Physiology II
Renal
Overview of the Major Components of Renal Function

(Ch. 1 and 2, Koeppen and Stanton)

Have a good understanding of the basic anatomy of the two major types of nephrons (juxtamedullary and cortical), particularly the different segments that constitute each type of nephron.  The nephron is the functional unit of the kidney, with each kidney containing 1.0 to 1.3 million nephrons. Each consists of a glomerular capillary network (glomerulus) that is surrounded by a Bowmans’s Capsule. Both of these together equal a renal corpuscle. The nephrons are named from where the nephron originates – Cortical or Juxatmedullary.
Each nephron consists of:

• Renal corpuscle – which consists of the glomerulus and Bowman’s Capsule. The renal corpuscle of the juxamedullary nephron is larger.
• Proximal convoluted tubule (PCT) – the first segment that leaves the glomerulus. The straight (non-convoluted) segment of the PCT is called the pars recta.
• Loop of Henle – This is further broken down to a descending limb, a thin ascending limb (this is seen only in the juxamedullary nephron) and a thick ascending limb (also described as medullary or cortical thick ascending limb – depending on its location in the kidney). Near the end of the thick ascending tubule the nephron passes between the afferent and efferent arterioles of the same nephron. This short segment is called the macula densa.
• Distal Convoluted tubule – Begins a short distance beyond the macula densa and extends to the point in the cortex where two or more nephrons join together to form a cortical collecting duct.
• Collecting duct – The collecting ducts enter the medulla and eventually drain down into the minor and major calyx’s.

Koeppen and Stanton. Renal Physiology, 2^nd Edition, Mosby: St Louis, p. 21 (Figure 2-3).

Have an understanding of the blood supply to the kidney, including the vasa recta and peritubular capillary networks, and their importance.  Blood enters the kidney via the renal artery which branches a number of times into segmental arteries which subdivide into interlobar arteries. Each interlobar artery penetrates the kidney through a "column of Berlin". Before reaching the cortical surface it divides into arc-shaped arcuate arteries which run parallel to the surface. Interlbular arteries (or cortical radial arteries) arise from the arcuate arteries and penetrate the cortex perpendicularly toward the surface. Branching from each interlobular artery are the afferent arterioles, which enter Bowman’s Capsule.

Juxamedullary nephrons have glomerulli which arise in the deep cortical regions. Their loops of Henle extend deeper into the kidney, going into the inner medulla varying distances. The efferent arteriole of the juxamedullary nephron continues as a peritubular capillary bed, but also gives rise to a series of vascular loops called the vasa recta. These descend to varying depths to the inner medulla and break up into capillary networks, especially around the ascending thin limb of the loop of Henle.

Guyton and Hall. Textbook of Medical Physiology, 9^th Edition. WB Saunders Company: Philadelphia, p. 318

Be clear in your mind on the difference between the terms "reabsorbtion", "secretion", and "filtration", which describe the handling of water and solute in the nephron, and the direction in which it they are transported.

• Reabsorbtion: transport of solutes or water that were already filtered into the nephron, out of the tubular lumen of the nephron into the surrounding interstitium and peritubular blood, returning to the circulation.
• Secretion: transport of solutes from the interstitium or peritubular blood into the tubular lumen of the nephron. This is applied to both passive and active transport in that direction.
• Filtration: substances entering the nephron by being filtered out of the glomerular capillaries to Bowman’s space to the proximal tubule.

Have an idea of what the juxtaglomerular apparatus (JGA) is, and what it’s role in renal function is in the so-called "renin-angiotensin-aldosterone" cascade.

The juxtaglomerular apparatus (JGA) is the part of the nephron where the distal tubule of a nephron loops back up and passes between its vascular pole (where the afferent arteriole enters and the efferent arteriole leaves the glomerulus). Specialized epithelial cells (macula densa cells) line the lumen of the early distal tubule. The macula densa cells contact the extraglomerular mesangial cells and granular cells of the afferent and efferent arterioles. The granular cells are modified smooth muscle cells that manufacture, store, and release renin. Changes in sodium composition and probably flow rate of the filtrate are sensed by the macula densa cells which send the signal to the granular cells to or renin secretion. Renin is usually secreted in response to a ¯ in plasma volume which will activate the renin-angiotensin-aldosterone cascade.

Know what the major body fluid compartments are (total body water [TBW], extracellular fluid [ECF], and intracellular fluid [ICF]), and how they relate to each other. Know how to calculate a rough estimate of their size if you were given a body weight in kilograms.

Sixty (60%) of total body weight is composed of water (approximately 42 liters in the average 70 kg adult male).
Total body water is distributes primarily between two compartments:

• Intracellular fluid (ICF) and Extracellular fluid (ECF)
• They are essentially divided only by the cell membrane.

About 28 liters (67%) of the 42 liters are contained within the cells (ICF).  The remaining 14 liters (33%) is comprised of the ECF.

The ECF is divided into 2 major subdivisions:

• Plasma: 3.5 liters (25%) of the ECF
• Interstitial fluid (ISF): 10.5 liters (75%) of the ECF

Also classified as ECF: crystallized water in bone and fluid in dense connective tissues (Other minor ECF compartments: GI, biliary, urinary tracts, intraocular, CSF, pleural, peritoneal, cartilage), pericardial fluids.

From Class Handout

Estimating volumes:
TBW (L) = 0.6 x body wt (kg)
ICF (L) = 0.4 x body wt (kg)
ECF (L) = 0.2 x body wt (kg)

Understand what osmolality and osmolarity are, and why water and solute shift between the ECF and ICF the way they do when either hypotonic, hypertonic, or isotonic fluids are added to the ECF, as shown in a figure 25.6 near the end of the first handout ("Overview of the Major Components of Renal Function").
Osmolality: the number of solute particles in 1 kg of water.
Osmolarity: the number of solute particles per 1 liter of water.

For dilute solutions, the difference between them is insignificant.
Measurements of osmolarity are temperature dependent, because the volume of water varies with temperature (volume is higher at higher temperature).
Osmolality is based on the mass of water so it is temperature independent.
Osmolality has units of osm/kg H₂O. Because of the dilute nature of physiologic solutions – osmolalities are expressed as milliosmoles per kilogram water (mOsm/kg H₂O).

• Isotonic – a solution that does not change the volume of the cell. Example: IV infusion of 0.9% NaCl, osmolality » 290 mOsm/kg H₂O to the ECF. The volume of ECF is increased. Osmolality of the fluid and ECF is the same, there is no driving force for fluid movement between compartments. The volume of the ICF is unchanged. Although Na⁺ can cross cell membranes, it is effectively restricted to the ECF by the activity of the Na⁺-K⁺-ATPase, which is present in all cells.
• Hypotonic – solution that causes cell to swell. Example: IV infusion of 0.45% NaCl: osmolality » 145 mOsm/kg H₂O to the ECF. This decreases the osmolality of the ECF, resulting in movement of water into the ICF. The volume of both compartments increases. Since both compartments have taken on excess water, the solutes in each compartment are dissolved in more water, resulting in osmolality of both the ECF and ICF.
• Hypertonic: the solution causes the cell to shrink. Example: IV infusion of 3% NaCl: osmolality » 1000 mOsm/kg H₂O. This increases the osmolality of the ECF, resulting in movement of water from the cells. After equilibrium, the osmolality of the ECF and ICF will be equal (solutes in ICF are more concentrated, and the osmolality of ECF increased because of addition of NaCl). The volume of the ECF will be increased and the volume of the ICF is decreased.

Summary:
Isotonic – change in volume only of ECF.
Hypotonic – change in increase of volume of each compartment, osmolality of each .
Hypertonic – total volume of ICF ; total volume of ECF ; osmolality of both .

Last Updated 09/06/01 08:53:30 PM
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