Difficulty: Intermediate | Prerequisites: Cell biology basics (cell membrane, diffusion, osmosis). Familiarity with microbiology concepts (pathogens, bacteria) is helpful for the immunity sections.
This material bridges cardiovascular physiology and the innate immune system. The cardiovascular side covers how the heart moves blood (cardiac output), how blood pressure is maintained (mean arterial pressure), and how the body detects changes in pressure (baroreceptors). The immunology side introduces the body's first-line defences: nonspecific inflammation and the white blood cells that respond before the adaptive immune system gets involved. These two systems overlap constantly in clinical medicine, and exams often test them together.
Cardiac output equals heart rate times stroke volume; mean arterial pressure equals cardiac output times total peripheral resistance. Baroreceptors sense pressure changes and trigger corrections. The innate immune system uses inflammation, neutrophils, macrophages, and natural killer cells as rapid, nonspecific defences against pathogens.
Cardiac output (definition)
The total volume of blood the heart ejects in one minute. Think of it as the heart's throughput: how much blood it pumps per minute.
Cardiac output (calculation)
Heart rate x stroke volume. If the heart beats 70 times per minute and ejects 70 mL per beat, cardiac output is 4,900 mL/min (roughly 5 L/min).
Mean arterial pressure (MAP) (definition)
The average pressure driving blood flow to the tissues during a single cardiac cycle. It represents the "effective" pressure the organs experience.
Mean arterial pressure (calculation)
Cardiac output x total peripheral resistance. Alternatively, MAP can be estimated as diastolic pressure + 1/3(systolic pressure minus diastolic pressure).
Total peripheral resistance (TPR)
The resistance to blood flow in the peripheral circulation, determined mainly by the diameter of arterioles. In simple terms, narrower vessels mean higher resistance and higher blood pressure.
Baroreceptors
Specialised sensory neurons that detect changes in blood pressure. They are located in the carotid sinus and aortic arch. When pressure rises, they fire more frequently, triggering reflexes that lower heart rate and dilate vessels.
Inflammatory response
A nonspecific defence against infection characterised by redness, heat, swelling, and pain. These signs result from increased blood flow and immune cell migration to the site of injury or infection.
Macrophages
Large immune cells that engulf bacteria and cellular debris by phagocytosis and also stimulate T cells by presenting antigens. Think of them as both the cleanup crew and the alarm system.
Neutrophils
The most abundant white blood cells. They are the first responders at an infection site, engulfing microbes by phagocytosis. They are short-lived but arrive in large numbers.
Natural killer (NK) cells
Innate immune cells that destroy virus-infected cells, tumour cells, and cells lacking MHC class I markers. They do not need prior exposure to a pathogen to act.
Cardiac output (CO) = heart rate (HR) x stroke volume (SV). It tells you how much blood the heart delivers per minute.
Mean arterial pressure (MAP) = CO x TPR. MAP represents the average driving pressure of the blood.
Both formulas are two-factor products, so a change in either factor shifts the result. For example, if heart rate rises but stroke volume stays the same, cardiac output increases.
Total peripheral resistance is controlled primarily by arteriolar diameter. Vasoconstriction raises TPR (and thus MAP); vasodilation lowers it.
Baroreceptors provide a negative-feedback loop:
Blood pressure rises: baroreceptors fire more, the brainstem signals the heart to slow and vessels to dilate, bringing pressure back down.
Blood pressure falls: baroreceptors fire less, the brainstem signals the heart to speed up and vessels to constrict, raising pressure.
This reflex operates continuously and is one of the fastest cardiovascular control mechanisms.
The inflammatory response is the body's immediate, nonspecific reaction to tissue damage or infection. Its cardinal signs (redness, heat, swelling, pain) result from vasodilation and increased capillary permeability at the site.
Neutrophils are the first white blood cells to arrive. They phagocytose (engulf and digest) microbes and die quickly, forming pus.
Macrophages arrive after neutrophils. They phagocytose pathogens and debris and also serve as antigen-presenting cells, activating T cells and bridging the innate and adaptive immune systems.
Natural killer (NK) cells patrol for cells that lack normal MHC class I surface markers, a sign of viral infection or cancerous transformation. They kill on contact without prior sensitisation.
Cardiac output:
CO = HR x SV
(Heart rate in beats/min) x (Stroke volume in mL/beat) = Cardiac output in mL/min
Mean arterial pressure:
MAP = CO x TPR
Alternative clinical estimate: MAP = Diastolic + 1/3 (Systolic minus Diastolic)
A normal resting MAP is roughly 70 to 105 mmHg.
Blood pressure medications work by targeting the variables in the MAP equation. Beta-blockers reduce heart rate (lowering CO), while vasodilators reduce TPR. Understanding the formula tells you exactly why each drug class works.
The inflammatory response is what you experience when you stub your toe or get a splinter: the redness, warmth, swelling, and pain are the immune system doing its job. Anti-inflammatory drugs like ibuprofen work by dampening this response.
Students sometimes confuse cardiac output with blood pressure. Cardiac output is volume per minute; blood pressure is the force that volume exerts on vessel walls. They are related (MAP = CO x TPR) but they are not the same thing.
Baroreceptors do not set blood pressure. They detect changes and trigger corrective reflexes. The set point is determined by central nervous system processing.
The inflammatory response is often mistaken for infection itself. Inflammation is the immune system's response to damage or invasion, not the pathogen's activity. You can have inflammation without infection (e.g. a sprained ankle).
NK cells are sometimes confused with T cells. NK cells are part of the innate immune system and act without prior exposure. T cells belong to the adaptive immune system and require antigen presentation.
The two formulas (CO = HR x SV and MAP = CO x TPR) are almost certain to appear. Be able to calculate one variable given the other two.
Expect a scenario question: "A patient's heart rate increases while stroke volume remains constant. What happens to cardiac output and MAP?"
Know the four cardinal signs of inflammation and what causes each.
Be able to distinguish the roles of neutrophils, macrophages, and NK cells. Exams frequently ask which cell arrives first (neutrophils), which bridges innate and adaptive immunity (macrophages), and which kills without prior sensitisation (NK cells).
Fill in the blank: Cardiac output = heart rate x ______. (stroke volume)
True or False: Baroreceptors increase heart rate when blood pressure rises. (False, they trigger a decrease in heart rate.)
Fill in the blank: The four signs of inflammation are redness, heat, ______, and pain. (swelling)
True or False: Natural killer cells require antigen presentation by macrophages before they can act. (False, NK cells act without prior sensitisation.)
Fill in the blank: MAP = ______ x total peripheral resistance. (cardiac output)
Q: A patient has a heart rate of 80 bpm and a stroke volume of 65 mL. Calculate their cardiac output. If their TPR is 20 mmHg/L/min, estimate their MAP.
A: CO = 80 x 65 = 5,200 mL/min (5.2 L/min). MAP = 5.2 x 20 = 104 mmHg.
Q: Describe what happens when baroreceptors detect a sudden drop in blood pressure.
A: Baroreceptors fire less frequently, signalling the cardiovascular centre in the brainstem. The sympathetic nervous system increases heart rate and promotes vasoconstriction, raising both cardiac output and TPR, which restores MAP.
Q: A wound becomes red, warm, swollen, and painful. Explain the physiological basis of each sign.
A: Redness and heat result from vasodilation increasing blood flow to the area. Swelling (oedema) results from increased capillary permeability allowing plasma to leak into tissues. Pain results from chemical mediators (e.g. prostaglandins, bradykinin) stimulating local nerve endings.
Q: Compare neutrophils, macrophages, and NK cells in terms of their primary function and whether they are part of the innate or adaptive immune system.
A: All three are innate. Neutrophils phagocytose microbes and are the first to arrive. Macrophages phagocytose pathogens and debris and also present antigens to T cells, bridging innate and adaptive immunity. NK cells destroy virus-infected and tumour cells by recognising the absence of MHC I markers.
This connects to cell biology because osmotic pressure (covered in those notes) is one of the forces governing fluid movement between blood vessels and tissues, directly affecting blood volume and therefore blood pressure.
It connects to microbiology because the innate immune cells described here are the first line of defence against the pathogens covered in the microbiology notes. Gram-negative endotoxins, for instance, are potent triggers of the inflammatory response.
The cardiovascular formulas also connect to clinical pharmacology: every major class of blood-pressure medication targets one of the variables in MAP = CO x TPR.
Cardiac output, CO, heart rate, stroke volume, mean arterial pressure, MAP, total peripheral resistance, TPR, arterioles, vasoconstriction, vasodilation, baroreceptors, carotid sinus, aortic arch, negative feedback, blood pressure regulation, inflammatory response, inflammation, cardinal signs, redness, heat, swelling, pain, oedema, vasodilation, capillary permeability, neutrophils, phagocytosis, macrophages, antigen-presenting cells, natural killer cells, NK cells, MHC class I, innate immunity, nonspecific defence, first-line defence, white blood cells, leukocytes, pus, phagocytes