Tonic
discharge
inhibited
Sensory neuron
Parasympathetic
neuron
Motor neuron – +
Higher CNS
input may
facilitate or
inhibit reflex. 1 2
Figure 19-18b, steps 1–2

36. Micturition Figure 19-18b, steps 1–3 Stretch receptors fire. Stretch
Parasympathetic neurons fire.
Motor neurons stop firing.
Smooth muscle contracts.
Internal sphincter passively
pulled open. External
sphincter relaxes.
(b)
Micturition
Internal
sphincter
External
Tonic
discharge
inhibited
Sensory neuron
Parasympathetic
neuron
Motor neuron – +
Higher CNS
input may
facilitate or
inhibit reflex. 1 2 3
Figure 19-18b, steps 1–3