Overview of Renal Physiology

PCB3702L Dr. Lisa Brinn Urinary System Lab

I. Overview of Renal Physiology – The kidneys do most of the work within the urinary system.

Other parts are mainly passageways and storage areas. The functions of the kidneys include

the following:

A. Excretion of wastes – By forming urine, kidneys help excrete wastes from body. Some

wastes excreted in urine result from metabolic reactions. These include urea and ammonia

from deamination of amino acids; creatinine from breakdown of creatine phosphate; uric

acid from catabolism of nucleic acids; and urobilin from breakdown of hemoglobin. These

products are collectively known as nitrogenous wastes because they are waste products

that contain nitrogen. Other wastes excreted in the urine are foreign substances that have

entered the body, such as drugs and environmental toxins.

B. Regulation of blood ionic composition – Kidneys help regulate the blood levels of several

ions by adjusting the of these ions that are excreted into urine, including sodium ions (Na+),

potassium ions (K+), calcium ions (Ca2+), chloride ions (Cl−), and phosphate ions (HPO4 2−).

C. Regulation of blood pH – Kidneys excrete a variable amount of hydrogen ions (H+) into the

urine and conserve bicarbonate ions (HCO3 −), which are an important buffer of H+ in blood.

Both activities help regulate blood pH.

D. Regulation of blood volume – Kidneys adjust blood volume by returning water to blood or

eliminating it in urine. An increase in blood volume increases blood pressure; a decrease in

blood volume decreases blood pressure.

E. Regulation of blood pressure – Kidneys help regulate blood pressure by secreting the

enzyme renin, which activates the renin–angiotensin–aldosterone pathway. Increased

renin causes increase in blood pressure.

F. Maintenance of blood osmolarity – By separately regulating loss of water and loss of

solutes in the urine, kidneys maintain a relatively constant blood osmolarity close to 300

milliosmoles per liter (mosmol/liter). (The osmolarity of a solution is a measure of the total

number of dissolved particles per liter of solution. The particles may be molecules, ions, or a

mixture of both. A similar term, osmolality, is the number of particles of solute/kg water.

Since it is easier to measure volumes of solutions than to determine the mass of water they

contain, osmolarity is used more commonly than osmolality).

G. Production of hormones – Kidneys produce two hormones. Calcitriol, the active form of

vitamin D, helps regulate calcium homeostasis, and erythropoietin stimulates the

production of erythrocytes.

II. Organization of the Kidneys

A. Kidneys

1. Paired bean-shaped organs just above the waist

2. Kidney structure

a. Contains an outer layer called the cortex

b. Contains an inner area called the medulla

c. Renal pyramids – Several cone-shaped structures located within the renal

medulla

d. Major and minor Calyces (singular is calyx)

e. Renal pelvis that leads urine into ureter

and then urinary bladder

3. Functional unit of the kidney is the nephron

a. Nephron parts:

1) Renal corpuscle – where blood plasma is filtered. The two components

of a renal corpuscle are the glomerulus (a capillary network) and

Bowman's capsule, or glomerular capsule, a double-walled epithelial

cup that surrounds the glomerular capillaries

2) Renal tubule – Once blood plasma is filtered at the renal corpuscle, the

filtered fluid passes into the renal tubule, which consists of a single

layer of epithelial cells that lines a lumen. The renal tubule has three

main sections: proximal tubule, loop of Henle, and distal tubule.

The renal corpuscle, proximal tubule, and distal tubule lie within the renal cortex; the loop of Henle

extends into the renal medulla, makes a hairpin turn, and then returns to the renal cortex. From the

renal corpuscle, filtered fluid first enters the proximal tubule. The term proximal denotes that this

part of the renal tubule is closest to the site where the renal tubule attaches to Bowman's capsule.

Most of the proximal tubule is convoluted, which means that it is coiled rather than straight. From the

proximal tubule, filtered fluid enters the loop of Henle, also known as the nephron loop. The first part

of the loop of Henle dips into the renal medulla, where it is called the descending limb. It then makes

that hairpin turn and returns to the renal cortex as the

ascending limb. From the loop of Henle, filtered fluid enters the

distal tubule. The term distal denotes that this part of the renal

tubule is farther away from the site where renal tubule attaches

to Bowman's capsule. The distal tubule is also convoluted.

The distal tubules of several nephrons empty into a single

collecting duct. Multiple collecting ducts in turn unite to form

larger ducts that drain into the minor calyces. Note that as fluid

passes through the nephron and collecting duct, it is referred to

as either filtrate, filtered fluid, or tubular fluid, and its composition can be modified. Once the fluid

exits the collecting ducts, it is referred to as urine, and its composition cannot be altered.

4. Types of nephrons

a. Cortical

1) 80% of the kidney's nephrons are cortical nephrons.

2) Their renal corpuscles lie in the outer portion of the renal cortex, and

they have short loops of Henle that lie mainly in the cortex and

penetrate only into the outer region of the renal medulla

b. Juxtamedullary

1) The other 20% of the nephrons are juxtamedullary nephrons.

2) Their renal corpuscles lie deep in the cortex, close to the medulla, and

they have a long loop of Henle that extends into the deepest region of

the medulla

3) Ascending limb of the

loop of Henle consists of

two portions: a thin

ascending limb followed

by a thick ascending limb

4) Nephrons with long loops

of Henle enable the

kidneys to excrete very

dilute or very

concentrated urine

5. Blood supply to kidney

a. Very extensive – as the kidneys remove wastes from the blood and regulate

its volume and ionic composition

b. Renal artery leads to the afferent arteriole, which:

c. Leads to glomerulus

d. Blood leaves glomerulus through efferent arteriole, which divides to form:

e. Peritubular capillaries that carry blood back to the renal veins and surround:

1) The tubular parts of both cortical and juxtamedullary nephrons that

are within the renal cortex.

2) Short loops of Henle of cortical nephrons.

f. Vasa recta – long loop-shaped capillaries that extend from some efferent

arterioles, that surround long loops of Henle of juxtamedullary nephrons

6. Juxtaglomerular apparatus

a. Important in regulating kidney function

b. Part of the distal tubule and afferent arteriole come in contact

c. Consist of:

1) Macula densa – specialized cells in this region

2) Juxtaglomerular cells (JG) – modified smooth muscle fibers

III. Overview of Renal Physiology

A. To produce urine you perform three basic tasks

1. Glomerular filtration – In the first step of urine production,

water and most solutes in blood plasma move across the wall of

glomerular capillaries into Bowman's capsule and then into the

renal tubule.

2. Tubular reabsorption – As filtered fluid flows through the renal

tubule and collecting duct, tubule and duct cells reabsorb about 99% of the filtered

water and many solutes. Reabsorption is the movement of substances from fluid in

the tubular lumen to blood in the peritubular capillaries. This process allows useful

substances to be returned to the bloodstream. The remaining 1% of filtered fluid

that is not reabsorbed contains substances that the body does not need (wastes,

drugs, excess ions, etc.) and will eventually be excreted into the urine.

3. Tubular secretion – As fluid flows through the renal tubule and collecting duct,

tubule and duct cells also secrete wastes and other substances that are not useful to

the body. Secretion is the transfer of substances from blood in the peritubular

capillaries to fluid in the tubular lumen. This process serves as an additional

mechanism for removing unneeded substances from the bloodstream.

IV. Glomerular Filtration

A. Occurs at the renal corpuscle

B. Consists of glomerulus and Bowman's capsule

1. Bowman’s capsule

a. Organized into two layers

1) Parietal layer – Single cell layer of epithelial cells

2) Visceral layer – Podocytes

2. Renal corpuscle

a. Contains filtration membrane

1) Made of glomerulus and visceral (podocyte) layer of Bowman’s capsule,

forming a leaky barrier

2) Permits filtration of:

i. Water and small solutes

3) Prevents filtration of:

i. Blood cells and nearly all plasma proteins

4) Substances filtered from the blood cross three barriers—the

endothelium of the glomerulus, the basement membrane of the

glomerulus, and the visceral layer of Bowman's

i. Endothelium of the glomerulus. Glomerular endothelial cells are

quite leaky because they have fenestrations (pores) that measure

70–100 nm in diameter. This size permits passage to water and all

solutes in blood plasma. However, the fenestrations are too small

to allow blood cells to filter through the endothelium. Located

among the glomerular capillaries and in the cleft between afferent

and efferent arterioles are mesangial cells.

ii. Basement membrane of the glomerulus. a porous layer of acellular

material between the endothelium and the podocytes, consists of

collagen fibers and negatively charged glycoproteins. The pores

within the basement membrane allow water and most small

solutes to pass through. However, the negative charges of the

glycoproteins repel plasma proteins, most of which are anionic; the

repulsion hinders filtration of these proteins.

iii. Visceral layer of Bowman's capsule. Recall that the visceral layer of

Bowman's capsule consists of podocytes, and each podocyte

contains footlike processes called pedicels that wrap around

glomerular capillaries. The spaces between pedicels are the

filtration slits. A thin membrane, the slit membrane, extends

across each filtration slit; it contains pores that permit the passage

of molecules that have a diameter smaller than 6–7 nm, including

water, glucose, vitamins, hormones, amino acids, urea, ammonia,

and ions. Negatively charged glycoproteins that cover the slit

membrane oppose the filtration of plasma proteins. Because of the

negative charges associated with the podocyte layer and basement

membrane and the small size of the slit membrane pores, less than

1% of albumin, the smallest and most abundant plasma protein,

can pass through the filtration membrane and enter the

glomerular filtrate.

5) The principle of filtration—the use of pressure to force fluids and

solutes through a membrane—is the same in glomerular capillaries as

in capillaries elsewhere in the body. However, the volume of fluid

filtered by the renal corpuscle is much larger than in other capillaries of

the body for three reasons:

i. Glomerular capillaries present a large surface area for filtration

because they are long and extensive. Mesangial cells regulate how

much of this surface area is available for filtration. When

mesangial cells are relaxed, surface area is maximal, and

glomerular filtration is very high. Contraction of mesangial cells

reduces available surface area, and glomerular filtration decreases.

ii. Filtration membrane is thin and porous. Despite having several

layers, the thickness of the filtration membrane is only 0.1 mm

(100 µm). Glomerular capillaries also are about 50 times leakier

than capillaries in most other tissues, mainly because of their large

fenestrations.

iii. Glomerular capillary blood pressure is high. Because the efferent

arteriole is smaller in diameter than the afferent arteriole,

resistance to the outflow of blood from the glomerulus is high. As a

result, blood pressure in glomerular capillaries is considerably

higher than in capillaries elsewhere in the body.

C. Glomerular Filtration Is Determined by the Balance of Four Pressures

1. Glomerular capillary hydrostatic pressure (PGC) – is the blood pressure in

glomerular capillaries. Generally, PGC is about 55 mmHg. It promotes filtration by

forcing water and solutes in blood plasma through the filtration membrane.

2. Bowman’s space hydrostatic pressure (PBS) – is the hydrostatic pressure exerted

against the filtrati

Collepals.com Plagiarism Free Papers

Are you looking for custom essay writing service or even dissertation writing services? Just request for our write my paper service, and we'll match you with the best essay writer in your subject! With an exceptional team of professional academic experts in a wide range of subjects, we can guarantee you an unrivaled quality of custom-written papers.

Get ZERO PLAGIARISM, HUMAN WRITTEN ESSAYS

Why Hire Collepals.com writers to do your paper?

Quality- We are experienced and have access to ample research materials.

We write plagiarism Free Content

Confidential- We never share or sell your personal information to third parties.

Support-Chat with us today! We are always waiting to answer all your questions.