The Kidneys

What is excretion?

The removal of metabolic waste from the body

What are the main functions of the kidneys?

1. Filtering waste products out of the blood

2. Reabsorbing useful solutes

3. Carrying out osmoregulation

What useful solutes do the kidneys reabsorb?

Glucose and amino acids

What is the point of osmoregulation?

To control the water potential of the blood

What supplies the kidneys with blood?

The renal artery

What is blood from the kidney drained by?

The renal vein

What do the kidneys produce?

Urine

What do kidneys remove?

Waste products from the blood

Where does urine pass through?

Kidneys → ureters → bladder → urethra

What is the purpose of the bladder?

For storage of urine

What is the outer region of the kidneys called?

Renal cortex

What is the inner region of the kidneys called?

Renal medulla

What is the very centre of the kidneys called?

Renal pelvis

What does the renal pelvis do?

Drains urine into the ureters

What is a nephron?

A tiny tubule in the kidneys

How many nephrons do the kidneys contain?

Thousands

What is the structure of a nephron (in order)?

Bowman’s capsule → proximal convoluted tubule → loop of Henle → distal convoluted tubule → collecting duct

What shape is the Bowman’s capsule?

Cup shaped

What arteriole does the renal artery join to?

Afferent arteriole

Which direction does the afferent arteriole move blood?

Into the kidneys

What does the afferent arteriole form?

A capillary network called the glomerulus

Where is the glomerulus located?

Within the renal capsule

What are the two parts of the loop of Henle?

The descending limb and the ascending limb

Which parts of the nephron are located in the renal cortex?

Bowman’s capsule, proximal convoluted tubule, distal convoluted tubule

Which parts of the nephron are located in the renal medulla?

Loop of Henle, collecting duct

How many stages are involved in the formation of urine?

4

What is the first stage of urine production?

Ultrafiltration

Where does ultrafiltration take place?

Bowman’s Capsule

Which is wider: efferent arteriole or afferent arteriole?

Afferent arteriole

What does blood flowing into the glomerulus do?

Increases the pressure in the glomerulus compared to the pressure in the Bowman’s capsule

Where does blood pressure move when the pressure increases in the glomerulus?

Forces blood plasma through the pores in the blood capillary, through the basement membrane, epithelial cells of the renal capsule

What does ultrafiltration form?

Glomerular filtrate

How are endothelial cells adapted for ultrafiltration?

Narrow gaps allowing blood plasma through

How is the basement membrane adapted for ultrafiltration?

Mesh of fibres which act as the filter only allowing certain molecules through

What are the fibres in the basement membrane made of?

Collagen and glycoproteins

Which molecules does the basement membrane allow through?

Molecules with a relative molecular mass less than 69,000,, so large proteins are NOT filtered out

What does the presence of proteins do to water potential?

Lowers it, ensures some water is retained

What are epithelial cells of the renal capsule called?

Podocytes

How are the epithelial cells (of the renal capsule) adapted for ultrafiltration?

Have finger-like projections in which fluid can pass between them into the lumen of the Bowman’s capsule

What substances are filtered out to form glomerular filtrate?

Water, amino acids, glucose, urea, inorganic ions (e.g. sodium, potassium, chloride)

What is the second stage of urine production?

Selective reabsorption

Where does selective reabsorption take place?

In the proximal convoluted tubule

What substances are reabsorbed in selective reabsorption?

Glucose, amino acids, some salts

How much of the glucose and amino acids are reabsorbed?

All of them

By what processes are substances reabsorbed?

Active transport and facilitated diffusion

What does the sodium-potassium pump do?

Actively moves Na+ from the cells lining the proximal convoluted tubule into the tissue fluid using ATP.

What process does Na+ move by in sodium potassium pump?

Active transport

What does the sodiumpotassium pump do to concentration?

Lowers the concentration of Na+ in the cell cytoplasm

Where does Na+ move (selective reabsorption)?

Transported into the cell of the PCT as well as glucose and amino acids

How are substances moved into cells of the PCT?

By a co-transporter protein by facilitated diffusion

Where do glucose and amino acids go in selective reabsorption?

Diffuse along a concentration gradient out of the epithelial cells into the tissue fluid, then into the blood capillary.

Why is most of the water reabsorbed at the PCT?

Because channel proteins allow water to pass through

What are adaptations of the epithelial cells of the PCT for absorption?

• Folding of membrane to increase surface area

• Presence of sodium-potassium pump for active transport

• Microvilli on epithelial cells - large surface area for absorption

• Membrane on lumen side contains many co-transporter proteins

What is the third stage of urine production?

Water reabsorption

Where does water reabsorption take place?

In the loop of Henle

What is the permeability to water in the descending limb?

Permeable

What is the permeability to water in the ascending limb?

Impermeable

What is the permeability to ions in the descending limb?

Na+ and Cl- ions diffuse into the descending limb

What is the permeability to ions in the ascending limb?

Actively transports Na+ and Cl- out (but they diffuse near the bottom)

What are the steps of water reabsorption (simple)?

• Na+ and Cl- actively transported out

• Water moves out of descending limb

• Some Na+ and Cl- diffuse back into descending limb

• Bottom of ascending limb, Na+ and Cl- diffuse out

• Water moves out of DCT and collecting duct into medulla

Where are Na+ and Cl- actively transported out of?

The ascending limb

Where are Na+ and Cl- actively transported to?

The tissue fluid in the medulla

Does water move out of the ascending limb?

No

What is the arrangement of the tubules described as?

A sharp hairpin