Kidneys and Excretion

Gross structure of kidney

Renal Arteries

• Branch off from abdominal aorta

• Supply kidneys w. blood

Renal Vein

Removes circulated blood from kidneys, drains into inferior vena cava

Cortex

• Dark outer layer

• Filters blood 

• Has dense capillary network carrying blood from renal artery to nephrons

Medulla

• Lighter than cortex

• Contains tubules of nephrons that form pyramids of kidney and collecting ducts

(Renal) Pelvis

• Where urine collects before passing down ureter 

Mechanism of kidneys

1. Blood enters via renal artery and passes thru capillaries in cortex

2. Ultrafiltration

3. Selective reabsorption

4. Remaining unwanted substances (urea) pass along tubules, along ureter to bladder, where expelled as urine 

5. Filters blood passes out of kidneys thru renal vein

Nephrons

• Where blood is filtered ; removes nitrogenous wastes, balances mineral ions and water 

• 1.5 mil in each kidney, providing huge SA for reabsorption of water, glucose, salts and other substances into blood

Structure of nephron

Bowman’s Capsule

• Contains glomerulus, a tangle of capillaries

• More blood goes in than out due to ultrafiltration process

Proximal convoluted tubule

• First, coiled region of tubule after B.C

• Found in cortex

• Where substances needed by body are reabsorbed into blood 

Loop of Henle

• Long loop of tubule

• Creates region w. v. high solute conc. in tissue fluid in medulla

•  Descending runs from cortex thru medulla to bend at bottom of loop

• Ascending travels up thru medulla to cortex

Distal convoluted tubule

• 2nd twisted tubule

• Fine-tuning of water balance occurs here

• Permeability of walls to water varies in response to ADH levels in blood

• Further reg, of ion and pH balance of blood occurs here

Collecting duct

• Where urine passes down through medulla to pelvis

• More fine-tuning of water balance occurs here

• These walls also sensitive to ADH

Nephron and blood

• Has network of capillaries around it; venule —> renal vein

• Blood that leaves = reduces levels of urea, levels of glucose & amino acids roughly same as when entered

• Mineral ion conc. in blood restored to ideal elevels

External structure of kidney

Internal histology of kidney

Ultrafiltration in glom

• Blood enters glomerulus thru wide afferent arteriole, leaves thru narrow efferent arteriole

• High pressure in capillaries forces liquid & small molecules in blood out of capillaries into B.C. via diffusion

  • RBCs & large proteins stay in capillary

• Fluid passes thru basement membrane

Basement membrane

Made of network of collagen fibres

• Blood cells and many proteins retained due to large size

Ultrafiltration post glom

• Podocytes ensure cells, platelets, large plasma cells don’t get thru to tubule

• Filtrate enters B.C. lumen 

  • Contains substances in same conc. as in blood plasma

  • Is hypotonic to blood plasma

  • Contains glucose, salt, urea

• Passed along rest of nephron 

Reabsorption in PCT 

• Glucose, amino acids, hormones, ions moved from filtrate into blood by active transport 

  • Mg2+, Ca2+

• 80% of all filtrate

• Approx, 85% of NaCl and wate reabsorbed

  • Na+ active, Cl- and water passive down conc. g.

Reabsorption amount is always the same

Cells lining PCT adaptations

1. Covered w. microvilli 

• Increases SA over which substances can be reabsorbed

2. Many mitochondria

• Provide ATP need in active transport systems (Na+)

3. Co-transporter proteins

Once substances have been removed from nephron where do the useful ones go?

Diffuse down steep conc. gradients into extensive capillary network that surrounds the tubules

• Maintained by constant blood flow thru capillaries

Flow along PCT, thru loop of Henle, along DCT

Loop of Henle

• Allows mammals to produce urine

• Diff areas have diff permeabilities to water

• Acts as counter current multiplier

  • Uses energy to produce conc. g. that results in movement of substances 

  • Cells use ATP to transport ions, producing diffusion g. in medulla  

What happens in descending limb?

• Water moves out of filtrate down conc. g. , into medulla

• Medulla has low water potential due to high conc. of ions

  • Moves by osmosis into arteriole (neighbouring capillaries)

Features of descending limb

• Leads from PCT

• Filtrate entering is isotonic w. blood

• Not permeable to Na+ and Cl- ions

• No active transport occurs here

• Fluid that reaches bend very conc. and hypertonic to blood in capillaries

Features of ascending limb

• v. permeable to Na+ and Cl- ions

• Actively pumped out into medulla = high Na+ and Cl- conc.

• Impermeable to water

• Fluid left in here becomes dilute 

What happens to the medulla after Na+ and Cl- actively removed from ascending limb?

Has v. high conc. of ions

• Essential for kidney to produce urine more conc. than blood 

What is the fluid like once it has reached the top of the ascending limb?

Is hypotonic to blood

Enters DCT and Collecting Duct

What do the cells lining the DCT have?

Many mitochondria to carry out active transport

What happens with the DCT if body lacks salt?

Na+ and Cl- ions actively pumped out, down electrochemical gradient