Renal System

Major Functions of the Renal System

Elimination of metabolic waste
Regulation of ion levels
Regulation of acid-base balance
Regulation of blood pressure and blood volume
Elimination of biologically active molecules

Glomerular Filtration Rate

What is GFR?

The rate at which filtrate is formed. Primarily influenced by glomerular hydrostatic pressure.

Mechanisms that alter GFR

Autoregulation (Myogenic response)- Contraction or relaxation of smooth muscle of afferent arteriole in response to stretch.
Tubuloglomerular feedback- Regulates GFR by by linking it to the NaCl (salt) concentration in the tubule fluid at the macula densa.
Sympathetic (neural/hormonal) control- Activated by blood pressure drop. Adrenal glands release norepinephrine causing arteriolar constriction lowering GFR. Renin is released by kidneys to raise BP.

Water Reabsorption

Where is water reabsorbed in the nephron?
1. Proximal convoluted tubule
2. Nephron loop
3. Distal convoluted tubule
4. Collecting duct

Hormonal Control of Water Reabsorption

Antidiuretic hormone (ADH)- Released by posterior pituitary. Acts on DCT and CT to increase aquaporin channels. Increases water reabsorption.

Aldosterone- Released by adrenal cortex. Acts on the distal convoluted tubule and the collecting duct, to upregulate Na+ channels, and Na+/K+ pumps. Increases water and Na+ reabsorption.

Atrial natriuretic peptide (ANP)- Produced by the muscle cells in the atria of the heart. Inhibits release of aldosterone and reabsorption of Na+ in the PCT and collecting duct. Decreases GFR and water reabsorption.

Renin / Angiotensin II- Released by kidney. Acts on angiotensinogen to convert it to angiotensin I. ACE converts Angiotensin I to II which triggers release of aldosterone. Response to low BP. Increases thirst and causes the kidneys to retain Na+ and water.

Nephron

Components :
- Glomerulus
- Bowman’s capsule
- Proximal convoluted tubule (PCT)
- Loop of Henle (descending & ascending limbs)
- Distal convoluted tubule (DCT)
- Collecting duct

pH Regulation and Acid-Base Balance

How does the renal system help regulate blood pH?

Through bicarbonate reabsorption and hydrogen ion excretion.

How does it work with the respiratory system to maintain acid–base balance? (hint: bicarbonate)

Both the renal and respiratory system have a bicarbonate buffer system. The respiratory system works more in the short term to manage CO2 levels through breathing. The renal system works long term by reabsorbing bicarbonate and excreting hydrogen ions.

- Respiratory acidosis → Blood pH is below normal due to a build up of CO2 levels in the lungs. Causes are COPD, asthma, etc.
- Respiratory alkalosis → Blood pH is above normal due to low CO2 levels. Caused by hyperventilation.
- Metabolic acidosis → Too much acid in the body, causes the blood pH to decrease. Ex: Diabetic acidosis, chronic kidney disease.
- Metabolic alkalosis → Body’s pH level rises due to an increase in bicarbonate or loss of hydrogen ions. Ex: Diarrhea, vomiting.

RAAS Pathway

Stimulus for renin release: Low blood pressure
Sequence of events:
1. Renin released by the juxtaglomerular cells in the kidneys
2. Converts angiotensinogen→ angiotensin I
3. ACE converts angiotensin I → angiotensin II
4. Angiotensin II causes increase in blood pressure(vasoconstriction + aldosterone release)
5. Aldosterone acts on the distal convoluted tubule and collecting duct to increase sodium and water reabsorption

What is GFR and how is it primarily regulated?

Glomerular filtration rate. Primarily regulated by glomerular hydrostatic pressure.

Compare the three mechanisms that control GFR

Myogenic response is the constriction or relaxation of smooth muscle in afferent arteriole in response to stretch. Tubuloglomerular feedback mechanism is the “back up” to the myogenic response and responds to increased blood pressure. Sympathetic response is stimulated through exercise or injury and decreases GFR.

Where in the nephron is the most water reabsorbed?

Proximal convoluted tubule

How do ADH and aldosterone differ in their effects on kidney function?

ADH leads to water reabsorption through an increase in aquaporin channels, while aldosterone leads to an increase in Na+ reabsorption which in turn leads to increased water reabsorption.

Describe the RAAS pathway in detail

A drop in blood pressure/blood volume causes the kidneys to release renin. The renin acts on angiotensinogin which is produced by the liver to convert it to angiotensin I. ACE is released by the lungs and converts angiotensin I to angiotensin II. Angiotensin II stimulates the adrenal gland to release aldosterone which stimulates the reabsorption of NaCl and H2O, as well as stimulates vasoconstriction.

How does the renal system correct metabolic acidosis?

By increasing acid excretion and producing new bicarbonate through metabolizing the amino acid glutamate.

What is the path of filtrate from glomerulus to urethra?

Glomerulus→Bowman’s Capsule→Proximal Convoluted Tubule→Loop of Henle→Distal Convoluted Tubule→Collecting duct→Renal Papilla→Minor calyx→Major calyx→Renal pelvis→Ureter→Bladder→Urethra

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