Week 4: Fluid Balance & Circulation 2

Learning Objectives

Students will be able to trace blood flow through both pulmonary and systemic circulatory systems
Students will be able to explain how blood functions as transport and delivery system for gases, nutrients, and wastes
Students will be able to distinguish between internal and external respiration and relate them to blood gas transport
Students will be able to describe heart anatomy, location, and the cardiac conduction system
Students will be able to interpret basic ECG waveforms and explain their physiological significance
Students will be able to explain the relationships between cardiac output, blood pressure, blood flow, blood volume, and vascular resistance
Students will be able to describe kidney structure and explain multiple functions including filtration, hormone production, and blood pressure regulation
Students will be able to describe urine formation, basic urinalysis findings, and how urinary microbiome disruption increases UTI risk

Understanding Circulation Systems

Your body has two connected circulatory loops working together like a figure-eight. The pulmonary circulation sends blood from your heart to your lungs to pick up oxygen and drop off carbon dioxide. The systemic circulation then carries this oxygen-rich blood from your heart to every other part of your body, delivering oxygen and nutrients while picking up waste.

Blood acts as both delivery truck and garbage collector - bringing oxygen from lungs, nutrients from digestion, and hormones from glands to cells, then removing carbon dioxide and waste products for disposal.

Your heart is the powerful pump at the center of this system, located slightly left of center in your chest and protected by your sternum. It has four chambers: two atria (upper) that receive blood and two ventricles (lower) that pump blood out.

The heart generates its own electrical signals through a special conduction system - the sinoatrial (SA) node acts as the natural pacemaker, sending signals that make your heart beat about 100,000 times per day. Doctors use ECGs (electrocardiograms) to visualize these electrical signals and check heart health.

Your kidneys are remarkable organs that do much more than make urine. They filter the body's plasma volume about 60 times per day, removing waste while keeping essential substances. They regulate blood pressure by controlling fluid volume, maintain proper salt balance, help balance pH, produce hormones for red blood cell production, activate vitamin D for bone health, and can even help support blood glucose during prolonged fasting.

Baby's Special Circulation: Before Birth

Before birth, babies have a completely different circulatory system! Since their lungs aren't working yet (they get oxygen from mom through the placenta), special shortcuts called "shunts" let blood bypass the lungs. The foramen ovale is a hole between the heart's upper chambers that lets blood flow directly from right to left, skipping the lungs entirely. The ductus arteriosus is a blood vessel connecting the main lung artery to the body's main artery (aorta), providing another shortcut. When babies are born and take their first breath, these shortcuts close up permanently - the foramen ovale becomes the fossa ovalis, and the ductus arteriosus becomes the ligamentum arteriosum.

Your Body's Temperature Control

Your cardiovascular system helps keep you at the perfect temperature! When you're too hot, blood vessels near your skin widen (vasodilation), letting more warm blood flow close to the surface so heat can radiate away. When you're cold, those same vessels narrow (vasoconstriction), reducing blood flow to your skin to conserve precious core body heat. This is why your face gets red when you're hot and pale when you're cold!

Kidney Health Check: What Your Doctor Measures

Doctors measure something called GFR (Glomerular Filtration Rate) to check how well your kidneys are working. A normal GFR is about 125 mL per minute in healthy young adults. If your GFR stays below 60 mL/min for three months or more, it may indicate chronic kidney disease. If it drops below 15 mL/min, it may mean kidney failure. About 90% of the water your kidneys filter gets reabsorbed automatically as "obligatory water reabsorption," with the rest adjusted by hormones such as ADH.

Advanced: Cardiovascular and Renal Physiology

The cardiovascular system comprises two distinct but interconnected circuits: pulmonary circulation (right ventricle to pulmonary arteries to lungs for gas exchange, oxygenated blood returns via pulmonary veins to left atrium) and systemic circulation (left ventricle to aorta distributing oxygenated blood to body tissues, deoxygenated blood returns via superior and inferior vena cava to right atrium). This double circulation ensures separation of oxygenated and deoxygenated blood, maximizing oxygen delivery efficiency.

Blood functions as transport medium facilitating: oxygen transport (bound to hemoglobin in erythrocytes), nutrient delivery (glucose, amino acids, lipids from digestive system), hormone distribution, waste removal (CO2 to lungs, urea to kidneys, bilirubin to liver), thermoregulation, and immune surveillance.

The heart, a muscular organ weighing approximately 300g, is located in the mediastinum with the apex pointing inferolaterally toward the left hip. It is protected by the sternum anteriorly and flanked by lungs laterally (cardiac notch accommodates heart position). The heart wall consists of epicardium (visceral pericardium), myocardium (cardiac muscle with intercalated discs enabling functional syncytium for coordinated contraction), and endocardium.

Four valves ensure unidirectional blood flow: atrioventricular valves (tricuspid right, bicuspid/mitral left) and semilunar valves (pulmonary and aortic). The cardiac conduction system generates and coordinates electrical impulses: sinoatrial (SA) node (primary pacemaker, 60-100 bpm), internodal pathways, atrioventricular (AV) node (brief delay allowing atrial contraction before ventricular contraction), bundle of His, right and left bundle branches, and Purkinje fibers (rapid conduction to ventricular myocardium).

ECG recordings display P wave (atrial depolarization), QRS complex (ventricular depolarization), and T wave (ventricular repolarization), enabling diagnosis of arrhythmias, ischemia, and other cardiac conditions. Cardiac output (CO = heart rate × stroke volume) averages 5 L/min at rest. Blood pressure depends on cardiac output and total peripheral resistance. The kidneys filter approximately 180 L of blood daily through nephrons, producing 1-2 L of urine while maintaining fluid-electrolyte balance and blood pressure via the renin-angiotensin-aldosterone system.

Foetal Circulation: Specialized Adaptations

Foetal circulation represents a specialized circulatory adaptation that bypasses the non-functional foetal lungs through two critical shunts. Oxygenated blood arrives from the placenta via the umbilical vein, carrying approximately 80% oxygen saturation compared to maternal arterial blood's 98%. The foramen ovale is an interatrial septal opening that allows oxygenated blood from the right atrium to shunt directly into the left atrium, bypassing the pulmonary circuit entirely. The ductus arteriosus connects the pulmonary trunk to the aortic arch, diverting deoxygenated blood away from the lungs. At birth, increased oxygen tension and prostaglandin metabolism trigger closure: the foramen ovale seals to form the fossa ovalis, while the ductus arteriosus constricts to become the ligamentum arteriosum. Failure of these structures to close results in significant congenital heart defects.

Cardiovascular Thermoregulation

The cardiovascular system plays a critical role in thermoregulation through cutaneous blood flow modulation. During hyperthermia, sympathetic cholinergic activation induces vasodilation of cutaneous arterioles, increasing skin blood flow from approximately 250 mL/min to 6-8 L/min, facilitating heat dissipation via radiation and convection. Conversely, during hypothermia, sympathetic adrenergic stimulation causes marked vasoconstriction, reducing skin blood flow to minimal levels to conserve core body temperature. This autonomic regulation maintains core temperature within ±0.5°C of the 37°C set point despite environmental fluctuations.

Clinical Kidney Function Assessment

Glomerular Filtration Rate (GFR) represents the gold standard for assessing kidney function, measuring the volume of fluid filtered by glomeruli per minute. Normal GFR in healthy young adults is approximately 125 mL/min (range 90-140 mL/min), declining with age at ~1 mL/min/year after age 40. Clinically, chronic kidney disease (CKD) is suggested when GFR remains <60 mL/min for ≥3 months, while values at or below 15 mL/min may indicate kidney failure. GFR estimation uses serum creatinine-based equations (e.g., CKD-EPI, MDRD). Approximately 90% of filtered water undergoes obligatory water reabsorption as solutes are reclaimed, while the remaining fraction is adjusted by antidiuretic hormone (ADH) in the collecting ducts.

🫀 Cardio-Renal Control Points

Key Sensors and Protective Structures

Baroreceptors in the carotid sinus and aortic arch sense stretch, so they help detect blood pressure. Chemoreceptors sense changes in oxygen, carbon dioxide, and pH. These signals go to the medullary cardiovascular center, which adjusts heart rate, vessel tone, and cardiac output.

The heart sits inside the pericardium, a protective sac that reduces friction as the heart beats. The heart muscle itself receives oxygen from the coronary circulation, so blocked coronary arteries can quickly damage myocardium.

Urination is coordinated by the micturition reflex. Stretch receptors in the bladder wall signal the spinal cord, the detrusor muscle contracts, and the internal urethral sphincter relaxes; higher brain centers can delay voiding until it is appropriate.

Urine pH matters because it reflects how the kidneys are helping acid-base balance. Very acidic or alkaline urine can also influence crystal formation, infection risk, and the action of some drugs.

Transport, Pressure, and Urine Checks

Transport: In, Through, Out

Week 4 is not only about blood moving in circles. The source material emphasises what the cardiovascular system moves into the body, through the body, and out of the body. Oxygen enters at the lungs, nutrients and water are absorbed from the gut, and the blood then distributes gases, hormones, proteins, immune cells, and stored nutrients to tissues. Wastes such as carbon dioxide, urea, bile pigments, excess salts, and heat are carried to organs that remove them.

External respiration happens in the lungs, where oxygen diffuses from alveoli into pulmonary capillaries and carbon dioxide diffuses into the alveoli. Internal respiration happens at tissues, where oxygen leaves systemic capillaries and carbon dioxide enters the blood. Water absorption depends on osmosis, and the liver also helps process and dispose of materials such as bilirubin.

Pressure, Flow, Volume, Resistance

The heart generates blood pressure, but pressure alone does not explain circulation. The source lecture links blood pressure, blood flow, blood volume, and vascular resistance. Cardiac output is the total blood flow each minute: CO = heart rate × stroke volume. Blood pressure is influenced by cardiac output, blood volume, and resistance. As resistance rises, flow falls.

Resistance depends mainly on three things: total vessel length, lumen diameter, and blood viscosity. The most important rapid change is vessel diameter: when a vessel narrows, resistance increases sharply and blood flow falls. This is why vasoconstriction can support blood pressure, while widespread vasodilation can contribute to dizziness or fainting.

Urine Formation and Urinalysis

Urine is formed in three linked steps. Filtration moves fluid from blood into the nephron at the renal corpuscle. Reabsorption returns useful substances and most water back to the blood. Secretion moves additional substances from blood into the tubule. More than 99% of filtrate is normally reabsorbed, so about 150-180 L of filtrate becomes only 1-2 L of urine per day.

Urinalysis does more than estimate GFR. Source-backed checks include albumin, glucose, erythrocytes, leukocytes, ketone bodies, pH, odour, and colour or clarity. Normal urine pH is roughly 4.5 to 8. Acidic urine can occur with states such as ketoacidosis or starvation, while alkaline urine can occur with impaired acid removal, vomiting, kidney failure, or urinary tract infection.

Urinary Microbiome and UTIs

The kidneys are normally considered sterile, but urine itself is not completely sterile. Normal voiding helps flush microorganisms from the bladder and urethra. In females, healthy microbiota such as Lactobacillus help make the environment less favourable for pathogens.

When normal microbial balance is disrupted, infection becomes more likely. Antibiotic use, reduced oestrogen, and anatomy can increase risk, and women are generally at greater risk of UTIs. E. coli causes most UTIs. This is why urinary health is not just about filtration, but also about microbial balance and normal urine flow.

🎥 Video Lectures

Week 4 Overview

Advanced circulation and kidney function. Notes-only topics below open in the topic panel instead of loading a video.

Topic Title

Select a topic from the list to view detailed information.

📄 Lecture Notes

Key Terms

Pulmonary Circulation

Circuit carrying deoxygenated blood from right ventricle to lungs for oxygenation and return to left atrium

Systemic Circulation

Circuit distributing oxygenated blood from left ventricle to body tissues and returning deoxygenated blood to right atrium

Hemoglobin

Iron-containing protein in erythrocytes binding oxygen in lungs and releasing it to tissues

Sinoatrial Node

Natural pacemaker located in right atrium; initiates electrical impulses for heart contraction at 60-100 bpm

Atrioventricular Node

Electrical relay station between atria and ventricles; introduces delay allowing atrial contraction before ventricular contraction

Purkinje Fibers

Specialized cardiac conduction fibers rapidly transmitting electrical impulses throughout ventricular myocardium

Electrocardiogram

Recording of heart's electrical activity showing P wave, QRS complex, and T wave; used to diagnose cardiac conditions

Cardiac Output

Volume of blood pumped by ventricle per minute; calculated as heart rate multiplied by stroke volume (average 5 L/min)

Stroke Volume

Volume of blood ejected from ventricle during single contraction; regulated by preload, afterload, and contractility

Blood Pressure

Force exerted by blood against vessel walls; systolic (contraction) over diastolic (relaxation) in mmHg

Intercalated Discs

Specialized junctions between cardiac muscle cells enabling rapid electrical transmission and mechanical coupling

Syncytium

Mass of cytoplasm with multiple nuclei functioning as coordinated unit; describes cardiac muscle contraction

Nephrons

Functional filtering units of kidneys; approximately 1 million per kidney processing blood to form urine

Glomerulus

Tuft of capillaries in nephron where blood filtration occurs; filtrate enters Bowman's capsule

Erythropoietin

Hormone produced by kidneys stimulating red blood cell production in bone marrow in response to low oxygen

Renin-Angiotensin-Aldosterone System

Hormonal cascade regulating blood pressure and fluid balance; initiated by kidney-released renin

Filtration

Process in kidneys where blood plasma passes through glomerular barrier into nephron tubule

Reabsorption

Process returning essential substances (water, glucose, ions) from filtrate back into blood

Calcitriol

Active form of vitamin D produced by kidneys; essential for calcium absorption and bone health

Aorta

Largest artery in body; carries oxygenated blood from left ventricle to systemic circulation

Internal Respiration

Gas exchange between systemic capillary blood and body tissues; oxygen moves into tissues and carbon dioxide moves into blood

External Respiration

Gas exchange between alveoli and pulmonary capillary blood in the lungs

Vascular Resistance

Opposition to blood flow caused by friction between blood and vessel walls; affected by vessel length, lumen diameter, and viscosity

Urinalysis

Routine assessment of urine quantity and quality, checking findings such as albumin, glucose, blood cells, leukocytes, ketones, pH, odour, and clarity

Urinary Microbiome

Normal microorganisms associated with the urinary tract; disruption of protective flora can increase UTI risk

UTI

Infection affecting part of the urinary tract; often caused by E. coli and more common when normal urinary defenses are disrupted

Interactive Activity

End of Week Test

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