The Kidney and formation of urine

Objectives• State the main functions of the kidney

• Label a diagram to illustrate the location of the kidneys, ureters and bladder within the body

• Label a diagram of the kidney (to illustrate the renal cortex, renal medulla, renal pyramid, renal pelvis, major and minor calyces, renal artery and vein and the ureter)

• Label a diagram of a nephron (to illustrate the afferent and efferent arteriole, glomerulus, glomerular capsule, proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct)

Objectives• List and briefly explain the three stages in the process of

urine formation

• State the main role of ADH and aldosterone in the formation of urine

• List the main constituents of normal urine

• State the average urine output daily

• Briefly discuss the importance of the kidney in relation to homeostasis

Main function of the Kidney• The main function of the urinary system is: Elimination of

water-soluble substances

• Each day the kidneys filter >150L of fluid from the blood plasma

• Most of this fluid (containing vital constituents such as electrolytes, nutrients and water) is recovered by the kidneys and returned to the blood

• This leaves only 1 - 2L of fluid (containing water soluble waste) to be excreted as urine

There are 2 kidneys, There are 2 kidneys,

situated at the back of the abdomensituated at the back of the abdomen

The kidneys and homeostasisThe kidneys are regulatory organs that function

to maintain a normal body state under variations in the environment (homeostasis)

Which of the graphs best represents homeostasis?

The Nephron

• Functional unit of the kidney is the nephron (about one million nephrons in each kidney)

Formation of Urine

Three Phases:-1. Glomerular Filtration

2. Selective Reabsorption

3. Secretion

Glomerular Filtration

What you think will happen to water and other small (water soluble) molecules such as glucose, water, salts, urea?

Answer: They will be forced into glomerular capsule

Glomerular Filtration

What will happen to large molecules (such as blood cells and plasma proteins)?

Answer: They will remain in the capillary

Glomerular Filtration

Capillary wall

Capsule wall

Tubule Peritubular capillary

Fluid in here = FILTRATE

good blood flowgood blood flow

high capillary pressureshigh capillary pressures

Reabsorption

An extensive network of capillaries around the renal An extensive network of capillaries around the renal tubulestubules provides a provides a large surface arealarge surface area for contact for contact

ReabsorptionHow essential substances are recovered by the

body (and not lost in the urine) = Reabsorption

This will be achieved by re-absorption (or not) of constituents from the filtrate in the tubules to the blood in the peri-tubular capillaries

Purpose of re-absorption is to reabsorb filtrate constituents needed by the body to maintain fluid and electrolyte balance and blood alkalinity

Formation of Urine: Re-absorption

Peritubular capillary

TubuleThe function of re-absorption is to:

•Retain substances that are needed by the body

•E.g. glucose, amino acids, water, salts

Reabsorption

What of the substances in the Filtrate would the body want to recover:

• Completely?

• Partially?

• Not at all?

Reabsorption

Complete reabsorption• Some constituents of the GF are completely

reabsorbed (actively) unless present in excessive quantities

• This is because they are needed by the body e.g. glucose 100% absorbed

Reabsorption

Partial Reabsorption• The re-absorption of some substances

varies according to the need e.g. Water and salts

• Anti Diuretic Hormone (ADH) regulates the amount of water reabsorbed by the kidney

ReabsorptionIncreased blood concentration

Detected by hypothalamus

Secretion of ADH

ADH released by the pituitary

More water reabsorbed

Decreased urine output and Decreased blood concentration

Reabsorption

Partial Reabsorption• The hormone aldosterone increases re-

absorption of sodium by the kidney (water follows)

• This will lead to a decreased urine output

ReabsorptionNo reabsorption• Waste products are absorbed only to a slight

extent (e.g. UREA) or not at all (e.g. CREATININE)

• Urea is the chief nitrogenous waste of mammals.

• Creatinine is a break-down product of creatine phosphate in muscle, and is usually produced at a fairly constant rate by the body.

The urea cycleBreakdown of amino acids (deamination)

AmmoniaAmmonia is extremely toxic!

The liver contains enzymes which quickly converts the ammonia into urea.(less toxic than ammonia)

Urea is removed efficiently by the KIDNEYS

Recap – Filtration and Reabsorption

Secretion

How excess waste (and / or toxic substances) is removed from the blood and added to the urine = Secretion

Addition of substances from the blood stream to the filtrate e.g. some drugs and H+ ions

Functions of secretion is to eliminate waste and maintain blood pH

Formation of Urine: SecretionPeritubular capillary

Toxins, Drugs, Waste, H+Tubule

The function of secretion is to:

* Remove substances that are toxic

* Maintain acid-base balance

Secretion is the opposite of re-absorption

Filtrate Urine Bladder

Filtration, Reabsorption, Secretion

Normal amounts and constituents of urineH2O 1 - 2 L/day - average

?? mls/day – minimum30 – 40 mls/hr - minimum

Urea

Creatinine

Uric acid

Na+, K+, Mg++

Formation of Urine: Filtration

Blood cells

Plasma proteins

Small molecules

Glomerular blood

Capillary wallCapsule wall

TubulePeritubular capillary

Fluid in here = FLTRATE

Formation of Urine: Re-absorption

Peritubular capillary Tubule

The function of re-absorption is to:

•Retain substances that are needed by the body

•E.g. glucose, amino acids, water, salts

Formation of Urine: SecretionPeritubular capillary

Toxins, Drugs, Waste, H+Tubule

The function of secretion is to:

* Remove substances that are toxic

* Maintain acid-base balance

Secretion is the opposite of re-absorption

Filtrate Urine Bladder

Elimination of Urine: MicturitionKidneys

Ureters

Bladder

Urethra

Case Study 1• Callum is 12 months old and has been having very

loose stools for a couple of days.

• What effect is this likely to have on his fluid balance?

• What mechanisms should enable homeostasis to be maintained?

• What advice would you give his mother?

Case Study 2• Stephen is 21 years old and has been drinking excessive

amounts of alcohol!

• How does alcohol effect his fluid balance?

• What advice would you give him to maintain homeostasis and health?

• What problems might you encounter in giving such advice?

• What strategies are in place to reduce the amount of alcohol consumed by young people?

Case Study 3• Sarah has been taking Clozapine to control the symptoms of her

schizophrenia. • Her CPN visits her and she confides in him that her housemates are

stealing her possessions during the night and have also been trying to poison her.

• She has started locking her door and is refusing to eat anything unless it is sealed and she has bought it herself from the local shop.

• The CPN suspects that Sarah has been taking cannabis (she has a history of drug abuse).

• How might the mental health team establish if Sarah is experiencing drug induced psychosis?

• What other possible explanations are there for this situation?

Case Study 4• Caroline is 29 and expecting twins. She is 30 weeks

into her pregnancy.• She visits the clinic for a routine assessment.• The nurse does a urine tests and finds that there is

protein present in Caroline’s sample.• Should there be protein in Caroline’s urine?• If not, why not?• Does this indicate a problem with Caroline’s

kidneys?• Where else might the protein have come from?

Case Study 5• Mrs Evans (who is 65 years of age) has Non Insulin

Dependent Diabetes Mellitus (NIDDM).• She regularly tests her blood glucose levels.• This morning, after breakfast, it was 9mmol/L.• Would you expect her urine to contain glucose?

(explain your answer)• After lunch it was 12mmol/L.• Now would you expect her urine to contain

glucose? (explain your answer).• What are the implications in terms of urine testing

for people with diabetes?