topic 8_ the kidney

7/28/2019 Topic 8_ the Kidney

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The Kidney


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What do the kidneys do?

remove toxic waste products

remove excess water and salts

play a part in controlling your blood pressure produce erythropoietin (epo for short) which

stimulates red cell production from the bone marrow -you get anaemic without this

help to keep calcium and phosphate in balance for

healthy bones maintain the blood in a neutral (non-acid) state


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Blood is brought to the kidney in the renal artery. The kidneys filter the

blood and then reabsorb useful materials such as glucose. After it has

been purified the blood returns to the circulation through the renal vein.


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Urine is taken from the kidneys to the bladder bythe ureters. The bladder stores the urine until itis convenient to expel it from the body. The mainwaste product removed in the urine is urea.

Urea is a waste product produced in the liverwhen surplus amino acids are broken down tobe used for energy. It is released into thebloodstream until it is filtered out and removedby the kidney.

Urine is produced in microscopic structures inthe kidney called nephrons. Each kidney hasmany millions of nephrons


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Blood is filtered in the glomerulus. The filtrate is

collected by the bowman's capsule and enters

the tubules. Useful substances such as glucose,

some salt and water are reabsorbed into the

blood. Reabsorption is done by blood capillarieswhich are closely wrapped round the tubules.

The waste, consisting of water, some salt and

urea is urine. The urine is collected by the

collecting duct, taken to the ureters and then tothe bladder.


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A is the glomerulus. It is a ball of blood capillaries in

which filtration takes place. Some of the liquid plasma isforced out of the capillaries whilst the solid cells andplatelets remain behind.

B is a tubule. The filtrate from the glomerulus flowsslowly through the tubule. As it does so the bloodcapillaries which surround it re-absorb the usefulsubstances such as glucose, some salt and some water.Everything which is not re-absorbed is release as urine.

C is the collecting duct. This collects the fluid from theends of many tubules, which is now urine, and takes it tothe ureters for transport to the bladder.


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This is a schematic diagram illustrating the kidneys' ability to separate

particles in the blood in order to maintain optimal body chemistry. Blood

enters the kidney through the renal artery. In the kidney, the blood

undergoes filtration and dialysis to separate the particles that will be

removed from the body (through the ureter to the bladder) from those that

will be returned to the circulating blood (through the renal vein).


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Nephron Segment Function

Renal Corpuscle:

Glomerulus

Bowman's

Capsule

Filtration:

Glomerulus filters proteins and cells from the

blood.

All other blood components pass into

Bowman's capsule, then into the tubule.

U-Shaped Tubule

Reabsorption and Secretion:Semipermeable membranes surrounding the

tubule allow selective passage of particles

back into the blood (reabsorption), or from the

blood into the tubule (secretion).

Collecting Duct

Collection:

Collects all material that has not returned to

the blood through the tubular membranes. This

material exits the kidney as urine.


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Renal Corpuscle

Blood first enters the kidney through the renal artery which branches into anetwork of tiny blood vessels called arterioles. These arterioles then carrythe blood into the tiny blood vessels of the glomerulus. It is here, in the renalcorpuscle, where filtration occurs.

The glomerulus filters proteins and cells, which are too large to pass

through the membrane channels of this specialized component, fromthe blood. These large particles remain in the blood vessels of theglomerulus, which join with other blood vessels so that the proteins remaincirculating in the blood throughout the body. The small particles (e.g., ions,sugars, and ammonia) pass through the membranes of the glomerulus intoBowman's capsule. These smaller components then enter the membrane-

enclosed tubule in essentially the same concentrations as they have in theblood.

Hence, the fluid entering the tubule is identical to the blood, exceptthat it contains no proteins or cells.


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Tubule

The tubule functions as a dialysis unit, in which the fluid inside thetubule is the internal solution and the blood (in capillaries surroundingthe tubule) acts as the external solution. Particles may pass throughthe membrane and return to the blood stream in the process known asreabsorption, which is analogous to the movement of particles fromthe internal to the external solution in the dialysis experiment youperformed in lab.

The reabsorption of many blood components is regulatedphysiologically, as discussed below. Alternatively, particles may passthrough the membrane from the blood into this tubule in the processknown as secretion, which is analogous to the movement of particlesfrom the external solution into the dialysis bag in the experiment you

performed in lab. The most important particles that are secreted fromthe blood back into the tubules are H+ and K+ ions, as well as organicions from foreign chemicals or the natural by-products of the body'smetabolism.


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Collecting Duct

The blood components that remain in the nephron whenthe fluid reaches the collecting duct are excreted fromthe body.The collecting duct from one nephron meets upwith many others to feed into the ureter. The ureters

(one from each kidney) enter the bladder, which leads tothe urethra, where the liquid waste is excreted from thebody. Hence, the material that is filtered and secretedfrom the blood into the tubule, less the amount thatis reabsorbed into the blood, is ultimately excreted

from the body


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Normal Renal Funct ion

Functions of the Kidney:

balances solute and water transport

excretes metabolic waste products

conserves nutrient

regulates acid-base balance

secretes hormones that help regulate bloodpressure, erythrocyte production, and calciummetabolism.

forms urine


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Formation of Urine in the Kidney

A kidney contains around one millionnephrons. In general, the kidneys can

adequately function with only one third of

the normal number of nephrons. Less than

that, the body will retain waste products,

especially urea and creatinine.


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Nephrons process the blood to make

urine. A tuft of capillaries called theglomerulus is contained in each nephron.

The glomerulus is surrounded by

Bowman's capsule. The capillaries areextremely porous, allowing large amounts

of solute-rich fluids to pass from the

capillaries into the capsule. This fluid is theraw material of urine.


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The secondary active secretion of H+ during Na+ reabsorption is calledcountertransport since the ions move in opposite directions. Normally, only20% of total Na+ reabsorption occurs during active secretion of H+ and K+.When H+ ions are not available for exchange as in alkalemia, K+ ions aresecreted. This is why alkalemia may lead to hypokalemia. In H+ and K+secretion, HCO3- ions are reabsorbed in place of Cl-. When chloride ionsare in short supply, there is an increased demand for H+ and K+ secretionto reabsorb sodium.


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The proximal tubules are responsible for the iso-osmotic

reabsorption of water, electrolytes, non-electolytes. Asmuch as 80% of the filtrate is reabsorbed into thecapillaries that line the tubules.

All glucose and amino acids filtered are completelyreabsorbed.

Almost all potassium is reabsorbed. Almost all uric acid is reabsorbed.

90% of bicarbonate is reabsorbed.

Two- thirds of filtered sodium is reabsorbed.

H2O, chloride, and urea are reabsorbed by passive

transport. Hydrogen ion is secreted, creatinine is secreted.

Most of the calcium and phosphate is reabsorbed.


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Past the PCT is the loop of Henle which consists of both ascendingand descending limbs. The descending limb is freely permeable towater, while the ascending is less permeable. In the Loop of Henle

there is continued reabsorption of water, sodium, chloride.

The distal convoluted tubule receives fluid from the loop of Henle.The distal tubules are important in the final regulation of waterbalance and acid-base balance since hydrogen ion is excreted withammonia as ammonium and with phosphate buffers. Bicarbonate is

regenerated in this process and retained in the body.

The collecting tubule then receives the newly formed urine from thenephrons. In the collecting duct water reabsorption is completed. Thefinal concentration of urine takes place here under the control of anti-diuretic hormone (ADH). In the presence of ADH, more water is

reabsorbed.

The urine flows through the minor and major calyces of the renalpelvis into the ureter. From the ureter, urine makes its way tobladder.


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Controlling Kidney Function

Water and ion volume and content in urine is controlled by hormones.

ADH. . . Antidiuretic hormone is secreted from the posterior pituitary.

The target tissue for this hormone is the distal convoluted tubule which reabsorbsmore water back into the blood in response to ADH

The effect of ADH is: less urine, greater blood volume, and greater blood pressure.

Aldosterone. . . This hormone is secreted from the adrenal cortex.

The target tissue for this hormone is the distal convoluted tubule which reabsorbsmore sodium (Na+) in response to aldosterone.

When more Na+ is reabsorbed water follows by osmosis.

The effect of aldosterone is less urine, greater blood volume, and greater bloodpressure


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Atrial Natriuretic factor. . . Hormone released from the right atrium

This hormone is released in response to high blood pressure.

It triggers increased urine production, which results in lower blood volume and lowered blood pressure.

Renin. . . Is released from the kidney in response to low blood pressure.

Renin causes angiotensinogen to be converted to angiotensin.

Angiotensin causes:

1. Vasoconstriction2. Aldosterone secretion3. ADH secretion

The combined affect is decreased urine output, increased blood volume, and increased bloodpressure.

Parathyroid Hormone. . . Is released from the parathyroid glands

This hormone increases calcium reabsorption from the distal convoluted tubule into the blood.


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Nervous control. . . Sympathetic

stimulation to the kidneys constricts

arteries

This results in decreased blood flow,

decreased filtrate and therefore decreased

urine production.


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Blood filtered by the glomerulus

GF = glomerulus filtering. TR = tubular reabsorption. TS = tubular

secretion


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Summary of Chemical Changes of

Filtered Blood

Reabsorbed: sugar, sodium, vitamins,

nutrients, water

Secreted: hydrogen ion, potassium,

ammonia, drugs, toxins


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Summary of Chemical Changes of Filtered Blood

Reabsorbed sugar, sodium, vitamins, nutrients, water

Secretedhydrogen ion, potassium, ammonia, drugs,

toxins


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Example Effects of the Kidney Function

SubstanceAverage percentageof filtered substance

re-absorbed

Average percentageof filtered substance

excreted

Water 99 1Sodium 99.5 0.5

Glucose 100 0

Urea (wasteproduct)

50 50

Phenol (waste

product)

0 100


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Inside each kidney, blood ischannelled to 1 million tiny filters,each one called a glomerulus (the

plural is glomeruli). Glomeruli areeach joined onto small tubes(tubules) to make nephrons, andeach day 150 litres of filtratepasses into the nephrons - 100times the average daily amount ofurine. As the fluid passes along

these tubes, most of it is takenback into the blood stream(reabsorbed), leaving only thosethings that are to be sent out in theurine. Control mechanisms canvary the amount of water, salt, andother substances that are left in theurine when it finally goes into the

ureter and down to the bladder.

topic 8_ the kidney

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