The Structure and Function of the Mammalian Kidney (chapter 5 of homeostasis)

Question 1

Anatomy

Answer 1

• Supplied with blood at arterial pressure by the renal arteries that branch off the abdominal aorta - blood that circulates through the kidneys is removed by the renal vein that drains into the inferior vena cava
• Kidneys are made up of millions of nephrons - act as filtering units
• Kidney tubules produce urine, which passes out of the kidney down ureters and is collected in the bladder. When the bladder is full, the sphincter at the exit of the bladder opens and urine passes out of the body down the urethra

Question 2

Kidney structure

Answer 2

• Cortex - dark outer layer, filtering of blood takes place here, has a dense network of capillaries (carries blood from the renal artery to nephrons)
• Medulla - lighter in colour, contains tubules of the nephrons that form the pyramids of the kidney and the collecting duct
• Pelvis - central chamber where the urine collects before passing down out the ureter

Question 3

Nephron structure - Bowman’s capsule

Answer 3

• Cup-shaped structure
• Contains the glomerulus (tangle of capillaries)
• More blood goes into the glomerulus than leaves it due to ultrafiltration

Question 4

Nephron structure - proximal convoluted tubule

Answer 4

• First coiled region of the tubule after the Bowman’s capsule
• Found in the cortex of the kidney
• Many substances needed by the body are reabsorbed into the blood from here - glucose, amino acids, vitamins, and hormones. Around 85% of the sodium chloride and water is reabsorbed too - sodium ions moved by active transport while chloride ions and water follow passively down the concentration gradients
• Cells lining the proximal convoluted tubule - covered in microvilli (increases SA - more substances can be reabsorbed), have many mitochondria (provides ATP needed in active transport systems)

Question 5

Nephron structure - loop of Henle

Answer 5

• Long loop of tubule that creates a region with a very high solute concentration in the tissue fluid deep in the kidney medulla
• The descending loop runs down from the cortex through the medulla - water moves out of the filtrate by osmosis down a concentration gradient (lower part is permeable to water) and into the surrounding capillaries (vasa recta)
• The ascending loop travels back up through the medulla to the cortex - permeable to chloride and sodium ions and they move out of the concentrated solution by diffusion down a concentration gradient. In the second section of the ascending limb - sodium and chloride ions are actively pumped out into the medulla tissue fluid against a concentration gradient, which produces high ion concentrations in the medulla tissue (countercurrent multiplier system)

Question 6

Nephron structure - distal convoluted tubule

Answer 6

• Second twisted tubule
• Many mitochondria - adapted to carry out active transport
• Fine-tuning of the water balance of the body takes place
• Permeability of the walls to water varies in response to levels of ADH in the blood
• Further regulation of ion balance and pH also takes place in this tubule

Question 7

Nephron structure - collecting duct

Answer 7

• Urine passes down the collecting duct through the medulla to the pelvis
• More fine-tuning of water balance takes place - the walls of this part of the tubule are sensitive the ADH too - water moves out of the collecting duct via diffusion down a concentration gradient as it passes through the renal medulla

Question 8

Length of a nephron

Answer 8

3cm

Question 9

How many nephrons are there in each kidney?

Answer 9

1.5 million

Question 10

What is the renal capsule?

Answer 10

The protective layer of fat around each kidney - well-cushioned from physical damage in the body

Question 11

Ultrafiltration

Answer 11

The process by which blood plasma is filtered through the walls of the Bowma’s capsule under pressure

Question 12

Ultrafiltration - process

Answer 12

1. Glomerulus is supplied with blood by a wide, incoming arteriole from the renal artery
2. The blood leaves through a narrower outward arteriole and as a result there is considerable pressure in the capillaries of the glomerulus
3. This forces blood out through the capillary wall
4. Then the fluid passes through the basement membrane - this is made up of a network of collagen fibres and other proteins that make up a second ‘sieve’

Question 13

Podocytes

Answer 13

• Found in the wall of the Bowman’s capsule
• Act as an additional filter
• Have extensions called pedicels that wrap around capillaries forming slits that make sure any cells, platelets, or large plasma proteins have managed to get through the epithelial cells and the basement membrane do not get into the tubule itself

Question 14

Reabsorption

Answer 14

• Many substances removed by ultrafiltration are needed by the body, e.g. glucose in cellular respiration
• The ultrafiltrate is hypertonic to (less concentrated than) the blood plasma
• The main function of the nephron after the Bowman’s capsule is to return most of the filtered substances back to the blood