58 - Functions of the renal tubules. Transport through the different tubular sections. Excretion of diluted and concentrated urine. Renal excretion.

renal tubules parts and functions

Renal tubule: a long series of ducts in the nephron

Functions of the renal tubules

1. Reabsorption: moving of solutes from the tubules back to the bloodstream

2.Secretion: transport of substances into the tubules in order to be excreted

achieved by active transport.

These functions are regulated by the selective permeability of different parts of the tubules to water,

sodium and the response of the distal tubules to hormones such as aldosterone, ADH and parathyroid

hormone

Parts of the renal tubule:

1. Proximal Convoluted Tubule: Responsible for reabsorption of the majority (~65–70%) of the filtrate volume filtered by the glomerulus.

2. Loop of Henle: mainly located in the medulla of the kidney and has two parts

- descending limb: water flows out by osmosis

- ascending limb—Impermeable to water.

• Actively pumps out Na⁺, K⁺, and Cl⁻, helping to create the osmotic gradient in the medulla → dilutes the filtrate.

3. Distal convoluted Tubule: located in the cortex of the kidney, reabsorbs calcium, sodium and

chloride, regulates the pH of urine by secreting protons

• Responsive to aldosterone, which increases Na⁺ reabsorption and K⁺ secretion.

4. Collecting duct -> passes through a renal pyramid, site of water resorbtion

Water moves out by osmosis, driven by the osmotic gradient established by the Loop of Henle.

water reab = regulated by ADH

Excretion of concentrated and diluted urine

low water intake (dehydration) -> plasma osmolality increases -> signal to osmoreceptors in the

hypothalamus -> increased release of ADH from the pituitary gland into the blood stream -> kidneys will

increase water absorption -> less water secreted with the urine -> concentrated urine with less water and

more solute = high osmolality

---

---

---

---

---

---

---

---

---

high water intake (over-hydration) -> low plasma osmolality -> signal to the osmoreceptors of the

hypothalamus -> less release of ADH from the posterior pituitary gland -> kidneys will absorb less water ->

more water secreted with the urine -> diluted urine with more water and less solute = low osmolality

Renal Excretion

- excretion of waste products is a major function of the kidneys,

1. Glomerular filtration – substances are filtered from the blood in the glomerulus into the Bowman's (glomerular) capsule.

2. Tubular secretion – substances are actively transported from peritubular capillaries into the nephron tubule lumen, bypassing filtration.

• Tubular reabsorption may also occur (for substances the body wants to retain), but waste products are not reabsorbed and continue to the urine.

• Renal clearance is the rate at which a substance is removed from the plasma by the kidney.

  • Each substance has a different clearance rate depending on whether it is filtered, reabsorbed, or secreted.

v