Human Blood, Circulation and Excretion: Structure and Function

Interstitial Fluid

Interstitial fluid: the liquid that surrounds the cells, which renews constantly and contains nutrients, oxygen and other substances for exchange.

Blood

Composition: ^{Plasma sanguini} (H₂O + molecules), including proteins, hormones, salts, minerals, nutrients and substances for exchange.
^{Cèl·lules sanguínies:} formed in the interior of the bones, from the bone marrow.

• Red blood cells: contain haemoglobin, a protein that carries oxygen.
• White blood cells: take part in the body's defence against and phagocytosis of microbes.
• Platelets: fragments of cells that help coagulate the blood and heal wounds.

Types of Blood Vessels

1. ^Arteries: A. Carry blood away from the heart. B. Branch into smaller arteries and arterioles; prone to arteriosclerosis.
2. ^Veins: A. Carry blood back to the heart. B. Form venules.
3. ^Capillaries: A. Microscopic. B. Branch from arterioles. C. Make direct contact with body cells, allowing the exchange of gases, substances and nutrients.

Heart Movements

1. ^1. Atrial systole: The atria contract and open the atrioventricular valves. Blood flows into the ventricles.
2. ^2. Ventricular systole: The two ventricles contract and push blood out through the sigmoid valves. The atrioventricular valves close.
3. ^3. Diastole: The heart relaxes and blood from the veins fills the atria and ventricles.

Cardiac cycle: atrial systole + ventricular systole + diastole.

Blood flow: veins → atria → ventricles → arteries.

Excretory Apparatus

• Liver: recycles red blood cells, removes cholesterol and toxic substances.
• Urinary apparatus: removes large quantities of excretory products.
• Respiratory apparatus: expels CO₂.
• Sudoriparous glands: expel sweat, which cools the skin and expels some excretory substances.

Urinary Apparatus

Parts: renal cortex (external part) and renal medulla (internal part), leading to the renal pelvis.

Kidney Functions

1. ^{Excretory function:} each kidney receives oxygenated blood from the renal artery and, after filtering it, returns the cleaned blood to the body through the renal vein, removing waste substances.
2. ^{Internal regulatory function:} keeps the composition of the internal environment in balance.
3. ^{Stimulation of red blood cell production:} helps produce red blood cells.

Urine Formation

Stages:

1. A. Glomerular filtration: the blood is filtered, passing H₂O, nutrients and waste products into the Bowman's capsule.
2. B. Tubular reabsorption: as the filtrate travels through the nephron tubules, most of the useful substances are reabsorbed: glucose, vitamins, amino acids, H₂O and salts.
3. C. Substances that are not useful are excreted as urine.