18 THE URINARY SYSTEM: RENAL FUNCTION
Functions of the Urinary System
Kidneys filter blood, and produce urine (fluid that is eliminated from the body).
Urine contains metabolic by-products and other substances properly described as “wastes”; it also
contains water and solutes (Na, K) that must be maintained at certain levels in the plasma and other
Kidney’s perform the following primary functions
o Regulation of plasma ionic composition: increasing/decreasing excretion of specific ions
regulates [ions] in the plasma (Na, K, Ca, Mg, Cl, H3O , phosphates).
o Regulation of plasma volume: controlling the rate of water excretion, regulating plasma
volume, which affects the total blood volume and blood pressure.
o Regulation of plasma osmolarity: due to regulating water excretion can regulate osmolarity.
o Regulation of plasma hydrogen ion concentration (pH): by regulating [bicarbonate] and [H ] in
the plasma; works with the lungs.
o Removal of metabolic waste products and foreign substances from the plasma: include
metabolic by-products (i.e. urea, uric acid generated during protein and nucleic acid
catabolism, respectively), and food additives, drugs, or pesticides.
Kidneys also regulate the volume and composition of the interstitial fluid, which effects the
intracellular fluid. Thus the kidneys ultimately control the volume and composition of all body’s fluids.
o Endocrine organs because they secrete the hormone erythropoietin (stimulates erythrocyte
production by the bone marrow) and the enzyme renin (production of AngII – regulating
salt/water balance for long-term control of blood pressure).
o Also activates vitamin D (to 1,25 (OH)2vitamin D 3 to regulate blood Ca and phosphate levels.
o Function during fasting to maintain a steady supply of plasma glucose through
gluconeogenesis (glycerol and certain amino acids synthesize glucose).
Anatomy of the Urinary System
STRUCTURES OF THE URINARY SYSTEM
Urinary system: 2 kidneys, 2 ureters, urinary bladder, and urethra.
Urine is conducted into the bladder by the ureters. Bladder stores urine until it is time to excrete it.
Urine moves through the urethra and out of the body.
They lie near the rear wall of the abdominal cavity just above the waistline.
A clear membrane that lines the abdominal cavity (peritoneum) does not include the kidneys. They
are located between the peritoneum and the wall of the abdominal cavity (retroperitoneal).
They receive blood from the renal arteries, which branch off the aorta and enter each kidney at the
renal hilus. The rich blood supply is crucial because it provides oxygen and nutrients (16% of total
body ATP usage), and enables the kidneys to remove unneeded solutes and water from the blood.
Each kidney weighs about 115-170g (1% of the body weight). They receive about 20% of the CO.
The blood returns to the general circulation through the renal veins.
MACROSCOPIC ANATOMY OF THE KIDNEY
2 major regions: a reddish-brown outer layer called the cortex, and an inner region called the medulla,
which is darker and has a striped appearance.
Medulla is subdivided into a number of conical sections called renal pyramids. At the tip of the
pyramids are papillae; tubules called collecting ducts drain into the common passageways called
minor calyces. The minor calyces form two or three larger passageways called major calyces, which
drain into a single funnel-shaped passage called the renal pelvis, the initial portion of the ureter. 18 THE URINARY SYSTEM: RENAL FUNCTION
Within a kidney’s many renal pyramids are more than a million subunits called nephrons. They are
functional units of the kidneys: filter the blood forming urine.
Obvious feature of the nephron is a long coiled tube (renal tubule) that forms a hairpin loop. During
the process of urine formation, fluid flows through the renal tubules (fluid composition is modified).
Fluid from individual tubules eventually drains into the collecting ducts. Composition of fluid is
further modified. Fluid that exits the collecting ducts is urine.
MICROSCOPIC ANATOMY OF THE KIDNEY
An individual nephron is composed of: a renal corpuscle that filters blood, and a renal tubule through
which the filtrate travels and becomes modified to form urine.
o Consists of 2 parts: a spherical structure at the inflow end of the renal tubules (Bowman’s
capsule) and a tuft of capillaries (glomerulus).
o It is the site where blood is filtered, enters the glomerular capillaries via afferent arteriole.
o Protein-free plasma filters across the walls of the capillaries into the Bowman’s capsule
(glomerular filtration). The remaining blood leaves the glomerulus via an efferent arteriole.
o The walls of the afferent and efferent arterioles contain smooth muscle that can contract/relax
in response to input from paracrines and the sympathetic nervous system, thereby regulating
their diameter and glomerular filtration
o The glomerular filtrate flows into the Bowman’s capsule into the proximal tubule consisting of
the proximal convoluted tubule and then the proximal straight tubule.
o The proximal tubule empties into the loop of Henle (hairpin loop). 3 sections: descending limb,
thin ascending limb, and thick ascending limb.
o From the ascending limb of the loop of Henle, the fluid flows into the distal convoluted tubule,
which resembles the proximal tubule in appearance but is shorter.
o Fluid enters a short straight terminal portion of the nephron (collecting tubule), which joins
the nephron with the collecting ducts.
CORTICAL AND JUXTAMEDULLARY NEPHRONS
o Majority of nephrons in the kidney are cortical nephrons; located entirely within the renal
cortex, only the tip dips in the medulla.
o Juxtamedullary nephrons (15-20% of all nephrons); located near the border between the
cortex and medulla.
o All glomerulus, proximal convoluted tubule, and distal convoluted tubule are located in the
cortex, whereas the loop of Henle dips deep into the renal medulla.
o Juxtamedullary nephrons function in maintaining osmotic gradient in the renal medulla
(crucial for highly concentrated urine).
o At the site where the initial portion of the distal tubule comes into contact with a nephron’s
afferent and efferent arterioles is a structure called the juxtaglomerular apparatus.
o 2 components: a specialized cluster of tubule’s epithelial cells (macula densa) and granular
cells, specialized in the wall of the afferent arterioles that have granular cytoplasms due to the
presence of numerous secretory granules containing renin.
BLOOD SUPPLY TO THE KIDNEY
o The renal artery branches into segmental arteries, which in turn branch into a number of
interlobar arteries that feed into another set of arteries called arcuate arteries.
o The arcuate arteries branch off into interlobular arteries, from which blood is carried to
individual nephrons by afferent arterioles. 18 THE URINARY SYSTEM: RENAL FUNCTION
o Pertibular capillaries: branch from the efferent arterioles of the cortical nephrons and are
located close to the renal tubules.
o Vasa recta: branch from the efferent arterioles of juxtamedullary nephrons and are networks
of blood vessels forming hairpin loops that run along the loops of Henle and collecting ducts.
o The peritubular capillaries and vasa recta drain into the interlobular veins, then arcuate veins,
and then the interlobar veins.
Basic Renal Exchange Processes
3 exchange processes occur within the renal nephrons:
1. Glomerular filtration: bulk flow of protein-free plasma forms the glomerular capillaries into
2. Reabsorption: selective transport of molecules forms the lumen of the renal tubules to the
interstitial fluid outside the tubules. Reabsorbed molecules eventually enter the peritubular
capillaries by diffusion and then return to circulation.
3. Secretion: selective transport of molecules from the peritubular fluid to the lumen. The
secreted molecules come from the plasma of the peritubular capillaries.
Final part is excretion: elimination of materials fro the body in the form of urine.
Filtration at the renal corpuscle is driven by Starling forces (hydrostatic and osmotic pressure
gradients), existing across the walls of glomerular capillaries.
The wall of Bowman’s capsule and the wall of the renal tubule are composed of a continuous layer of
In Bowman’s capsule, the epithelium folds upon itself to envelop the glomerular capillaries.
Below the epithelium is a basement membrane that acts as a primary filtration barrier for proteins.
The glomerular filtrate must cross 3 barriers to enter Bowman’s capsule (glomerular
1. Capillary endothelial cell layer
2. Basement membrane
3. Epithelial cells of Bowman’s capsule.
The epithelial cells that cover the glomerular capillaries have special extensions or foot processes,
giving them their name podocytes.
o As fluid moves out, it passes through gaps between the podocytes (slit pores).
o Sizes of slit pores are regulated by slit diaphragms.
Fenestrations in the capillary endothelium, slit pores in the capsule epithelium, and large surface area
of filtration barrier combine make the renal corpuscle favorable for bulk flow of protein-free fluid.
Glomerular filtration pressure:
1. Glomerular capillary hydrostatic pressure: P GCvors filtration and is equal to the blood
pressure in the glomerular capillaries (60mmHg). This pressure is substantially higher than in
other capillaries because of high resistance of efferent arteriole, which is located downstream
from the glomerular capillaries. General rule: presences of high resistance in any network of
vessels tend to raise the pressure upstream while lowering the pressure downstream.
2. Bowman’s capsule osmotic pressure: favors filtration. Because proteins are generally the
only solute that can’t move between plasma and Bowman’s capsule, proteins generate the
osmotic force. The presence of proteins in the interstitial fluid surrounding the glomerulus
tends to pull fluid out of the capillaries and into the capsule. Because very little protein leaves
the capillaries with the filtrate, protein concentration in Bowman’s capsule is very small, and
osmotic pressure is negligible under normal conditions. 18 THE URINARY SYSTEM: RENAL FUNCTION
Net pressure favoring filtration at the renal corpuscGC: PBC= 60 + 0 = 60mmHg
3. Bowman’s capsule hydrostatic pressure: P BCposes filtration and is typically about 15mmHg.
It is higher than the hydrostatic pressure in the interstitial fluid surrounding most capillary
beds because the relatively large volume of fluid that filters out of the glomerular capillaries is
funneled into the restricted space of Bowman’s capsule.
4. Glomerular osmotic pressure: GCopposes filtration because the presence of proteins in the
plasma tends to draw filtrate back into the glomerulus. ThGCis 29mmHg (normal =
25mmHg). Due to the blood flow that loses a substantial fraction of water due to filtration