An apparatus (100, 200, 300, 500, 1300), comprising: at least one memory (130, 1406) comprising instructions; and at least one processor (120, 1404) coupled to the at least one memory (130, 1406) to access the instructions and configured to execute the instructions, whereby the at least one processor (120, 1404) is configured to: access a dialysis system model comprising a compartmental model, the compartmental model comprising an arterial access compartment, an arterial tube compartment, a dialyzer compartment, a venous tube compartment, and a venous access compartment, the compartments associated with elements of a dialysis system for performing a dialysis treatment for a patient, determine a first hemoglobin concentration for the patient at a first ultrafiltration rate, determine a change in the first ultrafiltration rate to a second ultrafiltration rate, determine a second hemoglobin concentration of the patient at the second ultrafiltration rate, and determine an access recirculation value for the patient based on a mass balance process (550, 552, 554, 556, 558) for each of the compartments of the compartment model using the first hemoglobin concentration and the second hemoglobin concentration, wherein when executing the instructions the at least one processor (120, 1404) is configured to determine the mass balance process (550, 552, 554, 556, 558) for each of the compartments of the compartmental model using the following hemoglobin mass balance processes: d C a V a dt = Q s C s − 1 − AR Q b C a − Q s − 1 − AR Q b C a ′ d C at V at dt = 1 − AR Q b C a + AR ⋅ Q b C vt − Q b C at ′ d C d V d dt = Q b C at − Q b − Q uf C d ′ d C vt V vt dt = Q b − Q uf C d − AR ⋅ Q b C vt − Q b − Q uf − AR ⋅ Q b C vt ′ d C v V v dt = Q s − 1 − AR Q b C a + Q b − Q uf − AR ⋅ Q ′ b C vt − Q s − Q uf C v, where C a is a hemoglobin concentration in an arterial access compartment, V a is a volume of the arterial access compartment, C at is a hemoglobin concentration in an arterial tube compartment, V a t is a volume of the arterial tube compartment, C d is a hemoglobin concentration in a dialyzer compartment, V d is a volume of the dialyzer compartment, C vt is a hemoglobin concentration in a venous tube compartment, V vt is a volume of the venous tube compartment, C v is a hemoglobin concentration in a venous access compartment, V v is a volume of the venous access compartment, Q s is a systemic flow rate, Q b is a dialyzer flow rate, Q a is an arterial access flow rate, Q uf is an ultrafiltration rate, C s is a systemic hemoglobin concentration, and AR is the access recirculation value. at least one memory (130, 1406) comprising instructions; and at least one processor (120, 1404) coupled to the at least one memory (130, 1406) to access the instructions and configured to execute the instructions, whereby the at least one processor (120, 1404) is configured to: access a dialysis system model comprising a compartmental model, the compartmental model comprising an arterial access compartment, an arterial tube compartment, a dialyzer compartment, a venous tube compartment, and a venous access compartment, the compartments associated with elements of a dialysis system for performing a dialysis treatment for a patient, determine a first hemoglobin concentration for the patient at a first ultrafiltration rate, determine a change in the first ultrafiltration rate to a second ultrafiltration rate, determine a second hemoglobin concentration of the patient at the second ultrafiltration rate, and determine an access recirculation value for the patient based on a mass balance process (550, 552, 554, 556, 558) for each of the compartments of the compartment model using the first hemoglobin concentration and the second hemoglobin concentration, access a dialysis system model comprising a compartmental model, the compartmental model comprising an arterial access compartment, an arterial tube compartment, a dialyzer compartment, a venous tube compartment, and a venous access compartment, the compartments associated with elements of a dialysis system for performing a dialysis treatment for a patient, determine a first hemoglobin concentration for the patient at a first ultrafiltration rate, determine a change in the first ultrafiltration rate to a second ultrafiltration rate, determine a second hemoglobin concentration of the patient at the second ultrafiltration rate, and determine an access recirculation value for the patient based on a mass balance process (550, 552, 554, 556, 558) for each of the compartments of the compartment model using the first hemoglobin concentration and the second hemoglobin concentration, wherein when executing the instructions the at least one processor (120, 1404) is configured to determine the mass balance process (550, 552, 554, 556, 558) for each of the compartments of the compartmental model using the following hemoglobin mass balance processes: d C a V a dt = Q s C s − 1 − AR Q b C a − Q s − 1 − AR Q b C a ′ d C at V at dt = 1 − AR Q b C a + AR ⋅ Q b C vt − Q b C at ′ d C d V d dt = Q b C at − Q b − Q uf C d ′ d C vt V vt dt = Q b − Q uf C d − AR ⋅ Q b C vt − Q b − Q uf − AR ⋅ Q b C vt ′ d C v V v dt = Q s − 1 − AR Q b C a + Q b − Q uf − AR ⋅ Q ′ b C vt − Q s − Q uf C v,