Functions: circulation of oxygen, nutrients, hormones, ions and fluids, removal of metabolic waste
The Circulatory system transports oxygen, nutrients, hormones, ions, and fluids throughout the body while removing metabolic waste.
In the lungs, oxygen diffuses into the blood at alveolar capillaries and binds to hemoglobin in red blood cells, then travels to tissues for cellular respiration. Meanwhile, carbon dioxide, produced by cellular respiration, is converted by carbonic anhydrase into bicarbonate, dissolved in the blood, or bound to hemoglobin and plasma proteins. This carbon dioxide then diffuses back into alveolar capillaries and is exhaled.
Absorbed nutrients enter the bloodstream from the small intestine, while hormones released by endocrine glands reach target cells via circulation.
Fluids and ions in the blood are regulated by the kidney, which reabsorbs water and salt as needed. Urea, a primary waste product, is carried in the blood to the kidneys for excretion in urine.
Four-chambered heart: structure and function
The four-chambered heart enables both systemic circulation (supplying the body with oxygenated blood) and pulmonary circulation (sending deoxygenated blood to the lungs for oxygenation).
Deoxygenated blood returns via the vena cava to the right atrium, passes through the tricuspid valve into the right ventricle, and flows through the pulmonary valve into the pulmonary artery toward the lungs.
After oxygenation, blood travels back through the pulmonary vein to the left atrium, crosses the bicuspid (mitral) valve into the left ventricle, and finally exits through the aortic valve into the aorta, delivering oxygen-rich blood to the body.
Endothelial cells
Endothelial cells create a single-cell thick lining throughout all blood vessels, facilitating exchanges between the blood and nearby tissues. They regulate vascular tone and blood flow by producing both relaxing and contracting agents, such as nitric oxide, various peptides (including endothelin, urotensin, CNP, and adrenomedullin), along with adenosine and other purines.
Although circulating endothelial cells—mature cells shed from blood vessels—are typically found in very low numbers, their increased presence is often an indicator of vascular damage in various diseases.
Systolic and diastolic pressure
During a heartbeat, systolic pressure measures the force in blood vessels when the ventricles contract, while diastolic pressure reflects the pressure when the ventricles relax. These pressures ensure efficient movement of blood through the heart’s chambers, valves, and connecting vessels, sustaining vital oxygen and nutrient transport throughout the body.
Pulmonary and systemic circulation
Pulmonary circulation carries deoxygenated blood from the heart to the lungs, where alveoli reoxygenate it, then returns oxygenated blood to the heart. It is shorter than systemic circulation, resulting in lower resistance and blood pressure. Additionally, low oxygen levels in the lungs trigger vasoconstriction, redirecting blood to well-ventilated areas for efficient gas exchange. Conversely, systemic circulation delivers oxygenated blood from the heart to the body, and low oxygen levels in tissues cause vasodilation so that more blood reaches oxygen-starved regions.
Once blood leaves the heart, it flows through…
Composition and constriction of blood vessels by type
Arteries, especially elastic arteries like the aorta, have considerable elastic tissue to handle the pressure from each heartbeat.
Muscular arteries distribute blood to specific organs, containing more smooth muscle to regulate vessel diameter.
Capillaries, by contrast, lack smooth muscle and do not actively constrict, focusing instead on gas and solute exchange, while smaller bronchiole-like arterioles are the primary regulators of blood flow through constriction and dilation.
Venule vessels bridge the gap between capillaries and veins, gradually increasing in size but lacking the ability to constrict.
A vein typically features endothelium, smooth muscle, and connective tissue, returning blood to the heart at relatively low pressure. Valves in veins prevent backflow, and adaptations like the respiratory pump (changes in chest and abdominal pressure) and muscular pump (skeletal muscle contractions) aid venous return. Although often thinner-walled than arteries, veins can still constrict via their smooth muscle when stimulated.
Blood pressure and diffusion
Pressure in the circulation is highest in arteries (notably the aorta) and steadily decreases through arterioles, capillaries, venules, and finally the veins (lowest in the vena cava). Elastic recoil in arteries helps sustain blood flow even during diastole, preventing pressure from dropping to zero.
Within capillary beds, a single cell layer of endothelium enables efficient diffusion of gases and solutes. Depending on tissue requirements, these vessels can be continuous, fenestrated (with small pores), or sinusoidal (with large pores).
Diffusion occurs freely in thin-walled capillaries, supporting nutrient delivery and waste removal. Thermoregulation in these networks involves heat loss by radiation, conduction, or evaporative cooling through mechanisms such as vasodilation near the skin’s surface.
Blood pressure is regulated by changes in vessel diameter (vasoconstriction or vasodilation) and hormone levels (e.g., ADH, aldosterone, renin, adrenaline), which modify flow resistance. Factors contributing to peripheral resistance include blood viscosity, total vessel length, and lumen diameter; increased body mass or plaques within vessels can further raise resistance. This interplay of vessel structure, fluid dynamics, and external influences ensures adequate circulation and maintains homeostasis.
Composition of blood
Composition of blood includes:
Production and regulation of blood components
In coagulation, the process by which blood transforms from a liquid into a gel-like clot, the liver produces clotting factors like fibrinogen, which a cascade of reactions converts into fibrin to form a protective mesh over wounds. Platelets initiate this process by aggregating at a wound site, releasing enzymes and chemicals that activate clotting factors. Finally, the clot retracts and compacts, and after the blood vessel is repaired, it dissolves.
New erythrocytes are produced in the bone marrow, while the spleen removes aged and damaged ones, recycling iron and converting heme into bilirubin.
Blood osmolarity affects fluid movement: higher osmolarity pulls water into the bloodstream, whereas lower osmolarity draws it into tissues.
Hormones such as ADH (vasopressin) and aldosterone increase water and salt reabsorption in the kidneys, boosting blood volume. Blood volume is primarily managed by the kidneys regulating the loss of water and sodium in the urine.
The percentage of erythrocytes in blood, known as the hematocrit, is usually around 45%. Oxygen transport depends on hemoglobin’s affinity, which can be altered by factors like pH, temperature, and carbon dioxide levels.
Lastly, myoglobin binds oxygen more tightly than hemoglobin, allowing certain tissues to store extra oxygen for later use.
Nervous and endocrine control in blood pressure and transport
The nervous system and endocrine system work together to regulate blood transport throughout the body. The nervous system quickly adjusts heart rate and modulates vasoconstriction and vasodilation through autonomic signals, ensuring rapid responses to changes in activity or stress. At the same time, hormones released by the endocrine system—such as adrenaline and norepinephrine—fine-tune these adjustments, helping to increase blood pressure and promote vasoconstriction during acute stress.
Over longer periods, systems like the renin-angiotensin-aldosterone system and ADH help regulate blood volume and maintain stable blood pressure by controlling water and salt balance.
Baroreceptors monitor pressure changes and send feedback to both systems, ensuring that tissues receive adequate blood flow under varying conditions.
Sign up for free to take 8 quiz questions on this topic