Renal system

RENAL SYSTEM

Renal Lecture 1 Dr. Janan Alrefaee


Functions of the kidneys in homeostasis 1- Excretion of metabolic waste products: like urea, creatinine, uric acid, bilirubin and metabolites of various hormones. The kidneys also eliminate most toxins and other foreign substances that are either produced by the body or ingested. 2- Regulation of water and electrolyte balances. 3- Regulation of body fluid osmolality and electrolyte concentrations 4- Regulation of arterial pressure. 5- Regulation of acid-base balance. 6- Secretion, metabolism, and excretion of hormones as: The kidneys secrete erythropoietin and regulation of 1, 25–dihydroxyvitamin D3 production. 7- Gluconeogenesis: The kidneys synthesize glucose from amino acids.

Physiologic anatomy of the kidneys 1-General organization of the kidneys and urinary tract The two kidneys lie on the posterior wall of the abdomen, outside the peritoneal cavity. Each kidney of the adult human weighs about 150 grams and is about the size of a clenched fist. The kidney has outer cortex and the inner medulla. The renal pyramids base originates at the border between the cortex and medulla and terminates in the papilla, which projects into the renal pelvis. The outer border of the pelvis has major calyces that divide into minor calyces. The walls of the calyces, pelvis, and ureter contain contractile elements that propel the urine toward the bladder.

General organization of the kidneys and the urinary system.

2-Renal Blood Supply The renal artery enters the kidney through the hilum and then branches progressively to form the interlobar arteries, arcuate arteries, interlobular arteries (also called radial arteries) and afferent arterioles, which lead to the glomerular capillaries. The distal ends of the capillaries of each glomerulus coalesce to form the efferent arteriole, which leads to a second capillary network, the peritubular capillaries, that surrounds the renal tubules.

The renal circulation is unique in that it has two capillary beds, the glomerular and peritubular capillaries, which are arranged in series and separated by the efferent arterioles. The peritubular capillaries empty into the vessels of the venous system, which run parallel to the arteriolar vessels and progressively forms the interlobular vein, arcuate vein, interlobar vein, and renal vein, which leaves the kidney beside the renal artery and ureter.

3-The nephron is the functional unit of the kidney Each kidney in the human contains about 1 million nephrons, each capable of forming urine. The kidney cannot regenerate new nephrons and after age 40 the nephron number decrease with aging. Each nephron contains glomerulus & a long tubule. The glomerulus which is branching and anastomosing capillaries that have high hydrostatic pressure (about 60 mm Hg) than other capillaries.

The total glomerulus is encased in Bowman’s capsule. Fluid filtered from the glomerular capillaries flows into Bowman’s capsule, then into the proximal tubule (in the cortex), then into the loop of Henle, which dips into the renal medulla. Each loop consists of a descending and an ascending limb. The walls of the descending limb and the lower end of the ascending limb are very thin called the thin segment of the loop of Henle. After the ascending limb of the loop has returned partway back to the cortex, its wall becomes thicker called the thick segment of the ascending limb. At the end of the thick ascending limb is a short segment, which is a plaque in its wall, known as the macula densa. Beyond the macula densa, fluid enters the distal tubule in the renal cortex.



This is followed by the cortical collecting tubule then cortical collecting duct. The initial parts of 8 to 10 cortical collecting ducts join to form a single larger collecting duct that runs downward into the medulla and becomes the medullary collecting duct. The collecting ducts combine to form progressively larger ducts that empty into the renal pelvis through the tips of the renal papillae. In each kidney, there are about 250 of the very large collecting ducts, each of which collects urine from about 4000 nephrons.

Regional differences in nephron structure: cortical and juxtamedullary nephrons. Cortical nephrons have glomeruli located in the outer cortex, they have short loops of Henle that penetrate only a short distance into the medulla and have an extensive network of peritubular capillaries.


juxtamedullary nephrons (about 20 to 30 % of the nephrons) have glomeruli that lie deep in the renal cortex near the medulla. They have long loops of Henle that dip deeply into the medulla, in some cases all the way to the tips of the renal papillae. They have long efferent arterioles extend from the glomeruli down into the outer medulla and then divide into specialized peritubular capillaries called vasa recta that extend downward into the medulla, lying side by side with the loops of Henle. Like the loops of Henle, the vasa recta return toward the cortex and empty into the cortical veins.

Urine Formation Urinary excretion rate = Glomerular filtration rate - Tubular reabsorption rate + Tubular secretion rate Fig.1.6 shows the renal handling of four hypothetical substances.

Renal handling of four hypothetical substances.

Why large amounts of solutes are filtered (high GFR) and then reabsorbed by the kidneys? 1-A high GFR remove waste products from the body rapidly. 2- A high GFR allows the entire plasma to be filtered and processed about 60 times each day, because the plasma volume is about 3 liters and the GFR is about 180 L/day. This allows the kidneys to precisely and rapidly control the volume and composition of the body fluids.

Glomerular filtration—the first step in urine formation Urine formation begins with filtration of large amounts of fluid through the glomerular capillaries into Bowman’s capsule.

Composition of the glomerular filtrate The composition of the glomerular filtrate is the same as plasma except that: 1- It has no proteins and cellular elements, including red blood cells. 2- A few low-molecular-weight substances as calcium and fatty acids, that are not freely filtered because they are partially (half) bound to the plasma proteins.

Glomerular Capillary Membrane The glomerular capillary membrane has three major layers (instead of two in other capillaries): the endothelium, a basement membrane and epithelial cells (podocytes). Despite the three layers, it filters several hundred times as much as the usual capillary membrane which is due partly to its special characteristics. 1-The capillary endothelium is perforated by thousands of small holes called fenestrae. 2- The basement membrane has large spaces. 3- Epithelial cells layer are not continuous but have long footlike processes (podocytes) that encircle the outer surface of the capillaries. The foot processes are separated by slit pores.

Despite the high filtration rate of the glomerular capillary membrane, it has a high degree of selectivity. This selectivity of the glomerular capillary membrane depends on: 1- Size of the molecules (filterability is inversely related to the size of the molecules). 2-electrical charge of the molecules (negatively charged molecules are restricted during filtration unlike positively charged molecules of equal size, this due to the negative charges present in all 3 layers of the glomerular capillary membrane which restrict filtration of plasma proteins). The diameter of the albumin (protein) is about 6 nanometers, whereas the pores of the glomerular membrane are about 8 nanometers but albumin is restricted from filtration.


Determinants of the GFR GFR = Kf * Net filtration pressure Where: Kf is the glomerular capillary filtration coefficient The net filtration pressure represents the sum of the forces include forces include

The net filtration pressure represents the sum of the forces include forces include (1) glomerular hydrostatic pressure, (PG, 60), which promotes filtration; (2) the hydrostatic pressure in Bowman’s capsule (PB, 18) outside the capillaries, which opposes filtration; (3) the colloid osmotic pressure of the glomerular capillary plasma proteins (πG, 32) which opposes filtration; and (4) the colloid osmotic pressure of the proteins in Bowman’s capsule (πB, 0), which promotes filtration. (Under normal conditions, the concentration of protein in the glomerular filtrate is so low that the πB is considered to be zero.) Net filtration pressure = PG-PB- πG + πB Net filtration pressure = 60 –18 – 32+0 = +10 mm Hg