Lecture 3 – Glomerular Filtration and Micturition

1) What are some of the things the kidneys regulate and eliminate from the blood?

The kidneys regulate ion levels (Na⁺, K⁺, Ca²⁺, PO₄³⁻), acid-base balance (H⁺, HCO₃⁻), and blood pressure (fluid balance, renin release). They eliminate metabolic wastes (urea, uric acid) and biologically active molecules (hormones, drugs).

2) What are the secondary functions of the kidneys?

Secondary functions include formation of calcitriol, production of erythropoietin (EPO), and the ability to perform gluconeogenesis during extreme nutrient deprivation.

3) What is happening during glomerular filtration? What causes this to occur?

During glomerular filtration, water and solutes move passively from blood plasma in the glomerular capillaries to the capsular space. This occurs due to pressure differences between the glomerulus and the capsule.

4) When a substance is reabsorbed, it moves from where to where? Is this movement passive or active?

When a substance is reabsorbed, it moves from tubular fluid (in renal tubules/collecting ducts) to the blood (in peritubular capillaries/vasa recta). This can be active or passive transport.

5) When a substance is secreted, it is actively moved from where to where?

When a substance is secreted, it is actively moved from the blood (peritubular capillaries/vasa recta) into the tubular fluid (renal tubules/collecting ducts).

6) What is the filtration membrane composed of?

The filtration membrane is made of fenestrated endothelium, a basement membrane, and podocytes forming filtration slits.

7) What do mesangial cells do and where would you find them?

Mesangial cells are found between capillary loops in the glomerulus. They are not part of the filtration membrane and have phagocytic and contractile functions to help regulate filtration rate

8) How much filtrate is produced daily and what substances are NOT filtered through the filtration membrane?

About 180 L of filtrate is produced daily. Not filtered substances include formed elements and large proteins.

 

9) What causes there to be a higher hydrostatic (blood) pressure within the glomerulus in comparison to all other capillaries in the body?

The afferent arteriole has a larger lumen than the efferent arteriole, causing higher hydrostatic pressure in the glomerulus than in other capillaries.

10) What two pressures oppose the movement of water/solutes out of the blood and into the capsular space?

 The two opposing pressures are blood colloid osmotic pressure (OPg) and capsular hydrostatic pressure (HPc).

11) If HP_g is 65, OP_g is 30, and HP_c is 12, calculate the net filtration of the glomerulus.

NFP = HPg − (OPg + HPc) = 65 − (30 + 12) = 23 mm Hg

12) What is glomerular filtration rate? (define it).  How do NFP and GFR relate to each other?

Glomerular filtration rate (GFR) is the rate at which filtrate is formed (ml/min). NFP and GFR have a direct relationship — if NFP increases, GFR increases.

Explain why increased NFP leads to more solutes being found in the urine.

Increased NFP means more filtrate forms, leaving less time for reabsorption, so more solutes remain in urine.

Why can inulin (NOT insulin the hormone) be used to measure a patients GFR?

Inulin can measure GFR because it is freely filtered but not reabsorbed or secreted by the tubules, making it a reliable marker.

  What would be a ‘normal’ GFR? ______________ ml/min

Normal GFR = 125 ml/min

  In what two ways can the body influence GFR (not control it, but influence it)?

The body can influence GFR by changing afferent arteriole diameter or altering the filtration membrane surface area.

   What are the differences between regulation of GFR intrinsically versus extrinsically?

Intrinsic regulation (renal autoregulation) maintains a constant GFR within the kidney itself. Extrinsic regulation involves nervous or hormonal control (e.g., sympathetic system or ANP) that adjusts GFR based on body needs

What are the two mechanisms of renal autoregulation?

The two mechanisms are the myogenic response and the tubuloglomerular feedback mechanism.

 Which mechanism affects the pressure within the glomerulus? Which mechanism also affects the amount of surface area available for filtration?

The myogenic response affects glomerular pressure, while the tubuloglomerular feedback mechanism also affects surface area of the filtration membrane.

If systemic blood pressure increases, the afferent arteriole leading into the glomerulus will ….

If systemic BP increases, the afferent arteriole constricts to prevent excessive pressure and maintain stable GFR.

Which cells within the JG (juxtaglomerular) apparatus detect sodium and make the changes necessary for the tubuloglomerular feedback mechanism?

The macula densa cells in the JG apparatus detect Na⁺ levels and trigger changes for tubuloglomerular feedback.

What cells are needed to change the surface area of the filtration membrane?  If surface area goes up, what happens to GFR?

Mesangial cells change the surface area of the filtration membrane. If surface area increases, GFR increases.

Renal autoregulation only works within what range of MAP? Why?

Renal autoregulation works only when MAP is 80–180 mm Hg because outside that range, afferent arterioles are already maximally dilated or constricted, and regulation fails.

   The sympathetic division causes vasoconstriction of the afferent arteriole and contraction of the mesangial cells. How does this affect GFR?

The sympathetic division causes afferent arteriole vasoconstriction and mesangial cell contraction, which decrease GFR.

AND causes vasodilation of the afferent arteriole and relaxation of the mesangial cells. How does this affect GFR?

ANP causes afferent arteriole vasodilation and mesangial cell relaxation, which increase GFR

What reflex of the bladder does the sympathetic division stimulate? What reflex does the parasympathetic division stimulate?

The sympathetic division stimulates the storage reflex, and the parasympathetic division stimulates the micturition reflex.

What receptors in the bladder are responsible for initiating the micturition reflex?

Baroreceptors in the bladder wall detect stretch and initiate the micturition reflex.

  When the micturition reflex is stimulated, what contracts and what relaxes?

During the micturition reflex, the detrusor muscle contracts and the internal urethral sphincter relaxes.

 Which nerve do we use to consciously control when urination occurs? And what will relax when we consciously decide to urinate?

The pudendal nerve provides voluntary control of urination. When we choose to urinate, the external urethral sphincter relaxes.

  If we don’t urinate during the first micturition reflex, what happens?

If we don’t urinate during the first reflex, the detrusor muscle relaxes (stress-relaxation response) allowing the bladder to fill more, and the reflex repeats. If this continues, conscious control is lost and the parasympathetic division causes automatic urination.