Excretion and the Kidney

HL (Paper 1 and 2)

Excretion• What is excretion?

– Elimination of waste from the metabolic processes, to maintain homeostasis

• What are the products we excrete?– Undigested material (egested – feces)– Metabolites– CO2

– Urea

• Why do we eliminate waste?– Stop toxins from circulating in the body– Maintain homeostasis

The Kidney• Pair of reddish organs, smaller

than your fist, just under the last rib

• Blood flow– Each has a renal vein and renal

artery, which branches to a network of capillaries, forming the pyramid

– To empty urine, each has a ureter emptying into the bladder

The Kidney• Parts of the Kidney– Cortex – outer portion– Medulla – inner portion– Pyramid – collection of

capillaries– Pelvis – urine empties from

the collecting ducts, and collects before emptying into ureter to bladder

– Functional unit, called the nephron, microscopic structure

Functions of the Kidney• The main functions of the kidney are:

– To maintain water balance in the body.

– To remove nitrogenous metabolic wastes and salts. You call this ‘pee’, we call it Urea. Urine is the whole thing together.

– To allow the body to reabsorb substances it needs.

Urea

• Made from the breakdown of proteins, from the more toxic ammonia (NH3)

• Broken down in the liver

• Transported from liver, in blood to kidneys to be filtered and eliminated

Nephron – functional unit of the Kidney

• Principles of the functioning of the nephron:

– Ultra-filtration

– Reabsorption

– Secretion

Ultrafiltration• Achieved by pressure built up in the

glomerular vessels, due to the efferent vessel being more narrow exiting the Bowman’s capsule than the afferent vessel entering the Bowman’s capsule

• Fluid pushed into the Bowman’s capsule, is called the filtrate, consisting of ions, glucose and urea

Ultrafiltration

• Accomplished by:

– Wall of the glomerulus – fenestrations

– Basement membrane of the glomerulus

– Outer wall of the glomerulus - Podocytes

Glomerulus and Bowman’s Capsule

Osmoregulation and Reabsorption

• Osmoregulalation – the control of water balance of the blood, tissue and cytoplasm of a living organism.

• Achieved by the Proximal Convoluted Tubule and Loop of Henle– Using osmosis, diffusion and active transport

Osmoregulation and Reabsorption• Since the blood leaving the Bowman’s capsule

is more concentrated (low water), as the capillary comes close to the PCT:– osmosis drives the reabsorption of water– Sodium, glucose and other ions, are move by

active transport– Once the salts move into, more water moves by

osmosis• Large surface area is achieved by microvilli and

large amounts of mitochondria in microvilli cells, for active transport

Osmoregulation and Reabsorption• Filtrate leaving the PCT, is mostly water,

sodium, and urea, and goes into the Loop of Henle

• As filtrate goes into the descending loop, it goes into the medulla, which is concentrated with sodium

• More water is absorbed, into the capillaries along side the loop, along with some sodium, which stays in the tissue of the medulla

Osmoregulation and Reabsorption

• Ascending Loop is impermeable to water, and salts are removed by active transport– The sodium remains in the medulla tissue, to maintain

a concentration gradient• The concentration is maintained by the vasa recta,

vessels that run along the loop, exchanging salts• This process is called the vasa recta counter

current exchange• Filtrate leaving has only water and urea,

metabolites and some sodium

Secretion

• Filtrate goes into the distal convoluted tubule and collecting duct.

• DCT and CT are permeable to water, only if ADH is present

• If water is needed in body (ie. Low blood volume, High Osmotic Pressure)– ADH is secreted and water goes from the filtrate

to the blood– Concentrated Urine

Secretion

• If there is excess water (ie. High blood volume, low Osmotic Pressure)– no ADH will be secreted– Dilute Urine

• If the urine is too concentrated, this can lead to kidney stones