Heart Function, Blood Circulation, and Kidney Filtration
Heart Function and Blood Circulation
Phases of the Cardiac Cycle
Atrial Systole
The atria contract, pumping blood into the ventricles through the atrioventricular valves.
Ventricular Systole
The ventricles contract. Blood exits into the pulmonary artery and aorta as the semilunar valves open. Simultaneously, the atrioventricular valves close.
Diastole
The heart relaxes. Blood from the vena cavae and pulmonary veins enters the atria. Blood just pumped into the arteries cannot return to the ventricles due to the closure of the semilunar (sigmoid) valves.
Venous Valves (Swallow’s Nest)
These valves are located in the walls of veins. They only allow blood to flow towards the heart, preventing backflow.
Electrocardiogram (ECG/EKG)
An electrocardiogram is a graph that records the electrical activity during the heart’s cycle.
Blood Pressure
Maximum blood pressure (systolic pressure) represents the highest force exerted by blood against the walls of the arteries during ventricular contraction.
Circulatory Circuits
Systemic Circuit (Greater Circulation)
This circuit starts from the left ventricle, pumping oxygenated blood into the aorta. The aorta branches into arteries, then arterioles, and finally into capillaries that reach all body tissues and organs (except the lungs). In the capillaries, blood delivers nutrients and oxygen to cells and picks up waste products and carbon dioxide. The deoxygenated blood then returns to the right atrium via venules and veins, culminating in the two venae cavae.
Pulmonary Circuit (Lesser Circulation)
This circuit begins in the right ventricle, pumping deoxygenated blood into the pulmonary artery. This artery divides into two, one for each lung, where they further subdivide alongside the bronchi. Around the pulmonary alveoli, a dense capillary network facilitates gas exchange: carbon dioxide is released, and oxygen is taken up. Oxygenated blood originates in capillary venules, which merge into the four pulmonary veins. These veins empty into the left atrium. Blood then passes through the mitral valve into the left ventricle, starting the systemic circuit again.
Lymphatic and Excretory Systems
The Lymphatic System
Lymph
Lymph is a fluid circulating within lymphatic vessels. It is formed from interstitial fluid (plasma collected from tissues) containing white blood cells and is filtered through lymph nodes.
Lymphatic Capillaries and Vessels
Lymphatic capillaries are found in tissues. They are blind-ended (closed at one end) and collect interstitial fluid, which is then called lymph. These capillaries join to form larger lymphatic ducts and vessels, which eventually drain lymph back into the bloodstream.
The Kidneys and Urinary System
Kidney Structure and Function
The kidneys are bean-shaped organs located at the back of the abdomen, on either side of the spine. Each kidney consists of an outer layer called the renal cortex, an inner layer called the renal medulla, and a central cavity known as the renal pelvis.
The Nephron
Each kidney contains over a million functional units called nephrons. Nephrons are small tubules, closed at one end and surrounded by a blood capillary network. They are responsible for filtering blood and producing urine. These tubules end in collecting ducts, which empty into the renal pelvis.
Kidney Blood Supply and Other Roles
Each kidney receives oxygenated blood via a renal artery and drains deoxygenated, filtered blood via a renal vein back towards the heart. Besides excretion, kidneys perform other vital physiological functions. They actively regulate the internal environment (homeostasis) and secrete substances that stimulate red blood cell production and regulate blood pressure.
The Urinary Tract
These are the conduits for urine removal:
Ureters
Two narrow tubes, approximately 28 cm long, connecting each kidney to the bladder.
Urethra
The tube through which urine exits the body from the bladder.