Anatomy: Chapter 26: Urinary System

Proximal Convoluted tubule

approximately 14 mm long, 60 um in diameter. Wall is composed of simple cuboidal epithelium, cells rest on the basement membrane which forms the outer surface of the tubule. These cells have many microvilli projecting from the luminal surface of the cells.

Vasa Recta

associated with the juxtamedullar nephrons are specialized sets

Where does most of the reabsorption occur?

the proximal convoluted tubule. The cells here have an apical surface which makes up the inside surface of the tubule wall; a basal surface, which forms the outer wall of the tubule; and lateral surfaces which are bound to the surfaces of other cells of the tubule.

Functions of the kidneys

1) Excretion 2) Regulation of blood volume and pressure 3) Regulation of blood solute concentrations 4) Regulation of extracellular fluid 5) Stimulation of Red Blood Cell Synthesis 6) Activation of Vitamin D

Urine Production

1) Filtration-Filtration is the movement of materials across the filtration membrane into the Bowman capsule to form filtrate. 2) Tubular reabsorption-Solutes are reabsorbed across the wall of the renal tubule into the interstitial fluid by transport processes, such as active transport and cotransport. Water is reabsorbed across the wall of the renal tubule by osmosis. Water and solutes pass from the interstitial fluid into the peritubular capillaries. 3) Tubular secretion-solutes are secreted across the wall of the renal tubule into the filtrate.

Types of Filtration Pressure

1) Glomerular Capillary Pressure (GCP) is an outward pressure fro the blood pressing on the capillary walls. Simply blood pressure. GCP forces fluid and solutes out of the blood into the Bowman capsule. GCP is larger than any of the other capillaries of the body. Due to smaller arteriole diameter. 2) Capsular hydrostatic Pressure (CHP)- an inward pressure that opposes filtration. This pressure is due to the force of filtrate volume on the wall of the Bowman capsule. Due to volume in a tube or chamber. 3) Blood colloid osmotic pressure (BCOP)- is also an inward pressure that opposes filtration. It is due to the osmotic pressure of plasma proteins in the glomerular capillaries. The presence of these proteins draws fluid back into the glomerular presence of these proteins draws fluid back into the glomerular capillary from the Bowman capsule. Greater at the end of the glomerular capillary than at its beginning because there is a high protein concentration at the end of the glomerulus.

Filtration Pressure

1) Glomerular capillary pressure the blood pressure within the glomerulus, moves fluid from the blood into the Bowman capsule. 2) Capsular hydrostatic pressure, the fluid pressure inside the Bowman capsule moves fluid from then Bowman capsule into the blood. 3) Blood colloid osmotic pressure produced by the concentration of the blood proteins in the glomerular capillaries moves fluid from the Bowman capsule into the blood by osmosis. 4) Filtration pressure is equal to the glomerular capillary pressure minus the capsular hydrostatic and blood colloid osmotic pressures.

Filtration Membrane Structures

1) The fenestrated glomerular capillaries 2) The basement membrane between the capillary wall and the visceral layer of the Bowman capsule 3) Podocytes of the visceral layer of the Bowman capsule.

Renal Corpuscle

1) The renal corpuscle consists of the Bowman capsule and the glomerulus. The bowman capsule is the enlarged end of the renal tubule, which is indented to form a double-walled chamber. The Bowman capsule surrounds the glomerulus, which is a network of capillaries. Blood flows from the afferent arteriole into the glomerulus and leaves the glomerulus through the efferent arteriole. 2) The visceral layer of the Bowman capsule covers the glomerular capillaries. Fluid from the blood enters the Bowman capsule by passing through the capillary walls and the visceral layer of the Bowman capsule. From there, fluid passes into the proximal convoluted tubule of the nephron. The juxtaglomerular apparatus consists of cells from the wall of the afferent arteriole and the distal convoluted tubule. 3) The glomerulus is composed of fenestrated capillaries. The visceral layer of the Bowman capsule consists of specialized cells called potocytes. Spaces between the podocyte cell processes are called filtration slits. 4) The filtration membrane consists of the fenestrated glomerular capillaries endothelium, a basement membrane, and the podocytes cell processes. Fluid passes from the capillary through the filtration membrane into the bowman capsule.

Efferent Arteriole

transports the filtered blood away from the glomerulus.

Macula densa

A part of the distal convoluted tubule of the nephron lies between the afferent and efferent arterioles next to the renal corpuscle. In this section of the distal convoluted tubule, there is a group of specialized cells.

Filtration Membrane

Consists of capillary endothelium, the basement membrane, and the podocytes of the Bowman capsule. Preforms the first major step in urine production. Filtered fluid then enters the lumen or space inside the Bowman capsule.

Juxtaglomerular Cells

At the point where the afferent arteriole enters the renal corpuscle, it has a cuff of specialized smooth muscle cells around it. Renin secreted here plays an important role in the regulation of filtrate formation and blood pressure.

Filtration

Blood pressure int he glomerular capillaries forces fluid and small molecules out of the blood. Now its called filtrate. Nonselective and separates based only on size or charge of molecules. Does not remove everything in the blood and only those substances that fit through the filtration membrane.

Loop of Henle

Every loop of hence has a descending limb and as ascending limb. The first part of the descending limb is similar in structure to the proximal convoluted tubule. The portion that extends into the medulla becomes very thin near the bend of the loop.

Arteries and Veins of the Kidneys

The renal artery branches off the abdominal aorta and enters the renal sinus of each kidney. Segmental arteries diverge from the renal artery to form interlobar arteries, which ascend within the renal columns toward the renal cortex. Branches from the interlobar arteries diverge near the base of each pyramid and arch over the bases of the pyramids to form the arcuate arteries.

Reabsorption of Solutes in the proximal Convoluted Tubule

The support of molecules and ions across the epithelial lining of the proximal convoluted tubule depends on the active transport of Na+ in exchange for K+ across the basal membrane. Symport is the process by which carrier proteins move molecules or ions with Na+ across the apical membrane. The Na+ concentration gradient provides the energy for support. Amino acids, glucose, K+, Cl- and most other solutes are transported into eh tubule cells with Na+. Water enters and leaves the cell by osmosis. Glucose, amino acids, Na+, Cl-, and many other solutes leave the cells across the basal membrane by facilitated diffusion.

Renal Fraction

This portion of the total cardiac output that flows through the kidneys.

Hilum

a small area of the concave, medial side of the kidney that is continuous with an adipose and connective tissue filled cavity of the kidney, called the renal sinus.

Filtration membrane

a specialized structure that filters blood int he kidneys. It is located within the renal corpuscle in the renal cortex. Allows water and small molecules to leave the blood the blood while preventing blood cells and most proteins from leaving the blood. The basis of separation is both size and charge of the blood components. Principle component is the glomerulus. The glomerular capillaries are many times more permeable than a typical capillary

Histology of the Nephron

a) Proximal convoluted tubule-the luminal surface of the epithelial cells is lined with numerous microvilli. The basal surface of each cell rests on the basement membrane and each cell is bound to the adjacent cells by tight junctions. The basal margin of each epithelial cell has deep invaginations and numerous mitochondria are adjacent to the basal membrane. Active reabsorption and secretion are major functions. b) descending limb of hence- the thin segment of the descending limb is composed of simple squamous epithelial cells that have microvilli and contain a relatively small number o mitochondria. Water easily diffuses from the thin segment into the interstitial fluid. c) Distal convoluted tubule- the cells have sparse microvilli and numerous mitochondria, and they actively reabsorb Na+ and K+ and Cl-. d) Collecting duct- the cells have some microvilli and numerous mitochondria, and they actively reabsorb Na+, K+ and Cl-

Reabsorption in the Loop of Henle

a) The wall of the thin segment of the descending limb of the loop of Henle is permeable to water and, to a lesser extent, to solutes. The interstitial fluid in the medulla of the kidney and the blood in the vasa recta have a high solute concentration (high osmolality). Water therefore moves by osmosis from the tubule into the interstitial fluid and into the vasa recta. An additional 15 % of the filtrate volume is reabsorbed. To lesser extent, solutes diffuse from the vasa recta and interstitial fluid into the tubule. b) The thin segment of the ascending limb of the loop of Henle is not permeable to water but is permeable to solutes. The solutes diffuse out of the tubule adn into the more dilute interstitial fluid as the ascending limb projects toward the cortex. Then the solutes diffuse into the descending vasa recta.

Tubular reabsorption

cells throughout the nephron contain many transport proteins. These move water and some filtered molecules fro the filtrate back into the blood in the peritubular capillaries. This prevents them from being lost from the body as components of urine. Most of the filtered water and useful solutes have been returned to the blood by the time the filtrate has been modified to urine, whereas the remaining waste or excess substances and a small amount of water form urine. For certain solute there is a higher concentration in the urine compared to the plasma. This is a result of secretion and its explained in more detail.

Tubular secretion

certain tubule cells transport additional solutes from the blood into the filtrate. Some of these solutes may not have been filtered by the filtration membrane.

Glomerular Filtration Rate (GFR)

filtrate produced per minute.

Filtrate

fluid filtered from the glomerular capillaries

Glomerular capillaries

have fenestrae in which neither large proteins nor blood cells can fit through them. Have filtration slits that are between the cell processes of the podocytes of the visceral layer.

Cortical nephrons

have renal corpuscles that are distributed throughout the cortex. Loops are shorter and are closer to the outer edge of cortex.

Juxtamedullary nephrons

have renal corpuscles that are found near the medulla. Have long loops of Henle which are deep in medulla. Well adapted for water conservation. Only about 15 percent are this type

Visceral Layer

inner layer of the bowmans capsule.

Juxtaglomerular apparatus

located next to the glomerulus. Consists of a unique set of afferent arteriole cells and specialized cells in the distal convoluted cells that are in close contact with each other.

Autoregulation

maintenance of GFR is due to this process. Achieved through two processes: the myogenic mechanism and tubuloglomerular feedback. Myogenic mechanism is associated with the intrinsic properties of smooth muscle cells in the afferent and efferent arterioles. Smooth muscle cells act as stretch receptors, which detect changes in bp. When blood pressure decreases the smooth muscle in the wall of the afferent arteriole relaxes and the vessels dilate. Blood supply to the glomerulus and thus GFR fluctuates very little, even when mean arteriole pressure changes. The tubuloglomerular feedback mechanism correlates filtrate flow past the macula dense cells detect an increased flow rate, these cells send a signal to the juxtapositionglomerular apparatus to GFR.

Parietal Layer

outer layer of the bowmans capsule.

Interlobular arteries

project from the arcuate arteries into the cortex, and afferent arterioles are derived from the interlobular arteries or their branches. The afferent arterioles supply blood to the glomerular capillaries of the renal corpuscles. Efferent arterioles arise from the glomerular capillaries and carry blood away from the glomeruli. Each efferent arteriole exits the glomerulus, it gives rise to a plexus of capillaries, called peritubular capillaries, around the proximal and distal convoluted tubules.

Apical membrane

separates the lumen of the proximal convoluted tubule from the cytoplasm of epithelial cells. Each of these carrier proteins binds specifically to one of those substances to be transported and to Na+. The concentration gradient for Na+ provides energy that moves both the Na+ and the other molecules or ions from the

Afferent Arteriole

supplies blood to the glomerulus for filtration.

Bowman capsule

surrounding the glomerulus is an indented, double-walled chamber.

Renal fascia

surrounds the adipose tissue and helps anchor the kidneys to the abdominal wall.

Renal Capsule

surrounds the kidney, connective tissue. Capsule is a thick layer of adipose tissue which cushions and protects the kidneys.

Renal corpuscle

the glomerulus and bowman capsule form this together.

urinary system

the major excretory system of the body.

Tubular Reabsorption

the return of water and solutes in the filtrate to the blood. Nearly all 99% of the water and solutes are rapidly returned to the blood via the renal tubules, and because of this, toxins are quickly removed from the blood. Solutes reabsorbed from the lumen of the renal tubule to the interstitial fluid include amino acids, glucose and fructose as well as Na, K, Ca, HCO3 and Cl-.

Nephron

the unit of the kidney. 1.3 million in each kidney. Parts are as follows: 1) Renal Corpuscle in the cortex-filters the blood 2) Proximal Convoluted tubule in the cortex-returns filtered substances to the blood 3) The loop of Henle-conserves water and solutes 4) The distal convoluted tubule in the cortex-rids blood of additional wastes.

Podocytes

the visceral layer which has these specialized cells that wrap around the glomerular capillaries.