Complete the following:
$(a)$ Urinary excretion = tubular reabsorption + tubular secretion -..........
$(b)$ Dialysis fluid = Plasma -..........

$(A)$ Glomerular filtration (or filtrate)
$(b)$ Nitrogenous wastes (or excretory substances)
Explanation:
$(a)$ The process of urine formation involves three main steps: glomerular filtration, tubular reabsorption, and tubular secretion. The net amount excreted is calculated as: $\text{Urinary excretion} = \text{Glomerular filtration} + \text{Tubular secretion} - \text{Tubular reabsorption}$. However, based on the provided equation structure, the missing term is the initial filtrate formed at the glomerulus.
$(b)$ Dialysis fluid is designed to mimic the composition of blood plasma but lacks nitrogenous waste products (like urea, uric acid, and creatinine) so that these substances can diffuse out of the patient's blood into the dialysis fluid.