BPK 105 Study Guide - Final Guide: Proximal Tubule, Efferent Arteriole, Afferent Arterioles
27 Apr 2018
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Module 12 - Overview and Terminology
Overview
- The urinary system: essential in the maintenance of a healthy physiological content of
our blood and other body fluids.
- major excretory system of the body
- the kidneys eliminate toxins taken into the body, or produced by the body as byproducts
of metabolism. The kidneys are also essential in regulating blood volume, which is
important in maintaining cardiovascular function and regulating blood pressure. In
addition, specialized areas of the kidneysโ functional unit, the nephron, are involved in
homeostatic regulation of sodium, potassium, and hydrogen (pH) ion concentrations of
the blood in other fluid compartments of the body.
- the glomerulus, the first portion of the nephron, filters your entire blood volume about 35
times per day. In order not to have 180 L of urine to deal with each day, the remaining
portions of the nephron reabsorb 99% of what is filtered back into the blood. Though this
may seem like a waste of effort, filtering this large volume allows us to eliminate waste
products found in small concentrations in the blood.
- In addition, the ability to regulate water and ion balance as described above comes from
the capacity to regulate how much is reabsorbed, or saved from elimination from the
body as urine.
Terminology
1. cortex (Section 18.2, Figures 18.4, 18.9): Outer part of an organ such as the brain,
kidney, adrenal gland, or hair.
2. medulla (Section 18.2, Figures 18.4, 18.9): Center, or core, of an organ, such as the
adrenal gland, kidney, or hair.
3. afferent arteriole (Figures 18.5, 18.6, 18.7): Small artery in the renal cortex that
supplies ๎blood to the glomerulus๎.
4. efferent arteriole (Figures 18.5, 18.6, 18.7): Vessel that carries blood ๎from the
glomerulus ๎to the peritubular capillaries.
5. glomerulus (Section 18.2, Figure 18.6): Mass of๎ capillary loops๎ at the beginning of
each nephron, nearly surrounded by Bowmanโs capsule. First part of a nephron.
6. proximal tubule (Section 18.2, Figures 18.6, 18.10): [proximal convoluted tubule]
Convoluted portion of the nephron that extends from ๎Bowmanโs capsule to the
descending๎ limb of the loop of Henle.
7. descending loop of Henle (Section 18.2, Figure 18.11):
Loop of henle: ๎U-shaped๎ part of the ๎nephron๎ extending from the proximal to the distal
convoluted tubule and consisting of descending and ascending limbs; many of the loops of
Henle extend into the renal pyramids.
The thin segment of the descending limb of the loop of Henle is๎ permeable to water and, to a
lesser degree, solutes, ๎and the thin segment of the ascending limb is permeable to solutes,
but not to water.
The descending limb of the loop of Henle further concentrates the filtrate.
[The renal medulla contains very concentrated interstitial fluid that has large amounts of Na+,
Clโ, and urea.] The๎ wall of the thin segment๎ of the descending limb is permeable to water and
moderately permeable to solutes. As the filtrate passes through the descending limb of the loop
of Henle into the medulla of the kidney, ๎water moves out of the nephron by osmosis๎, and
some ๎solutes๎ move ๎into๎ the ๎nephron๎ by ๎diffusion๎. By the time the filtrate has passed through
the descending limb, another 15% of the filtrate volume has been reabsorbed, and the filtrate is
as concentrated as the interstitial fluid of the medulla. The reabsorbed water and solutes enter
the vasa recta (figure 18.11; see figure 18.9, step 2).
Because of the volume of filtrate reabsorbed in the proximal convoluted tubule and the
descending limb of the loop of Henle, only about 20% of the original filtrate volume remains.
Filtrate volume๎ is reduced by 65% in the proximal convoluted tubule and by 15% in the
descending limb of the loop of Henle.
8. ascending loop of Henle (Section 18.2, Figure 18.12): The fluid flows through the
ascending limb of the loop of Henle to the ๎distal๎ convoluted tubule.
The ascending limb of the loop of Henle dilutes the filtrate by removing solutes. The thin
segment of the ascending limb is not permeable to water, but it is permeable to solutes.
Consequently, solutes diffuse out of the nephron (figure 18.11; see figure 18.9, step 3).
The cuboidal epithelial cells of the thick segment of the ๎ascending๎ limb actively transport Na+
out of the nephron, and K+ and Clโ are cotransported with Na+. The thick segment of the
ascending๎ limb is not permeable to water. As a result, Na+, K+, and Clโ, but little water, are
removed from the filtrate (see figure 18.9, step 4). Because of the efficient removal of these
solutes, the highly concentrated filtrate that enters the ๎ascending๎ limb of the loop of Henle is
converted to a dilute solution by the time it reaches the distal convoluted tubule (figure 18.12;
see figure 18.9, step 5). As the filtrate enters the distal convoluted tubule, it is ๎more dilute t๎han
the interstitial fluid of the renal cortex. Also, because of the ๎volume of filtrate reabsorbed ๎in
the proximal convoluted tubule and the descending limb of the loop of Henle, only about 20% of
the original filtrate volume remains. The solutes transported from the ๎ascending๎ limb of the loop
of Henle enter the interstitial fluid of the medulla and help keep the ๎concentration of solutes๎ in
the ๎medulla๎ high. Excess solutes enter the vasa recta.
